avoid issuing -Wrestrict from folder (PR 93519)

PR 93519 reports a false positive -Wrestrict issued for an inlined call
to strcpy that carefully guards against self-copying.  This is caused
by the caller's arguments substituted into the call during inlining and
before dead code elimination.

The attached patch avoids this by removing -Wrestrict from the folder
and deferring folding perfectly overlapping (and so undefined) calls
to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
perfectly overlapping calls to memcpy are still folded early.

Tested on x86_64-linux.

Martin

On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
>
> PR 93519 reports a false positive -Wrestrict issued for an inlined call
> to strcpy that carefully guards against self-copying.  This is caused
> by the caller's arguments substituted into the call during inlining and
> before dead code elimination.
>
> The attached patch avoids this by removing -Wrestrict from the folder
> and deferring folding perfectly overlapping (and so undefined) calls
> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> perfectly overlapping calls to memcpy are still folded early.

Why do we bother to warn at all for this case?  Just DWIM here.  Warnings like
this can be emitted from the analyzer?

That is, I suggest to simply remove the bogus warning code from folding
(and _not_ fail the folding).

Richard.

> Tested on x86_64-linux.
>
> Martin

On 2/4/20 2:34 AM, Richard Biener wrote:
> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
>>
>> PR 93519 reports a false positive -Wrestrict issued for an inlined call
>> to strcpy that carefully guards against self-copying.  This is caused
>> by the caller's arguments substituted into the call during inlining and
>> before dead code elimination.
>>
>> The attached patch avoids this by removing -Wrestrict from the folder
>> and deferring folding perfectly overlapping (and so undefined) calls
>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
>> perfectly overlapping calls to memcpy are still folded early.
> 
> Why do we bother to warn at all for this case?  Just DWIM here.  Warnings like
> this can be emitted from the analyzer?
> 
> That is, I suggest to simply remove the bogus warning code from folding
> (and _not_ fail the folding).

The overlapping copy is ultimately folded into a no-op but the warning
points out that code that relies on it is undefined.  The code should
be fixed.  This is in line with one of the strategies we discussed and
(at least those of us in the room) agreed on for undefined behavior
back in Manchester: try to do the least harm but warn.

Martin

On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> > PR 93519 reports a false positive -Wrestrict issued for an inlined call
> > to strcpy that carefully guards against self-copying.  This is caused
> > by the caller's arguments substituted into the call during inlining and
> > before dead code elimination.
> > 
> > The attached patch avoids this by removing -Wrestrict from the folder
> > and deferring folding perfectly overlapping (and so undefined) calls
> > to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> > perfectly overlapping calls to memcpy are still folded early.
> 
> Why do we bother to warn at all for this case?  Just DWIM here.  Warnings like
> this can be emitted from the analyzer?
They potentially can, but the analyzer is and will almost always
certainly be considerably slower.  I would not expect it to be used
nearly as much as the core compiler.

WHether or not a particular warning makes sense in the core compiler or
analyzer would seem to me to depend on whether or not we can reasonably
issue warnings without interprocedural analysis.  double-free
realistically requires interprocedural analysis to be effective.  I'm
not sure Wrestrict really does.


> 
> That is, I suggest to simply remove the bogus warning code from folding
> (and _not_ fail the folding).
I haven't looked at the patch, but if we can get the warning out of the
folder that's certainly preferable.  And we could investigate deferring
self-copy removal.

Jeff

On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
>On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
>> > PR 93519 reports a false positive -Wrestrict issued for an inlined
>call
>> > to strcpy that carefully guards against self-copying.  This is
>caused
>> > by the caller's arguments substituted into the call during inlining
>and
>> > before dead code elimination.
>> > 
>> > The attached patch avoids this by removing -Wrestrict from the
>folder
>> > and deferring folding perfectly overlapping (and so undefined)
>calls
>> > to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
>> > perfectly overlapping calls to memcpy are still folded early.
>> 
>> Why do we bother to warn at all for this case?  Just DWIM here. 
>Warnings like
>> this can be emitted from the analyzer?
>They potentially can, but the analyzer is and will almost always
>certainly be considerably slower.  I would not expect it to be used
>nearly as much as the core compiler.
>
>WHether or not a particular warning makes sense in the core compiler or
>analyzer would seem to me to depend on whether or not we can reasonably
>issue warnings without interprocedural analysis.  double-free
>realistically requires interprocedural analysis to be effective.  I'm
>not sure Wrestrict really does.
>
>
>> 
>> That is, I suggest to simply remove the bogus warning code from
>folding
>> (and _not_ fail the folding).
>I haven't looked at the patch, but if we can get the warning out of the
>folder that's certainly preferable.  And we could investigate deferring
>self-copy removal.

I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature. 

Richard. 

>Jeff

On 2/4/20 12:15 PM, Richard Biener wrote:
> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
>> call
>>>> to strcpy that carefully guards against self-copying.  This is
>> caused
>>>> by the caller's arguments substituted into the call during inlining
>> and
>>>> before dead code elimination.
>>>>
>>>> The attached patch avoids this by removing -Wrestrict from the
>> folder
>>>> and deferring folding perfectly overlapping (and so undefined)
>> calls
>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
>>>> perfectly overlapping calls to memcpy are still folded early.
>>>
>>> Why do we bother to warn at all for this case?  Just DWIM here.
>> Warnings like
>>> this can be emitted from the analyzer?
>> They potentially can, but the analyzer is and will almost always
>> certainly be considerably slower.  I would not expect it to be used
>> nearly as much as the core compiler.
>>
>> WHether or not a particular warning makes sense in the core compiler or
>> analyzer would seem to me to depend on whether or not we can reasonably
>> issue warnings without interprocedural analysis.  double-free
>> realistically requires interprocedural analysis to be effective.  I'm
>> not sure Wrestrict really does.
>>
>>
>>>
>>> That is, I suggest to simply remove the bogus warning code from
>> folding
>>> (and _not_ fail the folding).
>> I haven't looked at the patch, but if we can get the warning out of the
>> folder that's certainly preferable.  And we could investigate deferring
>> self-copy removal.
> 
> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.

In this instance the code is reachable (or isn't obviously unreachable).
GCC doesn't remove it, but provides benign (and reasonable) semantics
for it(*).  To me, that's one aspect of quality.  Letting the user know
that the code is buggy is another.  I view that as at least as important
as folding the ill-effects away because it makes it possible to fix
the problem so the code works correctly even with compilers that don't
provide these benign semantics.

Martin

[*] This is in contrast to the usual (though arguably inferior) strategy
GCC employs when dealing with undefined calls to built-in functions:
call the library function even when it's certain the call will crash.

On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> On 2/4/20 12:15 PM, Richard Biener wrote:
> > On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> > > On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> > > > On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> > > > > PR 93519 reports a false positive -Wrestrict issued for an inlined
> > > call
> > > > > to strcpy that carefully guards against self-copying.  This is
> > > caused
> > > > > by the caller's arguments substituted into the call during inlining
> > > and
> > > > > before dead code elimination.
> > > > > 
> > > > > The attached patch avoids this by removing -Wrestrict from the
> > > folder
> > > > > and deferring folding perfectly overlapping (and so undefined)
> > > calls
> > > > > to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> > > > > perfectly overlapping calls to memcpy are still folded early.
> > > > 
> > > > Why do we bother to warn at all for this case?  Just DWIM here.
> > > Warnings like
> > > > this can be emitted from the analyzer?
> > > They potentially can, but the analyzer is and will almost always
> > > certainly be considerably slower.  I would not expect it to be used
> > > nearly as much as the core compiler.
> > > 
> > > WHether or not a particular warning makes sense in the core compiler or
> > > analyzer would seem to me to depend on whether or not we can reasonably
> > > issue warnings without interprocedural analysis.  double-free
> > > realistically requires interprocedural analysis to be effective.  I'm
> > > not sure Wrestrict really does.
> > > 
> > > 
> > > > That is, I suggest to simply remove the bogus warning code from
> > > folding
> > > > (and _not_ fail the folding).
> > > I haven't looked at the patch, but if we can get the warning out of the
> > > folder that's certainly preferable.  And we could investigate deferring
> > > self-copy removal.
> > 
> > I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> 
> In this instance the code is reachable (or isn't obviously unreachable).
> GCC doesn't remove it, but provides benign (and reasonable) semantics
> for it(*).  To me, that's one aspect of quality.  Letting the user know
> that the code is buggy is another.  I view that as at least as important
> as folding the ill-effects away because it makes it possible to fix
> the problem so the code works correctly even with compilers that don't
> provide these benign semantics.
If you look at the guts of what happens at the point where we issue the
warning from within gimple_fold_builtin_strcpy we have:

> DCH_to_char (char * in, char * out, int collid)
> {
>   int type;
>   char * D.2148;
>   char * dest;
>   char * num;
>   long unsigned int _4;
>   char * _5;
> 
> ;;   basic block 2, loop depth 0
> ;;    pred:       ENTRY
> ;;    succ:       4
> 
> ;;   basic block 4, loop depth 0
> ;;    pred:       2
> ;;    succ:       5
> 
> ;;   basic block 5, loop depth 0
> ;;    pred:       4
> ;;    succ:       6
> 
> ;;   basic block 6, loop depth 0
> ;;    pred:       5
>   if (0 != 0)
>     goto <bb 7>; [53.47%]
>   else
>     goto <bb 8>; [46.53%]
> ;;    succ:       7
> ;;                8
> 
> ;;   basic block 7, loop depth 0
> ;;    pred:       6
>   strcpy (out_1(D), out_1(D));
> ;;    succ:       8
> 
> ;;   basic block 8, loop depth 0
> ;;    pred:       6
> ;;                7
>   _4 = __builtin_strlen (out_1(D));
>   _5 = out_1(D) + _4;
>   __builtin_memcpy (_5, "foo", 4);
> ;;    succ:       3
> 
> ;;   basic block 3, loop depth 0
> ;;    pred:       8
>   return;
> ;;    succ:       EXIT
> 
> }
> 

Which shows the code is obviously unreachable in the case we're warning
about.  You can't see this in the dumps because it's exposed by
inlining, then cleaned up before writing the dump file.

ISTM this would be a case we could handle with the __builtin_warning
stuff. 

I think the question is do we want to do anything about it this cycle?
 

If so, I think Martin's approach is quite reasonable.  It disables
folding away the self-copies from gimple-fold and moves the warning
into the expander.  So if there's such a call in the IL at expansion
time we get a warning (-O0).

I'd hazard a guess that the diagnostic was added to the strlen pass to
capture the missed warning when we're optimizing and the self-copy has
survived until that point. There's a couple issues that raises though.

First, it's insufficient.  DSE (for example) can do self-copy removal,
so it needs the same handling.  There may be others places too.

Second, if the code becomes unreachable after strlen, then we've got
new false positive issues.

It's the classic problems we have with all middle end based warnings.

But I could live with those if we can show that using __builtin_warning
to handle this stuff in gcc-11 works...  ISTM we emit the
__builtin_warning call before any self-copy like that, whenever we
happen to spot them.  They'll naturally get removed if the path becomes
unreachable.  We'd warn during expansion for calls to
__builtin_warning.  We could even optionally warn when removing a call
to __bulitin_warning.

Thoughts?

Jeff

On 2/4/20 2:31 PM, Jeff Law wrote:
> On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
>> On 2/4/20 12:15 PM, Richard Biener wrote:
>>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
>>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
>>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
>>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
>>>> call
>>>>>> to strcpy that carefully guards against self-copying.  This is
>>>> caused
>>>>>> by the caller's arguments substituted into the call during inlining
>>>> and
>>>>>> before dead code elimination.
>>>>>>
>>>>>> The attached patch avoids this by removing -Wrestrict from the
>>>> folder
>>>>>> and deferring folding perfectly overlapping (and so undefined)
>>>> calls
>>>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
>>>>>> perfectly overlapping calls to memcpy are still folded early.
>>>>>
>>>>> Why do we bother to warn at all for this case?  Just DWIM here.
>>>> Warnings like
>>>>> this can be emitted from the analyzer?
>>>> They potentially can, but the analyzer is and will almost always
>>>> certainly be considerably slower.  I would not expect it to be used
>>>> nearly as much as the core compiler.
>>>>
>>>> WHether or not a particular warning makes sense in the core compiler or
>>>> analyzer would seem to me to depend on whether or not we can reasonably
>>>> issue warnings without interprocedural analysis.  double-free
>>>> realistically requires interprocedural analysis to be effective.  I'm
>>>> not sure Wrestrict really does.
>>>>
>>>>
>>>>> That is, I suggest to simply remove the bogus warning code from
>>>> folding
>>>>> (and _not_ fail the folding).
>>>> I haven't looked at the patch, but if we can get the warning out of the
>>>> folder that's certainly preferable.  And we could investigate deferring
>>>> self-copy removal.
>>>
>>> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
>>
>> In this instance the code is reachable (or isn't obviously unreachable).
>> GCC doesn't remove it, but provides benign (and reasonable) semantics
>> for it(*).  To me, that's one aspect of quality.  Letting the user know
>> that the code is buggy is another.  I view that as at least as important
>> as folding the ill-effects away because it makes it possible to fix
>> the problem so the code works correctly even with compilers that don't
>> provide these benign semantics.
> If you look at the guts of what happens at the point where we issue the
> warning from within gimple_fold_builtin_strcpy we have:
> 
>> DCH_to_char (char * in, char * out, int collid)
>> {
>>    int type;
>>    char * D.2148;
>>    char * dest;
>>    char * num;
>>    long unsigned int _4;
>>    char * _5;
>>
>> ;;   basic block 2, loop depth 0
>> ;;    pred:       ENTRY
>> ;;    succ:       4
>>
>> ;;   basic block 4, loop depth 0
>> ;;    pred:       2
>> ;;    succ:       5
>>
>> ;;   basic block 5, loop depth 0
>> ;;    pred:       4
>> ;;    succ:       6
>>
>> ;;   basic block 6, loop depth 0
>> ;;    pred:       5
>>    if (0 != 0)
>>      goto <bb 7>; [53.47%]
>>    else
>>      goto <bb 8>; [46.53%]
>> ;;    succ:       7
>> ;;                8
>>
>> ;;   basic block 7, loop depth 0
>> ;;    pred:       6
>>    strcpy (out_1(D), out_1(D));
>> ;;    succ:       8
>>
>> ;;   basic block 8, loop depth 0
>> ;;    pred:       6
>> ;;                7
>>    _4 = __builtin_strlen (out_1(D));
>>    _5 = out_1(D) + _4;
>>    __builtin_memcpy (_5, "foo", 4);
>> ;;    succ:       3
>>
>> ;;   basic block 3, loop depth 0
>> ;;    pred:       8
>>    return;
>> ;;    succ:       EXIT
>>
>> }
>>
> 
> Which shows the code is obviously unreachable in the case we're warning
> about.  You can't see this in the dumps because it's exposed by
> inlining, then cleaned up before writing the dump file.

In the specific case of the bug the code is of course eliminated
because it's guarded by the if (s != d).  I was referring to
the general (unguarded) case of:

   char *s = "", *p;

   int main (void)
   {
     p = strcpy (s, s);
     puts (p);
   }

where GCC folds the assignment 'p = strcpy(s, s);' to effectively
p = s;  That's perfectly reasonable but it could equally as well
leave the call alone, as it does when s is null, for instance.

I think folding it away is not only reasonable but preferable to
making the invalid call, but it's done only rarely.  Most of
the time GCC does emit the undefined access (it does that with
calls to library functions as well as with direct stores and
reads).  (I am hoping we can change that in the future so that
these kinds of problems are handled with some consistency.)

> 
> ISTM this would be a case we could handle with the __builtin_warning
> stuff.
> 
> I think the question is do we want to do anything about it this cycle?
>   
> 
> If so, I think Martin's approach is quite reasonable.  It disables
> folding away the self-copies from gimple-fold and moves the warning
> into the expander.  So if there's such a call in the IL at expansion
> time we get a warning (-O0).
> 
> I'd hazard a guess that the diagnostic was added to the strlen pass to
> capture the missed warning when we're optimizing and the self-copy has
> survived until that point. There's a couple issues that raises though.
> 
> First, it's insufficient.  DSE (for example) can do self-copy removal,
> so it needs the same handling.  There may be others places too.
> 
> Second, if the code becomes unreachable after strlen, then we've got
> new false positive issues.
> 
> It's the classic problems we have with all middle end based warnings.
> 
> But I could live with those if we can show that using __builtin_warning
> to handle this stuff in gcc-11 works...  ISTM we emit the
> __builtin_warning call before any self-copy like that, whenever we
> happen to spot them.  They'll naturally get removed if the path becomes
> unreachable.  We'd warn during expansion for calls to
> __builtin_warning.  We could even optionally warn when removing a call
> to __bulitin_warning.
> 
> Thoughts?

The patch has pretty much the same effect as emitting __builtin_warning
from the folder would.  It defers the folding until much later, and if
the code isn't eliminated, it issues a warning and folds the call away.


Martin

On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
>
> On 2/4/20 2:31 PM, Jeff Law wrote:
> > On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> >> On 2/4/20 12:15 PM, Richard Biener wrote:
> >>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> >>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> >>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> >>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
> >>>> call
> >>>>>> to strcpy that carefully guards against self-copying.  This is
> >>>> caused
> >>>>>> by the caller's arguments substituted into the call during inlining
> >>>> and
> >>>>>> before dead code elimination.
> >>>>>>
> >>>>>> The attached patch avoids this by removing -Wrestrict from the
> >>>> folder
> >>>>>> and deferring folding perfectly overlapping (and so undefined)
> >>>> calls
> >>>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> >>>>>> perfectly overlapping calls to memcpy are still folded early.
> >>>>>
> >>>>> Why do we bother to warn at all for this case?  Just DWIM here.
> >>>> Warnings like
> >>>>> this can be emitted from the analyzer?
> >>>> They potentially can, but the analyzer is and will almost always
> >>>> certainly be considerably slower.  I would not expect it to be used
> >>>> nearly as much as the core compiler.
> >>>>
> >>>> WHether or not a particular warning makes sense in the core compiler or
> >>>> analyzer would seem to me to depend on whether or not we can reasonably
> >>>> issue warnings without interprocedural analysis.  double-free
> >>>> realistically requires interprocedural analysis to be effective.  I'm
> >>>> not sure Wrestrict really does.
> >>>>
> >>>>
> >>>>> That is, I suggest to simply remove the bogus warning code from
> >>>> folding
> >>>>> (and _not_ fail the folding).
> >>>> I haven't looked at the patch, but if we can get the warning out of the
> >>>> folder that's certainly preferable.  And we could investigate deferring
> >>>> self-copy removal.
> >>>
> >>> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> >>
> >> In this instance the code is reachable (or isn't obviously unreachable).
> >> GCC doesn't remove it, but provides benign (and reasonable) semantics
> >> for it(*).  To me, that's one aspect of quality.  Letting the user know
> >> that the code is buggy is another.  I view that as at least as important
> >> as folding the ill-effects away because it makes it possible to fix
> >> the problem so the code works correctly even with compilers that don't
> >> provide these benign semantics.
> > If you look at the guts of what happens at the point where we issue the
> > warning from within gimple_fold_builtin_strcpy we have:
> >
> >> DCH_to_char (char * in, char * out, int collid)
> >> {
> >>    int type;
> >>    char * D.2148;
> >>    char * dest;
> >>    char * num;
> >>    long unsigned int _4;
> >>    char * _5;
> >>
> >> ;;   basic block 2, loop depth 0
> >> ;;    pred:       ENTRY
> >> ;;    succ:       4
> >>
> >> ;;   basic block 4, loop depth 0
> >> ;;    pred:       2
> >> ;;    succ:       5
> >>
> >> ;;   basic block 5, loop depth 0
> >> ;;    pred:       4
> >> ;;    succ:       6
> >>
> >> ;;   basic block 6, loop depth 0
> >> ;;    pred:       5
> >>    if (0 != 0)
> >>      goto <bb 7>; [53.47%]
> >>    else
> >>      goto <bb 8>; [46.53%]
> >> ;;    succ:       7
> >> ;;                8
> >>
> >> ;;   basic block 7, loop depth 0
> >> ;;    pred:       6
> >>    strcpy (out_1(D), out_1(D));
> >> ;;    succ:       8
> >>
> >> ;;   basic block 8, loop depth 0
> >> ;;    pred:       6
> >> ;;                7
> >>    _4 = __builtin_strlen (out_1(D));
> >>    _5 = out_1(D) + _4;
> >>    __builtin_memcpy (_5, "foo", 4);
> >> ;;    succ:       3
> >>
> >> ;;   basic block 3, loop depth 0
> >> ;;    pred:       8
> >>    return;
> >> ;;    succ:       EXIT
> >>
> >> }
> >>
> >
> > Which shows the code is obviously unreachable in the case we're warning
> > about.  You can't see this in the dumps because it's exposed by
> > inlining, then cleaned up before writing the dump file.
>
> In the specific case of the bug the code is of course eliminated
> because it's guarded by the if (s != d).  I was referring to
> the general (unguarded) case of:
>
>    char *s = "", *p;
>
>    int main (void)
>    {
>      p = strcpy (s, s);
>      puts (p);
>    }
>
> where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> p = s;  That's perfectly reasonable but it could equally as well
> leave the call alone, as it does when s is null, for instance.
>
> I think folding it away is not only reasonable but preferable to
> making the invalid call, but it's done only rarely.  Most of
> the time GCC does emit the undefined access (it does that with
> calls to library functions as well as with direct stores and
> reads).  (I am hoping we can change that in the future so that
> these kinds of problems are handled with some consistency.)
>
> >
> > ISTM this would be a case we could handle with the __builtin_warning
> > stuff.
> >
> > I think the question is do we want to do anything about it this cycle?
> >
> >
> > If so, I think Martin's approach is quite reasonable.  It disables
> > folding away the self-copies from gimple-fold and moves the warning
> > into the expander.  So if there's such a call in the IL at expansion
> > time we get a warning (-O0).
> >
> > I'd hazard a guess that the diagnostic was added to the strlen pass to
> > capture the missed warning when we're optimizing and the self-copy has
> > survived until that point. There's a couple issues that raises though.
> >
> > First, it's insufficient.  DSE (for example) can do self-copy removal,
> > so it needs the same handling.  There may be others places too.
> >
> > Second, if the code becomes unreachable after strlen, then we've got
> > new false positive issues.
> >
> > It's the classic problems we have with all middle end based warnings.
> >
> > But I could live with those if we can show that using __builtin_warning
> > to handle this stuff in gcc-11 works...  ISTM we emit the
> > __builtin_warning call before any self-copy like that, whenever we
> > happen to spot them.  They'll naturally get removed if the path becomes
> > unreachable.  We'd warn during expansion for calls to
> > __builtin_warning.  We could even optionally warn when removing a call
> > to __bulitin_warning.
> >
> > Thoughts?
>
> The patch has pretty much the same effect as emitting __builtin_warning
> from the folder would.  It defers the folding until much later, and if
> the code isn't eliminated, it issues a warning and folds the call away.

But it affects subsequent optimizations - the call is more expensive
in any size heuristics, it posses an (alias-set zero) memory write
barrier (unless you start to optimize no-op copies in the alias oracle),
it is a _call_ - passes like the vectorizer are not happy about a call.
It prevents SRA of the accessed object, ...

So no, leaving in the call is _not_ equivalent to sticking in a
__builtin_warning() call (or however we actually implement it,
I'd prefer a stmt in the "debug" category so it's simply ignored
or elided by most passes by means of existing code).

That said, I'd prefer to not do anything about this bug.  Iff then
in the inliner try doing a CFG cleanup before folding stmts
(it's doing delayed folding anyway).  But not for GCC 10.
One could also mark stmts with no-warning before the inliner
folds them (and then mark back) to avoid those classes of
folding warnings.

Richard.

>
> Martin

On 2/5/20 1:19 AM, Richard Biener wrote:
> On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
>>
>> On 2/4/20 2:31 PM, Jeff Law wrote:
>>> On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
>>>> On 2/4/20 12:15 PM, Richard Biener wrote:
>>>>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
>>>>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
>>>>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
>>>>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
>>>>>> call
>>>>>>>> to strcpy that carefully guards against self-copying.  This is
>>>>>> caused
>>>>>>>> by the caller's arguments substituted into the call during inlining
>>>>>> and
>>>>>>>> before dead code elimination.
>>>>>>>>
>>>>>>>> The attached patch avoids this by removing -Wrestrict from the
>>>>>> folder
>>>>>>>> and deferring folding perfectly overlapping (and so undefined)
>>>>>> calls
>>>>>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
>>>>>>>> perfectly overlapping calls to memcpy are still folded early.
>>>>>>>
>>>>>>> Why do we bother to warn at all for this case?  Just DWIM here.
>>>>>> Warnings like
>>>>>>> this can be emitted from the analyzer?
>>>>>> They potentially can, but the analyzer is and will almost always
>>>>>> certainly be considerably slower.  I would not expect it to be used
>>>>>> nearly as much as the core compiler.
>>>>>>
>>>>>> WHether or not a particular warning makes sense in the core compiler or
>>>>>> analyzer would seem to me to depend on whether or not we can reasonably
>>>>>> issue warnings without interprocedural analysis.  double-free
>>>>>> realistically requires interprocedural analysis to be effective.  I'm
>>>>>> not sure Wrestrict really does.
>>>>>>
>>>>>>
>>>>>>> That is, I suggest to simply remove the bogus warning code from
>>>>>> folding
>>>>>>> (and _not_ fail the folding).
>>>>>> I haven't looked at the patch, but if we can get the warning out of the
>>>>>> folder that's certainly preferable.  And we could investigate deferring
>>>>>> self-copy removal.
>>>>>
>>>>> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
>>>>
>>>> In this instance the code is reachable (or isn't obviously unreachable).
>>>> GCC doesn't remove it, but provides benign (and reasonable) semantics
>>>> for it(*).  To me, that's one aspect of quality.  Letting the user know
>>>> that the code is buggy is another.  I view that as at least as important
>>>> as folding the ill-effects away because it makes it possible to fix
>>>> the problem so the code works correctly even with compilers that don't
>>>> provide these benign semantics.
>>> If you look at the guts of what happens at the point where we issue the
>>> warning from within gimple_fold_builtin_strcpy we have:
>>>
>>>> DCH_to_char (char * in, char * out, int collid)
>>>> {
>>>>     int type;
>>>>     char * D.2148;
>>>>     char * dest;
>>>>     char * num;
>>>>     long unsigned int _4;
>>>>     char * _5;
>>>>
>>>> ;;   basic block 2, loop depth 0
>>>> ;;    pred:       ENTRY
>>>> ;;    succ:       4
>>>>
>>>> ;;   basic block 4, loop depth 0
>>>> ;;    pred:       2
>>>> ;;    succ:       5
>>>>
>>>> ;;   basic block 5, loop depth 0
>>>> ;;    pred:       4
>>>> ;;    succ:       6
>>>>
>>>> ;;   basic block 6, loop depth 0
>>>> ;;    pred:       5
>>>>     if (0 != 0)
>>>>       goto <bb 7>; [53.47%]
>>>>     else
>>>>       goto <bb 8>; [46.53%]
>>>> ;;    succ:       7
>>>> ;;                8
>>>>
>>>> ;;   basic block 7, loop depth 0
>>>> ;;    pred:       6
>>>>     strcpy (out_1(D), out_1(D));
>>>> ;;    succ:       8
>>>>
>>>> ;;   basic block 8, loop depth 0
>>>> ;;    pred:       6
>>>> ;;                7
>>>>     _4 = __builtin_strlen (out_1(D));
>>>>     _5 = out_1(D) + _4;
>>>>     __builtin_memcpy (_5, "foo", 4);
>>>> ;;    succ:       3
>>>>
>>>> ;;   basic block 3, loop depth 0
>>>> ;;    pred:       8
>>>>     return;
>>>> ;;    succ:       EXIT
>>>>
>>>> }
>>>>
>>>
>>> Which shows the code is obviously unreachable in the case we're warning
>>> about.  You can't see this in the dumps because it's exposed by
>>> inlining, then cleaned up before writing the dump file.
>>
>> In the specific case of the bug the code is of course eliminated
>> because it's guarded by the if (s != d).  I was referring to
>> the general (unguarded) case of:
>>
>>     char *s = "", *p;
>>
>>     int main (void)
>>     {
>>       p = strcpy (s, s);
>>       puts (p);
>>     }
>>
>> where GCC folds the assignment 'p = strcpy(s, s);' to effectively
>> p = s;  That's perfectly reasonable but it could equally as well
>> leave the call alone, as it does when s is null, for instance.
>>
>> I think folding it away is not only reasonable but preferable to
>> making the invalid call, but it's done only rarely.  Most of
>> the time GCC does emit the undefined access (it does that with
>> calls to library functions as well as with direct stores and
>> reads).  (I am hoping we can change that in the future so that
>> these kinds of problems are handled with some consistency.)
>>
>>>
>>> ISTM this would be a case we could handle with the __builtin_warning
>>> stuff.
>>>
>>> I think the question is do we want to do anything about it this cycle?
>>>
>>>
>>> If so, I think Martin's approach is quite reasonable.  It disables
>>> folding away the self-copies from gimple-fold and moves the warning
>>> into the expander.  So if there's such a call in the IL at expansion
>>> time we get a warning (-O0).
>>>
>>> I'd hazard a guess that the diagnostic was added to the strlen pass to
>>> capture the missed warning when we're optimizing and the self-copy has
>>> survived until that point. There's a couple issues that raises though.
>>>
>>> First, it's insufficient.  DSE (for example) can do self-copy removal,
>>> so it needs the same handling.  There may be others places too.
>>>
>>> Second, if the code becomes unreachable after strlen, then we've got
>>> new false positive issues.
>>>
>>> It's the classic problems we have with all middle end based warnings.
>>>
>>> But I could live with those if we can show that using __builtin_warning
>>> to handle this stuff in gcc-11 works...  ISTM we emit the
>>> __builtin_warning call before any self-copy like that, whenever we
>>> happen to spot them.  They'll naturally get removed if the path becomes
>>> unreachable.  We'd warn during expansion for calls to
>>> __builtin_warning.  We could even optionally warn when removing a call
>>> to __bulitin_warning.
>>>
>>> Thoughts?
>>
>> The patch has pretty much the same effect as emitting __builtin_warning
>> from the folder would.  It defers the folding until much later, and if
>> the code isn't eliminated, it issues a warning and folds the call away.
> 
> But it affects subsequent optimizations - the call is more expensive
> in any size heuristics, it posses an (alias-set zero) memory write
> barrier (unless you start to optimize no-op copies in the alias oracle),
> it is a _call_ - passes like the vectorizer are not happy about a call.
> It prevents SRA of the accessed object, ...

This is a strcpy call copying over itself.  It's undefined code,
and so hardly anything that's common or so performance sensitive
to make a noticeable difference.

> So no, leaving in the call is _not_ equivalent to sticking in a
> __builtin_warning() call (or however we actually implement it,
> I'd prefer a stmt in the "debug" category so it's simply ignored
> or elided by most passes by means of existing code).
> 
> That said, I'd prefer to not do anything about this bug.  Iff then
> in the inliner try doing a CFG cleanup before folding stmts
> (it's doing delayed folding anyway).  But not for GCC 10.
> One could also mark stmts with no-warning before the inliner
> folds them (and then mark back) to avoid those classes of
> folding warnings.

I think this would very unfortunate for GCC 10.  The user's code
is clearly correct -- they take pains to avoid the overlapping
copy by guarding against it just before it -- and GCC simply emits
an invalid warning because of how it does inlining.  All that will
be accomplished by not fixing it is we will release a worse quality
compiler than we otherwise can, unnecessarily eroding our users'
confidence in the value of GCC's diagnostic.

I'll look into your suggestions for the inliner in stage 1 but
please reconsider for GCC 10.

Martin

On Wed, 2020-02-05 at 09:19 +0100, Richard Biener wrote:
> On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
> > On 2/4/20 2:31 PM, Jeff Law wrote:
> > > On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> > > > On 2/4/20 12:15 PM, Richard Biener wrote:
> > > > > On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> > > > > > On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> > > > > > > On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> > > > > > > > PR 93519 reports a false positive -Wrestrict issued for an inlined
> > > > > > call
> > > > > > > > to strcpy that carefully guards against self-copying.  This is
> > > > > > caused
> > > > > > > > by the caller's arguments substituted into the call during inlining
> > > > > > and
> > > > > > > > before dead code elimination.
> > > > > > > > 
> > > > > > > > The attached patch avoids this by removing -Wrestrict from the
> > > > > > folder
> > > > > > > > and deferring folding perfectly overlapping (and so undefined)
> > > > > > calls
> > > > > > > > to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> > > > > > > > perfectly overlapping calls to memcpy are still folded early.
> > > > > > > 
> > > > > > > Why do we bother to warn at all for this case?  Just DWIM here.
> > > > > > Warnings like
> > > > > > > this can be emitted from the analyzer?
> > > > > > They potentially can, but the analyzer is and will almost always
> > > > > > certainly be considerably slower.  I would not expect it to be used
> > > > > > nearly as much as the core compiler.
> > > > > > 
> > > > > > WHether or not a particular warning makes sense in the core compiler or
> > > > > > analyzer would seem to me to depend on whether or not we can reasonably
> > > > > > issue warnings without interprocedural analysis.  double-free
> > > > > > realistically requires interprocedural analysis to be effective.  I'm
> > > > > > not sure Wrestrict really does.
> > > > > > 
> > > > > > 
> > > > > > > That is, I suggest to simply remove the bogus warning code from
> > > > > > folding
> > > > > > > (and _not_ fail the folding).
> > > > > > I haven't looked at the patch, but if we can get the warning out of the
> > > > > > folder that's certainly preferable.  And we could investigate deferring
> > > > > > self-copy removal.
> > > > > 
> > > > > I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> > > > 
> > > > In this instance the code is reachable (or isn't obviously unreachable).
> > > > GCC doesn't remove it, but provides benign (and reasonable) semantics
> > > > for it(*).  To me, that's one aspect of quality.  Letting the user know
> > > > that the code is buggy is another.  I view that as at least as important
> > > > as folding the ill-effects away because it makes it possible to fix
> > > > the problem so the code works correctly even with compilers that don't
> > > > provide these benign semantics.
> > > If you look at the guts of what happens at the point where we issue the
> > > warning from within gimple_fold_builtin_strcpy we have:
> > > 
> > > > DCH_to_char (char * in, char * out, int collid)
> > > > {
> > > >    int type;
> > > >    char * D.2148;
> > > >    char * dest;
> > > >    char * num;
> > > >    long unsigned int _4;
> > > >    char * _5;
> > > > 
> > > > ;;   basic block 2, loop depth 0
> > > > ;;    pred:       ENTRY
> > > > ;;    succ:       4
> > > > 
> > > > ;;   basic block 4, loop depth 0
> > > > ;;    pred:       2
> > > > ;;    succ:       5
> > > > 
> > > > ;;   basic block 5, loop depth 0
> > > > ;;    pred:       4
> > > > ;;    succ:       6
> > > > 
> > > > ;;   basic block 6, loop depth 0
> > > > ;;    pred:       5
> > > >    if (0 != 0)
> > > >      goto <bb 7>; [53.47%]
> > > >    else
> > > >      goto <bb 8>; [46.53%]
> > > > ;;    succ:       7
> > > > ;;                8
> > > > 
> > > > ;;   basic block 7, loop depth 0
> > > > ;;    pred:       6
> > > >    strcpy (out_1(D), out_1(D));
> > > > ;;    succ:       8
> > > > 
> > > > ;;   basic block 8, loop depth 0
> > > > ;;    pred:       6
> > > > ;;                7
> > > >    _4 = __builtin_strlen (out_1(D));
> > > >    _5 = out_1(D) + _4;
> > > >    __builtin_memcpy (_5, "foo", 4);
> > > > ;;    succ:       3
> > > > 
> > > > ;;   basic block 3, loop depth 0
> > > > ;;    pred:       8
> > > >    return;
> > > > ;;    succ:       EXIT
> > > > 
> > > > }
> > > > 
> > > 
> > > Which shows the code is obviously unreachable in the case we're warning
> > > about.  You can't see this in the dumps because it's exposed by
> > > inlining, then cleaned up before writing the dump file.
> > 
> > In the specific case of the bug the code is of course eliminated
> > because it's guarded by the if (s != d).  I was referring to
> > the general (unguarded) case of:
> > 
> >    char *s = "", *p;
> > 
> >    int main (void)
> >    {
> >      p = strcpy (s, s);
> >      puts (p);
> >    }
> > 
> > where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> > p = s;  That's perfectly reasonable but it could equally as well
> > leave the call alone, as it does when s is null, for instance.
> > 
> > I think folding it away is not only reasonable but preferable to
> > making the invalid call, but it's done only rarely.  Most of
> > the time GCC does emit the undefined access (it does that with
> > calls to library functions as well as with direct stores and
> > reads).  (I am hoping we can change that in the future so that
> > these kinds of problems are handled with some consistency.)
> > 
> > > ISTM this would be a case we could handle with the __builtin_warning
> > > stuff.
> > > 
> > > I think the question is do we want to do anything about it this cycle?
> > > 
> > > 
> > > If so, I think Martin's approach is quite reasonable.  It disables
> > > folding away the self-copies from gimple-fold and moves the warning
> > > into the expander.  So if there's such a call in the IL at expansion
> > > time we get a warning (-O0).
> > > 
> > > I'd hazard a guess that the diagnostic was added to the strlen pass to
> > > capture the missed warning when we're optimizing and the self-copy has
> > > survived until that point. There's a couple issues that raises though.
> > > 
> > > First, it's insufficient.  DSE (for example) can do self-copy removal,
> > > so it needs the same handling.  There may be others places too.
> > > 
> > > Second, if the code becomes unreachable after strlen, then we've got
> > > new false positive issues.
> > > 
> > > It's the classic problems we have with all middle end based warnings.
> > > 
> > > But I could live with those if we can show that using __builtin_warning
> > > to handle this stuff in gcc-11 works...  ISTM we emit the
> > > __builtin_warning call before any self-copy like that, whenever we
> > > happen to spot them.  They'll naturally get removed if the path becomes
> > > unreachable.  We'd warn during expansion for calls to
> > > __builtin_warning.  We could even optionally warn when removing a call
> > > to __bulitin_warning.
> > > 
> > > Thoughts?
> > 
> > The patch has pretty much the same effect as emitting __builtin_warning
> > from the folder would.  It defers the folding until much later, and if
> > the code isn't eliminated, it issues a warning and folds the call away.
> 
> But it affects subsequent optimizations - the call is more expensive
> in any size heuristics, it posses an (alias-set zero) memory write
> barrier (unless you start to optimize no-op copies in the alias oracle),
> it is a _call_ - passes like the vectorizer are not happy about a call.
> It prevents SRA of the accessed object, ...
Yes, it does affect subsequent optimizations, but realistically how
often do we have self-assignments?  And how often to we have them via
the str* functions.

Additionally these are undefined except for memmove.  I find it hard to
believe these are appearing it performance sensitive code.

So I'd claim it's equivalent from a practical standpoint.

I guess I could do a Fedora build with a diagnostic to tell us how
often this happens.  Would you be willing to reconsider if the data
shows this just doesn't happen often enough to matter?

Jeff
>

On Wed, Feb 5, 2020 at 4:55 PM Martin Sebor <msebor@gmail.com> wrote:
>
> On 2/5/20 1:19 AM, Richard Biener wrote:
> > On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
> >>
> >> On 2/4/20 2:31 PM, Jeff Law wrote:
> >>> On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> >>>> On 2/4/20 12:15 PM, Richard Biener wrote:
> >>>>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> >>>>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> >>>>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> >>>>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
> >>>>>> call
> >>>>>>>> to strcpy that carefully guards against self-copying.  This is
> >>>>>> caused
> >>>>>>>> by the caller's arguments substituted into the call during inlining
> >>>>>> and
> >>>>>>>> before dead code elimination.
> >>>>>>>>
> >>>>>>>> The attached patch avoids this by removing -Wrestrict from the
> >>>>>> folder
> >>>>>>>> and deferring folding perfectly overlapping (and so undefined)
> >>>>>> calls
> >>>>>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> >>>>>>>> perfectly overlapping calls to memcpy are still folded early.
> >>>>>>>
> >>>>>>> Why do we bother to warn at all for this case?  Just DWIM here.
> >>>>>> Warnings like
> >>>>>>> this can be emitted from the analyzer?
> >>>>>> They potentially can, but the analyzer is and will almost always
> >>>>>> certainly be considerably slower.  I would not expect it to be used
> >>>>>> nearly as much as the core compiler.
> >>>>>>
> >>>>>> WHether or not a particular warning makes sense in the core compiler or
> >>>>>> analyzer would seem to me to depend on whether or not we can reasonably
> >>>>>> issue warnings without interprocedural analysis.  double-free
> >>>>>> realistically requires interprocedural analysis to be effective.  I'm
> >>>>>> not sure Wrestrict really does.
> >>>>>>
> >>>>>>
> >>>>>>> That is, I suggest to simply remove the bogus warning code from
> >>>>>> folding
> >>>>>>> (and _not_ fail the folding).
> >>>>>> I haven't looked at the patch, but if we can get the warning out of the
> >>>>>> folder that's certainly preferable.  And we could investigate deferring
> >>>>>> self-copy removal.
> >>>>>
> >>>>> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> >>>>
> >>>> In this instance the code is reachable (or isn't obviously unreachable).
> >>>> GCC doesn't remove it, but provides benign (and reasonable) semantics
> >>>> for it(*).  To me, that's one aspect of quality.  Letting the user know
> >>>> that the code is buggy is another.  I view that as at least as important
> >>>> as folding the ill-effects away because it makes it possible to fix
> >>>> the problem so the code works correctly even with compilers that don't
> >>>> provide these benign semantics.
> >>> If you look at the guts of what happens at the point where we issue the
> >>> warning from within gimple_fold_builtin_strcpy we have:
> >>>
> >>>> DCH_to_char (char * in, char * out, int collid)
> >>>> {
> >>>>     int type;
> >>>>     char * D.2148;
> >>>>     char * dest;
> >>>>     char * num;
> >>>>     long unsigned int _4;
> >>>>     char * _5;
> >>>>
> >>>> ;;   basic block 2, loop depth 0
> >>>> ;;    pred:       ENTRY
> >>>> ;;    succ:       4
> >>>>
> >>>> ;;   basic block 4, loop depth 0
> >>>> ;;    pred:       2
> >>>> ;;    succ:       5
> >>>>
> >>>> ;;   basic block 5, loop depth 0
> >>>> ;;    pred:       4
> >>>> ;;    succ:       6
> >>>>
> >>>> ;;   basic block 6, loop depth 0
> >>>> ;;    pred:       5
> >>>>     if (0 != 0)
> >>>>       goto <bb 7>; [53.47%]
> >>>>     else
> >>>>       goto <bb 8>; [46.53%]
> >>>> ;;    succ:       7
> >>>> ;;                8
> >>>>
> >>>> ;;   basic block 7, loop depth 0
> >>>> ;;    pred:       6
> >>>>     strcpy (out_1(D), out_1(D));
> >>>> ;;    succ:       8
> >>>>
> >>>> ;;   basic block 8, loop depth 0
> >>>> ;;    pred:       6
> >>>> ;;                7
> >>>>     _4 = __builtin_strlen (out_1(D));
> >>>>     _5 = out_1(D) + _4;
> >>>>     __builtin_memcpy (_5, "foo", 4);
> >>>> ;;    succ:       3
> >>>>
> >>>> ;;   basic block 3, loop depth 0
> >>>> ;;    pred:       8
> >>>>     return;
> >>>> ;;    succ:       EXIT
> >>>>
> >>>> }
> >>>>
> >>>
> >>> Which shows the code is obviously unreachable in the case we're warning
> >>> about.  You can't see this in the dumps because it's exposed by
> >>> inlining, then cleaned up before writing the dump file.
> >>
> >> In the specific case of the bug the code is of course eliminated
> >> because it's guarded by the if (s != d).  I was referring to
> >> the general (unguarded) case of:
> >>
> >>     char *s = "", *p;
> >>
> >>     int main (void)
> >>     {
> >>       p = strcpy (s, s);
> >>       puts (p);
> >>     }
> >>
> >> where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> >> p = s;  That's perfectly reasonable but it could equally as well
> >> leave the call alone, as it does when s is null, for instance.
> >>
> >> I think folding it away is not only reasonable but preferable to
> >> making the invalid call, but it's done only rarely.  Most of
> >> the time GCC does emit the undefined access (it does that with
> >> calls to library functions as well as with direct stores and
> >> reads).  (I am hoping we can change that in the future so that
> >> these kinds of problems are handled with some consistency.)
> >>
> >>>
> >>> ISTM this would be a case we could handle with the __builtin_warning
> >>> stuff.
> >>>
> >>> I think the question is do we want to do anything about it this cycle?
> >>>
> >>>
> >>> If so, I think Martin's approach is quite reasonable.  It disables
> >>> folding away the self-copies from gimple-fold and moves the warning
> >>> into the expander.  So if there's such a call in the IL at expansion
> >>> time we get a warning (-O0).
> >>>
> >>> I'd hazard a guess that the diagnostic was added to the strlen pass to
> >>> capture the missed warning when we're optimizing and the self-copy has
> >>> survived until that point. There's a couple issues that raises though.
> >>>
> >>> First, it's insufficient.  DSE (for example) can do self-copy removal,
> >>> so it needs the same handling.  There may be others places too.
> >>>
> >>> Second, if the code becomes unreachable after strlen, then we've got
> >>> new false positive issues.
> >>>
> >>> It's the classic problems we have with all middle end based warnings.
> >>>
> >>> But I could live with those if we can show that using __builtin_warning
> >>> to handle this stuff in gcc-11 works...  ISTM we emit the
> >>> __builtin_warning call before any self-copy like that, whenever we
> >>> happen to spot them.  They'll naturally get removed if the path becomes
> >>> unreachable.  We'd warn during expansion for calls to
> >>> __builtin_warning.  We could even optionally warn when removing a call
> >>> to __bulitin_warning.
> >>>
> >>> Thoughts?
> >>
> >> The patch has pretty much the same effect as emitting __builtin_warning
> >> from the folder would.  It defers the folding until much later, and if
> >> the code isn't eliminated, it issues a warning and folds the call away.
> >
> > But it affects subsequent optimizations - the call is more expensive
> > in any size heuristics, it posses an (alias-set zero) memory write
> > barrier (unless you start to optimize no-op copies in the alias oracle),
> > it is a _call_ - passes like the vectorizer are not happy about a call.
> > It prevents SRA of the accessed object, ...
>
> This is a strcpy call copying over itself.  It's undefined code,
> and so hardly anything that's common or so performance sensitive
> to make a noticeable difference.
>
> > So no, leaving in the call is _not_ equivalent to sticking in a
> > __builtin_warning() call (or however we actually implement it,
> > I'd prefer a stmt in the "debug" category so it's simply ignored
> > or elided by most passes by means of existing code).
> >
> > That said, I'd prefer to not do anything about this bug.  Iff then
> > in the inliner try doing a CFG cleanup before folding stmts
> > (it's doing delayed folding anyway).  But not for GCC 10.
> > One could also mark stmts with no-warning before the inliner
> > folds them (and then mark back) to avoid those classes of
> > folding warnings.
>
> I think this would very unfortunate for GCC 10.  The user's code
> is clearly correct -- they take pains to avoid the overlapping
> copy by guarding against it just before it -- and GCC simply emits
> an invalid warning because of how it does inlining.  All that will
> be accomplished by not fixing it is we will release a worse quality
> compiler than we otherwise can, unnecessarily eroding our users'
> confidence in the value of GCC's diagnostic.
>
> I'll look into your suggestions for the inliner in stage 1 but
> please reconsider for GCC 10.

One possibility is the attached but that adds an extra CFG cleanup
(also untested but on the testcase).  Another possibility would be
to re-do fold_marked_statements in terms of cleanup_control_flow_pre (),
recognizing unreachable regions on the way and simply avoid folding
stmts in blocks that will be then removed by the pending CFG cleanup.

I'll see whether I can cook that up quickly.

> Martin

On Thu, Feb 6, 2020 at 11:06 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Wed, Feb 5, 2020 at 4:55 PM Martin Sebor <msebor@gmail.com> wrote:
> >
> > On 2/5/20 1:19 AM, Richard Biener wrote:
> > > On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
> > >>
> > >> On 2/4/20 2:31 PM, Jeff Law wrote:
> > >>> On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> > >>>> On 2/4/20 12:15 PM, Richard Biener wrote:
> > >>>>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> > >>>>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> > >>>>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> > >>>>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
> > >>>>>> call
> > >>>>>>>> to strcpy that carefully guards against self-copying.  This is
> > >>>>>> caused
> > >>>>>>>> by the caller's arguments substituted into the call during inlining
> > >>>>>> and
> > >>>>>>>> before dead code elimination.
> > >>>>>>>>
> > >>>>>>>> The attached patch avoids this by removing -Wrestrict from the
> > >>>>>> folder
> > >>>>>>>> and deferring folding perfectly overlapping (and so undefined)
> > >>>>>> calls
> > >>>>>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> > >>>>>>>> perfectly overlapping calls to memcpy are still folded early.
> > >>>>>>>
> > >>>>>>> Why do we bother to warn at all for this case?  Just DWIM here.
> > >>>>>> Warnings like
> > >>>>>>> this can be emitted from the analyzer?
> > >>>>>> They potentially can, but the analyzer is and will almost always
> > >>>>>> certainly be considerably slower.  I would not expect it to be used
> > >>>>>> nearly as much as the core compiler.
> > >>>>>>
> > >>>>>> WHether or not a particular warning makes sense in the core compiler or
> > >>>>>> analyzer would seem to me to depend on whether or not we can reasonably
> > >>>>>> issue warnings without interprocedural analysis.  double-free
> > >>>>>> realistically requires interprocedural analysis to be effective.  I'm
> > >>>>>> not sure Wrestrict really does.
> > >>>>>>
> > >>>>>>
> > >>>>>>> That is, I suggest to simply remove the bogus warning code from
> > >>>>>> folding
> > >>>>>>> (and _not_ fail the folding).
> > >>>>>> I haven't looked at the patch, but if we can get the warning out of the
> > >>>>>> folder that's certainly preferable.  And we could investigate deferring
> > >>>>>> self-copy removal.
> > >>>>>
> > >>>>> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> > >>>>
> > >>>> In this instance the code is reachable (or isn't obviously unreachable).
> > >>>> GCC doesn't remove it, but provides benign (and reasonable) semantics
> > >>>> for it(*).  To me, that's one aspect of quality.  Letting the user know
> > >>>> that the code is buggy is another.  I view that as at least as important
> > >>>> as folding the ill-effects away because it makes it possible to fix
> > >>>> the problem so the code works correctly even with compilers that don't
> > >>>> provide these benign semantics.
> > >>> If you look at the guts of what happens at the point where we issue the
> > >>> warning from within gimple_fold_builtin_strcpy we have:
> > >>>
> > >>>> DCH_to_char (char * in, char * out, int collid)
> > >>>> {
> > >>>>     int type;
> > >>>>     char * D.2148;
> > >>>>     char * dest;
> > >>>>     char * num;
> > >>>>     long unsigned int _4;
> > >>>>     char * _5;
> > >>>>
> > >>>> ;;   basic block 2, loop depth 0
> > >>>> ;;    pred:       ENTRY
> > >>>> ;;    succ:       4
> > >>>>
> > >>>> ;;   basic block 4, loop depth 0
> > >>>> ;;    pred:       2
> > >>>> ;;    succ:       5
> > >>>>
> > >>>> ;;   basic block 5, loop depth 0
> > >>>> ;;    pred:       4
> > >>>> ;;    succ:       6
> > >>>>
> > >>>> ;;   basic block 6, loop depth 0
> > >>>> ;;    pred:       5
> > >>>>     if (0 != 0)
> > >>>>       goto <bb 7>; [53.47%]
> > >>>>     else
> > >>>>       goto <bb 8>; [46.53%]
> > >>>> ;;    succ:       7
> > >>>> ;;                8
> > >>>>
> > >>>> ;;   basic block 7, loop depth 0
> > >>>> ;;    pred:       6
> > >>>>     strcpy (out_1(D), out_1(D));
> > >>>> ;;    succ:       8
> > >>>>
> > >>>> ;;   basic block 8, loop depth 0
> > >>>> ;;    pred:       6
> > >>>> ;;                7
> > >>>>     _4 = __builtin_strlen (out_1(D));
> > >>>>     _5 = out_1(D) + _4;
> > >>>>     __builtin_memcpy (_5, "foo", 4);
> > >>>> ;;    succ:       3
> > >>>>
> > >>>> ;;   basic block 3, loop depth 0
> > >>>> ;;    pred:       8
> > >>>>     return;
> > >>>> ;;    succ:       EXIT
> > >>>>
> > >>>> }
> > >>>>
> > >>>
> > >>> Which shows the code is obviously unreachable in the case we're warning
> > >>> about.  You can't see this in the dumps because it's exposed by
> > >>> inlining, then cleaned up before writing the dump file.
> > >>
> > >> In the specific case of the bug the code is of course eliminated
> > >> because it's guarded by the if (s != d).  I was referring to
> > >> the general (unguarded) case of:
> > >>
> > >>     char *s = "", *p;
> > >>
> > >>     int main (void)
> > >>     {
> > >>       p = strcpy (s, s);
> > >>       puts (p);
> > >>     }
> > >>
> > >> where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> > >> p = s;  That's perfectly reasonable but it could equally as well
> > >> leave the call alone, as it does when s is null, for instance.
> > >>
> > >> I think folding it away is not only reasonable but preferable to
> > >> making the invalid call, but it's done only rarely.  Most of
> > >> the time GCC does emit the undefined access (it does that with
> > >> calls to library functions as well as with direct stores and
> > >> reads).  (I am hoping we can change that in the future so that
> > >> these kinds of problems are handled with some consistency.)
> > >>
> > >>>
> > >>> ISTM this would be a case we could handle with the __builtin_warning
> > >>> stuff.
> > >>>
> > >>> I think the question is do we want to do anything about it this cycle?
> > >>>
> > >>>
> > >>> If so, I think Martin's approach is quite reasonable.  It disables
> > >>> folding away the self-copies from gimple-fold and moves the warning
> > >>> into the expander.  So if there's such a call in the IL at expansion
> > >>> time we get a warning (-O0).
> > >>>
> > >>> I'd hazard a guess that the diagnostic was added to the strlen pass to
> > >>> capture the missed warning when we're optimizing and the self-copy has
> > >>> survived until that point. There's a couple issues that raises though.
> > >>>
> > >>> First, it's insufficient.  DSE (for example) can do self-copy removal,
> > >>> so it needs the same handling.  There may be others places too.
> > >>>
> > >>> Second, if the code becomes unreachable after strlen, then we've got
> > >>> new false positive issues.
> > >>>
> > >>> It's the classic problems we have with all middle end based warnings.
> > >>>
> > >>> But I could live with those if we can show that using __builtin_warning
> > >>> to handle this stuff in gcc-11 works...  ISTM we emit the
> > >>> __builtin_warning call before any self-copy like that, whenever we
> > >>> happen to spot them.  They'll naturally get removed if the path becomes
> > >>> unreachable.  We'd warn during expansion for calls to
> > >>> __builtin_warning.  We could even optionally warn when removing a call
> > >>> to __bulitin_warning.
> > >>>
> > >>> Thoughts?
> > >>
> > >> The patch has pretty much the same effect as emitting __builtin_warning
> > >> from the folder would.  It defers the folding until much later, and if
> > >> the code isn't eliminated, it issues a warning and folds the call away.
> > >
> > > But it affects subsequent optimizations - the call is more expensive
> > > in any size heuristics, it posses an (alias-set zero) memory write
> > > barrier (unless you start to optimize no-op copies in the alias oracle),
> > > it is a _call_ - passes like the vectorizer are not happy about a call.
> > > It prevents SRA of the accessed object, ...
> >
> > This is a strcpy call copying over itself.  It's undefined code,
> > and so hardly anything that's common or so performance sensitive
> > to make a noticeable difference.
> >
> > > So no, leaving in the call is _not_ equivalent to sticking in a
> > > __builtin_warning() call (or however we actually implement it,
> > > I'd prefer a stmt in the "debug" category so it's simply ignored
> > > or elided by most passes by means of existing code).
> > >
> > > That said, I'd prefer to not do anything about this bug.  Iff then
> > > in the inliner try doing a CFG cleanup before folding stmts
> > > (it's doing delayed folding anyway).  But not for GCC 10.
> > > One could also mark stmts with no-warning before the inliner
> > > folds them (and then mark back) to avoid those classes of
> > > folding warnings.
> >
> > I think this would very unfortunate for GCC 10.  The user's code
> > is clearly correct -- they take pains to avoid the overlapping
> > copy by guarding against it just before it -- and GCC simply emits
> > an invalid warning because of how it does inlining.  All that will
> > be accomplished by not fixing it is we will release a worse quality
> > compiler than we otherwise can, unnecessarily eroding our users'
> > confidence in the value of GCC's diagnostic.
> >
> > I'll look into your suggestions for the inliner in stage 1 but
> > please reconsider for GCC 10.
>
> One possibility is the attached but that adds an extra CFG cleanup
> (also untested but on the testcase).  Another possibility would be
> to re-do fold_marked_statements in terms of cleanup_control_flow_pre (),
> recognizing unreachable regions on the way and simply avoid folding
> stmts in blocks that will be then removed by the pending CFG cleanup.
>
> I'll see whether I can cook that up quickly.

Like this.  Indenting not fixed and untested apart from on the testcase.
Disadvantage is we're walking _all_ blocks (at least skipping stmts).
Technically the iteration scheme should probably change to push
edges rather than edge iterators so we can avoid the repeated find_taken_edge
call.

Anyway, just a prototype.

Note SSA propagators / VN avoid to fold stmts that are obviously
unreachable so it somewhat makes sense to beat the inliner to do
the same.

Richard.

> > Martin

On Thu, Feb 6, 2020 at 11:33 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Thu, Feb 6, 2020 at 11:06 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > On Wed, Feb 5, 2020 at 4:55 PM Martin Sebor <msebor@gmail.com> wrote:
> > >
> > > On 2/5/20 1:19 AM, Richard Biener wrote:
> > > > On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
> > > >>
> > > >> On 2/4/20 2:31 PM, Jeff Law wrote:
> > > >>> On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> > > >>>> On 2/4/20 12:15 PM, Richard Biener wrote:
> > > >>>>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> > > >>>>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> > > >>>>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> > > >>>>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
> > > >>>>>> call
> > > >>>>>>>> to strcpy that carefully guards against self-copying.  This is
> > > >>>>>> caused
> > > >>>>>>>> by the caller's arguments substituted into the call during inlining
> > > >>>>>> and
> > > >>>>>>>> before dead code elimination.
> > > >>>>>>>>
> > > >>>>>>>> The attached patch avoids this by removing -Wrestrict from the
> > > >>>>>> folder
> > > >>>>>>>> and deferring folding perfectly overlapping (and so undefined)
> > > >>>>>> calls
> > > >>>>>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> > > >>>>>>>> perfectly overlapping calls to memcpy are still folded early.
> > > >>>>>>>
> > > >>>>>>> Why do we bother to warn at all for this case?  Just DWIM here.
> > > >>>>>> Warnings like
> > > >>>>>>> this can be emitted from the analyzer?
> > > >>>>>> They potentially can, but the analyzer is and will almost always
> > > >>>>>> certainly be considerably slower.  I would not expect it to be used
> > > >>>>>> nearly as much as the core compiler.
> > > >>>>>>
> > > >>>>>> WHether or not a particular warning makes sense in the core compiler or
> > > >>>>>> analyzer would seem to me to depend on whether or not we can reasonably
> > > >>>>>> issue warnings without interprocedural analysis.  double-free
> > > >>>>>> realistically requires interprocedural analysis to be effective.  I'm
> > > >>>>>> not sure Wrestrict really does.
> > > >>>>>>
> > > >>>>>>
> > > >>>>>>> That is, I suggest to simply remove the bogus warning code from
> > > >>>>>> folding
> > > >>>>>>> (and _not_ fail the folding).
> > > >>>>>> I haven't looked at the patch, but if we can get the warning out of the
> > > >>>>>> folder that's certainly preferable.  And we could investigate deferring
> > > >>>>>> self-copy removal.
> > > >>>>>
> > > >>>>> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> > > >>>>
> > > >>>> In this instance the code is reachable (or isn't obviously unreachable).
> > > >>>> GCC doesn't remove it, but provides benign (and reasonable) semantics
> > > >>>> for it(*).  To me, that's one aspect of quality.  Letting the user know
> > > >>>> that the code is buggy is another.  I view that as at least as important
> > > >>>> as folding the ill-effects away because it makes it possible to fix
> > > >>>> the problem so the code works correctly even with compilers that don't
> > > >>>> provide these benign semantics.
> > > >>> If you look at the guts of what happens at the point where we issue the
> > > >>> warning from within gimple_fold_builtin_strcpy we have:
> > > >>>
> > > >>>> DCH_to_char (char * in, char * out, int collid)
> > > >>>> {
> > > >>>>     int type;
> > > >>>>     char * D.2148;
> > > >>>>     char * dest;
> > > >>>>     char * num;
> > > >>>>     long unsigned int _4;
> > > >>>>     char * _5;
> > > >>>>
> > > >>>> ;;   basic block 2, loop depth 0
> > > >>>> ;;    pred:       ENTRY
> > > >>>> ;;    succ:       4
> > > >>>>
> > > >>>> ;;   basic block 4, loop depth 0
> > > >>>> ;;    pred:       2
> > > >>>> ;;    succ:       5
> > > >>>>
> > > >>>> ;;   basic block 5, loop depth 0
> > > >>>> ;;    pred:       4
> > > >>>> ;;    succ:       6
> > > >>>>
> > > >>>> ;;   basic block 6, loop depth 0
> > > >>>> ;;    pred:       5
> > > >>>>     if (0 != 0)
> > > >>>>       goto <bb 7>; [53.47%]
> > > >>>>     else
> > > >>>>       goto <bb 8>; [46.53%]
> > > >>>> ;;    succ:       7
> > > >>>> ;;                8
> > > >>>>
> > > >>>> ;;   basic block 7, loop depth 0
> > > >>>> ;;    pred:       6
> > > >>>>     strcpy (out_1(D), out_1(D));
> > > >>>> ;;    succ:       8
> > > >>>>
> > > >>>> ;;   basic block 8, loop depth 0
> > > >>>> ;;    pred:       6
> > > >>>> ;;                7
> > > >>>>     _4 = __builtin_strlen (out_1(D));
> > > >>>>     _5 = out_1(D) + _4;
> > > >>>>     __builtin_memcpy (_5, "foo", 4);
> > > >>>> ;;    succ:       3
> > > >>>>
> > > >>>> ;;   basic block 3, loop depth 0
> > > >>>> ;;    pred:       8
> > > >>>>     return;
> > > >>>> ;;    succ:       EXIT
> > > >>>>
> > > >>>> }
> > > >>>>
> > > >>>
> > > >>> Which shows the code is obviously unreachable in the case we're warning
> > > >>> about.  You can't see this in the dumps because it's exposed by
> > > >>> inlining, then cleaned up before writing the dump file.
> > > >>
> > > >> In the specific case of the bug the code is of course eliminated
> > > >> because it's guarded by the if (s != d).  I was referring to
> > > >> the general (unguarded) case of:
> > > >>
> > > >>     char *s = "", *p;
> > > >>
> > > >>     int main (void)
> > > >>     {
> > > >>       p = strcpy (s, s);
> > > >>       puts (p);
> > > >>     }
> > > >>
> > > >> where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> > > >> p = s;  That's perfectly reasonable but it could equally as well
> > > >> leave the call alone, as it does when s is null, for instance.
> > > >>
> > > >> I think folding it away is not only reasonable but preferable to
> > > >> making the invalid call, but it's done only rarely.  Most of
> > > >> the time GCC does emit the undefined access (it does that with
> > > >> calls to library functions as well as with direct stores and
> > > >> reads).  (I am hoping we can change that in the future so that
> > > >> these kinds of problems are handled with some consistency.)
> > > >>
> > > >>>
> > > >>> ISTM this would be a case we could handle with the __builtin_warning
> > > >>> stuff.
> > > >>>
> > > >>> I think the question is do we want to do anything about it this cycle?
> > > >>>
> > > >>>
> > > >>> If so, I think Martin's approach is quite reasonable.  It disables
> > > >>> folding away the self-copies from gimple-fold and moves the warning
> > > >>> into the expander.  So if there's such a call in the IL at expansion
> > > >>> time we get a warning (-O0).
> > > >>>
> > > >>> I'd hazard a guess that the diagnostic was added to the strlen pass to
> > > >>> capture the missed warning when we're optimizing and the self-copy has
> > > >>> survived until that point. There's a couple issues that raises though.
> > > >>>
> > > >>> First, it's insufficient.  DSE (for example) can do self-copy removal,
> > > >>> so it needs the same handling.  There may be others places too.
> > > >>>
> > > >>> Second, if the code becomes unreachable after strlen, then we've got
> > > >>> new false positive issues.
> > > >>>
> > > >>> It's the classic problems we have with all middle end based warnings.
> > > >>>
> > > >>> But I could live with those if we can show that using __builtin_warning
> > > >>> to handle this stuff in gcc-11 works...  ISTM we emit the
> > > >>> __builtin_warning call before any self-copy like that, whenever we
> > > >>> happen to spot them.  They'll naturally get removed if the path becomes
> > > >>> unreachable.  We'd warn during expansion for calls to
> > > >>> __builtin_warning.  We could even optionally warn when removing a call
> > > >>> to __bulitin_warning.
> > > >>>
> > > >>> Thoughts?
> > > >>
> > > >> The patch has pretty much the same effect as emitting __builtin_warning
> > > >> from the folder would.  It defers the folding until much later, and if
> > > >> the code isn't eliminated, it issues a warning and folds the call away.
> > > >
> > > > But it affects subsequent optimizations - the call is more expensive
> > > > in any size heuristics, it posses an (alias-set zero) memory write
> > > > barrier (unless you start to optimize no-op copies in the alias oracle),
> > > > it is a _call_ - passes like the vectorizer are not happy about a call.
> > > > It prevents SRA of the accessed object, ...
> > >
> > > This is a strcpy call copying over itself.  It's undefined code,
> > > and so hardly anything that's common or so performance sensitive
> > > to make a noticeable difference.
> > >
> > > > So no, leaving in the call is _not_ equivalent to sticking in a
> > > > __builtin_warning() call (or however we actually implement it,
> > > > I'd prefer a stmt in the "debug" category so it's simply ignored
> > > > or elided by most passes by means of existing code).
> > > >
> > > > That said, I'd prefer to not do anything about this bug.  Iff then
> > > > in the inliner try doing a CFG cleanup before folding stmts
> > > > (it's doing delayed folding anyway).  But not for GCC 10.
> > > > One could also mark stmts with no-warning before the inliner
> > > > folds them (and then mark back) to avoid those classes of
> > > > folding warnings.
> > >
> > > I think this would very unfortunate for GCC 10.  The user's code
> > > is clearly correct -- they take pains to avoid the overlapping
> > > copy by guarding against it just before it -- and GCC simply emits
> > > an invalid warning because of how it does inlining.  All that will
> > > be accomplished by not fixing it is we will release a worse quality
> > > compiler than we otherwise can, unnecessarily eroding our users'
> > > confidence in the value of GCC's diagnostic.
> > >
> > > I'll look into your suggestions for the inliner in stage 1 but
> > > please reconsider for GCC 10.
> >
> > One possibility is the attached but that adds an extra CFG cleanup
> > (also untested but on the testcase).  Another possibility would be
> > to re-do fold_marked_statements in terms of cleanup_control_flow_pre (),
> > recognizing unreachable regions on the way and simply avoid folding
> > stmts in blocks that will be then removed by the pending CFG cleanup.
> >
> > I'll see whether I can cook that up quickly.
>
> Like this.  Indenting not fixed and untested apart from on the testcase.
> Disadvantage is we're walking _all_ blocks (at least skipping stmts).
> Technically the iteration scheme should probably change to push
> edges rather than edge iterators so we can avoid the repeated find_taken_edge
> call.
>
> Anyway, just a prototype.
>
> Note SSA propagators / VN avoid to fold stmts that are obviously
> unreachable so it somewhat makes sense to beat the inliner to do
> the same.

I'm testing this variant but I'm quite sure the cgraph verifier will trip on
us not updating cgraph edges for "unreachable" but changed (for example
indirect -> direct call) calls.  Somewhat moot of course but it appears
we want to keep it happy at all cost ...(?)  We even do "special"

  delete_unreachable_blocks_update_callgraph (id.dst_node, false);

  if (flag_checking)
    id.dst_node->verify ();

rather than relying on CFG cleanup we'll do afterwards anyway.
Maybe that's for a reason?  Honza?  Might be for the early inliner?
I see (see first posted variant) that regular delete_basic_block
doesn't bother to remove cgraph edges.

Anyhow, will report back if the simple PRE walk skipping unreachable
regions "works" in practice  (bootstrap/regtest).

Richard.

> Richard.
>
> > > Martin

On Thu, Feb 6, 2020 at 12:14 PM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Thu, Feb 6, 2020 at 11:33 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > On Thu, Feb 6, 2020 at 11:06 AM Richard Biener
> > <richard.guenther@gmail.com> wrote:
> > >
> > > On Wed, Feb 5, 2020 at 4:55 PM Martin Sebor <msebor@gmail.com> wrote:
> > > >
> > > > On 2/5/20 1:19 AM, Richard Biener wrote:
> > > > > On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
> > > > >>
> > > > >> On 2/4/20 2:31 PM, Jeff Law wrote:
> > > > >>> On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> > > > >>>> On 2/4/20 12:15 PM, Richard Biener wrote:
> > > > >>>>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> > > > >>>>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> > > > >>>>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> > > > >>>>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
> > > > >>>>>> call
> > > > >>>>>>>> to strcpy that carefully guards against self-copying.  This is
> > > > >>>>>> caused
> > > > >>>>>>>> by the caller's arguments substituted into the call during inlining
> > > > >>>>>> and
> > > > >>>>>>>> before dead code elimination.
> > > > >>>>>>>>
> > > > >>>>>>>> The attached patch avoids this by removing -Wrestrict from the
> > > > >>>>>> folder
> > > > >>>>>>>> and deferring folding perfectly overlapping (and so undefined)
> > > > >>>>>> calls
> > > > >>>>>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> > > > >>>>>>>> perfectly overlapping calls to memcpy are still folded early.
> > > > >>>>>>>
> > > > >>>>>>> Why do we bother to warn at all for this case?  Just DWIM here.
> > > > >>>>>> Warnings like
> > > > >>>>>>> this can be emitted from the analyzer?
> > > > >>>>>> They potentially can, but the analyzer is and will almost always
> > > > >>>>>> certainly be considerably slower.  I would not expect it to be used
> > > > >>>>>> nearly as much as the core compiler.
> > > > >>>>>>
> > > > >>>>>> WHether or not a particular warning makes sense in the core compiler or
> > > > >>>>>> analyzer would seem to me to depend on whether or not we can reasonably
> > > > >>>>>> issue warnings without interprocedural analysis.  double-free
> > > > >>>>>> realistically requires interprocedural analysis to be effective.  I'm
> > > > >>>>>> not sure Wrestrict really does.
> > > > >>>>>>
> > > > >>>>>>
> > > > >>>>>>> That is, I suggest to simply remove the bogus warning code from
> > > > >>>>>> folding
> > > > >>>>>>> (and _not_ fail the folding).
> > > > >>>>>> I haven't looked at the patch, but if we can get the warning out of the
> > > > >>>>>> folder that's certainly preferable.  And we could investigate deferring
> > > > >>>>>> self-copy removal.
> > > > >>>>>
> > > > >>>>> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> > > > >>>>
> > > > >>>> In this instance the code is reachable (or isn't obviously unreachable).
> > > > >>>> GCC doesn't remove it, but provides benign (and reasonable) semantics
> > > > >>>> for it(*).  To me, that's one aspect of quality.  Letting the user know
> > > > >>>> that the code is buggy is another.  I view that as at least as important
> > > > >>>> as folding the ill-effects away because it makes it possible to fix
> > > > >>>> the problem so the code works correctly even with compilers that don't
> > > > >>>> provide these benign semantics.
> > > > >>> If you look at the guts of what happens at the point where we issue the
> > > > >>> warning from within gimple_fold_builtin_strcpy we have:
> > > > >>>
> > > > >>>> DCH_to_char (char * in, char * out, int collid)
> > > > >>>> {
> > > > >>>>     int type;
> > > > >>>>     char * D.2148;
> > > > >>>>     char * dest;
> > > > >>>>     char * num;
> > > > >>>>     long unsigned int _4;
> > > > >>>>     char * _5;
> > > > >>>>
> > > > >>>> ;;   basic block 2, loop depth 0
> > > > >>>> ;;    pred:       ENTRY
> > > > >>>> ;;    succ:       4
> > > > >>>>
> > > > >>>> ;;   basic block 4, loop depth 0
> > > > >>>> ;;    pred:       2
> > > > >>>> ;;    succ:       5
> > > > >>>>
> > > > >>>> ;;   basic block 5, loop depth 0
> > > > >>>> ;;    pred:       4
> > > > >>>> ;;    succ:       6
> > > > >>>>
> > > > >>>> ;;   basic block 6, loop depth 0
> > > > >>>> ;;    pred:       5
> > > > >>>>     if (0 != 0)
> > > > >>>>       goto <bb 7>; [53.47%]
> > > > >>>>     else
> > > > >>>>       goto <bb 8>; [46.53%]
> > > > >>>> ;;    succ:       7
> > > > >>>> ;;                8
> > > > >>>>
> > > > >>>> ;;   basic block 7, loop depth 0
> > > > >>>> ;;    pred:       6
> > > > >>>>     strcpy (out_1(D), out_1(D));
> > > > >>>> ;;    succ:       8
> > > > >>>>
> > > > >>>> ;;   basic block 8, loop depth 0
> > > > >>>> ;;    pred:       6
> > > > >>>> ;;                7
> > > > >>>>     _4 = __builtin_strlen (out_1(D));
> > > > >>>>     _5 = out_1(D) + _4;
> > > > >>>>     __builtin_memcpy (_5, "foo", 4);
> > > > >>>> ;;    succ:       3
> > > > >>>>
> > > > >>>> ;;   basic block 3, loop depth 0
> > > > >>>> ;;    pred:       8
> > > > >>>>     return;
> > > > >>>> ;;    succ:       EXIT
> > > > >>>>
> > > > >>>> }
> > > > >>>>
> > > > >>>
> > > > >>> Which shows the code is obviously unreachable in the case we're warning
> > > > >>> about.  You can't see this in the dumps because it's exposed by
> > > > >>> inlining, then cleaned up before writing the dump file.
> > > > >>
> > > > >> In the specific case of the bug the code is of course eliminated
> > > > >> because it's guarded by the if (s != d).  I was referring to
> > > > >> the general (unguarded) case of:
> > > > >>
> > > > >>     char *s = "", *p;
> > > > >>
> > > > >>     int main (void)
> > > > >>     {
> > > > >>       p = strcpy (s, s);
> > > > >>       puts (p);
> > > > >>     }
> > > > >>
> > > > >> where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> > > > >> p = s;  That's perfectly reasonable but it could equally as well
> > > > >> leave the call alone, as it does when s is null, for instance.
> > > > >>
> > > > >> I think folding it away is not only reasonable but preferable to
> > > > >> making the invalid call, but it's done only rarely.  Most of
> > > > >> the time GCC does emit the undefined access (it does that with
> > > > >> calls to library functions as well as with direct stores and
> > > > >> reads).  (I am hoping we can change that in the future so that
> > > > >> these kinds of problems are handled with some consistency.)
> > > > >>
> > > > >>>
> > > > >>> ISTM this would be a case we could handle with the __builtin_warning
> > > > >>> stuff.
> > > > >>>
> > > > >>> I think the question is do we want to do anything about it this cycle?
> > > > >>>
> > > > >>>
> > > > >>> If so, I think Martin's approach is quite reasonable.  It disables
> > > > >>> folding away the self-copies from gimple-fold and moves the warning
> > > > >>> into the expander.  So if there's such a call in the IL at expansion
> > > > >>> time we get a warning (-O0).
> > > > >>>
> > > > >>> I'd hazard a guess that the diagnostic was added to the strlen pass to
> > > > >>> capture the missed warning when we're optimizing and the self-copy has
> > > > >>> survived until that point. There's a couple issues that raises though.
> > > > >>>
> > > > >>> First, it's insufficient.  DSE (for example) can do self-copy removal,
> > > > >>> so it needs the same handling.  There may be others places too.
> > > > >>>
> > > > >>> Second, if the code becomes unreachable after strlen, then we've got
> > > > >>> new false positive issues.
> > > > >>>
> > > > >>> It's the classic problems we have with all middle end based warnings.
> > > > >>>
> > > > >>> But I could live with those if we can show that using __builtin_warning
> > > > >>> to handle this stuff in gcc-11 works...  ISTM we emit the
> > > > >>> __builtin_warning call before any self-copy like that, whenever we
> > > > >>> happen to spot them.  They'll naturally get removed if the path becomes
> > > > >>> unreachable.  We'd warn during expansion for calls to
> > > > >>> __builtin_warning.  We could even optionally warn when removing a call
> > > > >>> to __bulitin_warning.
> > > > >>>
> > > > >>> Thoughts?
> > > > >>
> > > > >> The patch has pretty much the same effect as emitting __builtin_warning
> > > > >> from the folder would.  It defers the folding until much later, and if
> > > > >> the code isn't eliminated, it issues a warning and folds the call away.
> > > > >
> > > > > But it affects subsequent optimizations - the call is more expensive
> > > > > in any size heuristics, it posses an (alias-set zero) memory write
> > > > > barrier (unless you start to optimize no-op copies in the alias oracle),
> > > > > it is a _call_ - passes like the vectorizer are not happy about a call.
> > > > > It prevents SRA of the accessed object, ...
> > > >
> > > > This is a strcpy call copying over itself.  It's undefined code,
> > > > and so hardly anything that's common or so performance sensitive
> > > > to make a noticeable difference.
> > > >
> > > > > So no, leaving in the call is _not_ equivalent to sticking in a
> > > > > __builtin_warning() call (or however we actually implement it,
> > > > > I'd prefer a stmt in the "debug" category so it's simply ignored
> > > > > or elided by most passes by means of existing code).
> > > > >
> > > > > That said, I'd prefer to not do anything about this bug.  Iff then
> > > > > in the inliner try doing a CFG cleanup before folding stmts
> > > > > (it's doing delayed folding anyway).  But not for GCC 10.
> > > > > One could also mark stmts with no-warning before the inliner
> > > > > folds them (and then mark back) to avoid those classes of
> > > > > folding warnings.
> > > >
> > > > I think this would very unfortunate for GCC 10.  The user's code
> > > > is clearly correct -- they take pains to avoid the overlapping
> > > > copy by guarding against it just before it -- and GCC simply emits
> > > > an invalid warning because of how it does inlining.  All that will
> > > > be accomplished by not fixing it is we will release a worse quality
> > > > compiler than we otherwise can, unnecessarily eroding our users'
> > > > confidence in the value of GCC's diagnostic.
> > > >
> > > > I'll look into your suggestions for the inliner in stage 1 but
> > > > please reconsider for GCC 10.
> > >
> > > One possibility is the attached but that adds an extra CFG cleanup
> > > (also untested but on the testcase).  Another possibility would be
> > > to re-do fold_marked_statements in terms of cleanup_control_flow_pre (),
> > > recognizing unreachable regions on the way and simply avoid folding
> > > stmts in blocks that will be then removed by the pending CFG cleanup.
> > >
> > > I'll see whether I can cook that up quickly.
> >
> > Like this.  Indenting not fixed and untested apart from on the testcase.
> > Disadvantage is we're walking _all_ blocks (at least skipping stmts).
> > Technically the iteration scheme should probably change to push
> > edges rather than edge iterators so we can avoid the repeated find_taken_edge
> > call.
> >
> > Anyway, just a prototype.
> >
> > Note SSA propagators / VN avoid to fold stmts that are obviously
> > unreachable so it somewhat makes sense to beat the inliner to do
> > the same.
>
> I'm testing this variant but I'm quite sure the cgraph verifier will trip on
> us not updating cgraph edges for "unreachable" but changed (for example
> indirect -> direct call) calls.  Somewhat moot of course but it appears
> we want to keep it happy at all cost ...(?)  We even do "special"
>
>   delete_unreachable_blocks_update_callgraph (id.dst_node, false);
>
>   if (flag_checking)
>     id.dst_node->verify ();
>
> rather than relying on CFG cleanup we'll do afterwards anyway.
> Maybe that's for a reason?  Honza?  Might be for the early inliner?
> I see (see first posted variant) that regular delete_basic_block
> doesn't bother to remove cgraph edges.

It seems to work fine on

void foo() {}

static inline void bar (int i, int j, void (*p)())
{
  if (i != j)
    p();
}

void baz(int i)
{
  bar (i, i, foo);
}

where I see early inlining inlining bar, changing the call to foo()
(and not noticing it will be unreachable - for simpler if (0) cases
the inliner replaces the call to foo with a call to __builtin_unreachable!),
not folding it and passing verification.  Guess the cgraph edge updating
code in copy_bb deals with this so we should be safe not folding
stmts.

That said, testing still running.

Richard.

> Anyhow, will report back if the simple PRE walk skipping unreachable
> regions "works" in practice  (bootstrap/regtest).
>
> Richard.
>
> > Richard.
> >
> > > > Martin

On Thu, Feb 6, 2020 at 2:00 PM Jeff Law <law@redhat.com> wrote:
>
> On Wed, 2020-02-05 at 09:19 +0100, Richard Biener wrote:
> > On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
> > > On 2/4/20 2:31 PM, Jeff Law wrote:
> > > > On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> > > > > On 2/4/20 12:15 PM, Richard Biener wrote:
> > > > > > On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> > > > > > > On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> > > > > > > > On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> > > > > > > > > PR 93519 reports a false positive -Wrestrict issued for an inlined
> > > > > > > call
> > > > > > > > > to strcpy that carefully guards against self-copying.  This is
> > > > > > > caused
> > > > > > > > > by the caller's arguments substituted into the call during inlining
> > > > > > > and
> > > > > > > > > before dead code elimination.
> > > > > > > > >
> > > > > > > > > The attached patch avoids this by removing -Wrestrict from the
> > > > > > > folder
> > > > > > > > > and deferring folding perfectly overlapping (and so undefined)
> > > > > > > calls
> > > > > > > > > to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> > > > > > > > > perfectly overlapping calls to memcpy are still folded early.
> > > > > > > >
> > > > > > > > Why do we bother to warn at all for this case?  Just DWIM here.
> > > > > > > Warnings like
> > > > > > > > this can be emitted from the analyzer?
> > > > > > > They potentially can, but the analyzer is and will almost always
> > > > > > > certainly be considerably slower.  I would not expect it to be used
> > > > > > > nearly as much as the core compiler.
> > > > > > >
> > > > > > > WHether or not a particular warning makes sense in the core compiler or
> > > > > > > analyzer would seem to me to depend on whether or not we can reasonably
> > > > > > > issue warnings without interprocedural analysis.  double-free
> > > > > > > realistically requires interprocedural analysis to be effective.  I'm
> > > > > > > not sure Wrestrict really does.
> > > > > > >
> > > > > > >
> > > > > > > > That is, I suggest to simply remove the bogus warning code from
> > > > > > > folding
> > > > > > > > (and _not_ fail the folding).
> > > > > > > I haven't looked at the patch, but if we can get the warning out of the
> > > > > > > folder that's certainly preferable.  And we could investigate deferring
> > > > > > > self-copy removal.
> > > > > >
> > > > > > I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> > > > >
> > > > > In this instance the code is reachable (or isn't obviously unreachable).
> > > > > GCC doesn't remove it, but provides benign (and reasonable) semantics
> > > > > for it(*).  To me, that's one aspect of quality.  Letting the user know
> > > > > that the code is buggy is another.  I view that as at least as important
> > > > > as folding the ill-effects away because it makes it possible to fix
> > > > > the problem so the code works correctly even with compilers that don't
> > > > > provide these benign semantics.
> > > > If you look at the guts of what happens at the point where we issue the
> > > > warning from within gimple_fold_builtin_strcpy we have:
> > > >
> > > > > DCH_to_char (char * in, char * out, int collid)
> > > > > {
> > > > >    int type;
> > > > >    char * D.2148;
> > > > >    char * dest;
> > > > >    char * num;
> > > > >    long unsigned int _4;
> > > > >    char * _5;
> > > > >
> > > > > ;;   basic block 2, loop depth 0
> > > > > ;;    pred:       ENTRY
> > > > > ;;    succ:       4
> > > > >
> > > > > ;;   basic block 4, loop depth 0
> > > > > ;;    pred:       2
> > > > > ;;    succ:       5
> > > > >
> > > > > ;;   basic block 5, loop depth 0
> > > > > ;;    pred:       4
> > > > > ;;    succ:       6
> > > > >
> > > > > ;;   basic block 6, loop depth 0
> > > > > ;;    pred:       5
> > > > >    if (0 != 0)
> > > > >      goto <bb 7>; [53.47%]
> > > > >    else
> > > > >      goto <bb 8>; [46.53%]
> > > > > ;;    succ:       7
> > > > > ;;                8
> > > > >
> > > > > ;;   basic block 7, loop depth 0
> > > > > ;;    pred:       6
> > > > >    strcpy (out_1(D), out_1(D));
> > > > > ;;    succ:       8
> > > > >
> > > > > ;;   basic block 8, loop depth 0
> > > > > ;;    pred:       6
> > > > > ;;                7
> > > > >    _4 = __builtin_strlen (out_1(D));
> > > > >    _5 = out_1(D) + _4;
> > > > >    __builtin_memcpy (_5, "foo", 4);
> > > > > ;;    succ:       3
> > > > >
> > > > > ;;   basic block 3, loop depth 0
> > > > > ;;    pred:       8
> > > > >    return;
> > > > > ;;    succ:       EXIT
> > > > >
> > > > > }
> > > > >
> > > >
> > > > Which shows the code is obviously unreachable in the case we're warning
> > > > about.  You can't see this in the dumps because it's exposed by
> > > > inlining, then cleaned up before writing the dump file.
> > >
> > > In the specific case of the bug the code is of course eliminated
> > > because it's guarded by the if (s != d).  I was referring to
> > > the general (unguarded) case of:
> > >
> > >    char *s = "", *p;
> > >
> > >    int main (void)
> > >    {
> > >      p = strcpy (s, s);
> > >      puts (p);
> > >    }
> > >
> > > where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> > > p = s;  That's perfectly reasonable but it could equally as well
> > > leave the call alone, as it does when s is null, for instance.
> > >
> > > I think folding it away is not only reasonable but preferable to
> > > making the invalid call, but it's done only rarely.  Most of
> > > the time GCC does emit the undefined access (it does that with
> > > calls to library functions as well as with direct stores and
> > > reads).  (I am hoping we can change that in the future so that
> > > these kinds of problems are handled with some consistency.)
> > >
> > > > ISTM this would be a case we could handle with the __builtin_warning
> > > > stuff.
> > > >
> > > > I think the question is do we want to do anything about it this cycle?
> > > >
> > > >
> > > > If so, I think Martin's approach is quite reasonable.  It disables
> > > > folding away the self-copies from gimple-fold and moves the warning
> > > > into the expander.  So if there's such a call in the IL at expansion
> > > > time we get a warning (-O0).
> > > >
> > > > I'd hazard a guess that the diagnostic was added to the strlen pass to
> > > > capture the missed warning when we're optimizing and the self-copy has
> > > > survived until that point. There's a couple issues that raises though.
> > > >
> > > > First, it's insufficient.  DSE (for example) can do self-copy removal,
> > > > so it needs the same handling.  There may be others places too.
> > > >
> > > > Second, if the code becomes unreachable after strlen, then we've got
> > > > new false positive issues.
> > > >
> > > > It's the classic problems we have with all middle end based warnings.
> > > >
> > > > But I could live with those if we can show that using __builtin_warning
> > > > to handle this stuff in gcc-11 works...  ISTM we emit the
> > > > __builtin_warning call before any self-copy like that, whenever we
> > > > happen to spot them.  They'll naturally get removed if the path becomes
> > > > unreachable.  We'd warn during expansion for calls to
> > > > __builtin_warning.  We could even optionally warn when removing a call
> > > > to __bulitin_warning.
> > > >
> > > > Thoughts?
> > >
> > > The patch has pretty much the same effect as emitting __builtin_warning
> > > from the folder would.  It defers the folding until much later, and if
> > > the code isn't eliminated, it issues a warning and folds the call away.
> >
> > But it affects subsequent optimizations - the call is more expensive
> > in any size heuristics, it posses an (alias-set zero) memory write
> > barrier (unless you start to optimize no-op copies in the alias oracle),
> > it is a _call_ - passes like the vectorizer are not happy about a call.
> > It prevents SRA of the accessed object, ...
> Yes, it does affect subsequent optimizations, but realistically how
> often do we have self-assignments?  And how often to we have them via
> the str* functions.
>
> Additionally these are undefined except for memmove.  I find it hard to
> believe these are appearing it performance sensitive code.
>
> So I'd claim it's equivalent from a practical standpoint.

But if the situation is so rare why does emitting the diagnostic matter then...

Also it's not only optimization but eventually further diagnostic that only
gets uncovered by optimizing such self-assignments.

> I guess I could do a Fedora build with a diagnostic to tell us how
> often this happens.  Would you be willing to reconsider if the data
> shows this just doesn't happen often enough to matter?

From my side its more on principle.

But see my two proposed patches to address the actual testcase
(second one doing a PRE walk for fold_marked_stmts is the one I prefer).

Richard.

> Jeff
> >
>

On 2/6/20 6:16 AM, Richard Biener wrote:
> On Thu, Feb 6, 2020 at 2:00 PM Jeff Law <law@redhat.com> wrote:
>>
>> On Wed, 2020-02-05 at 09:19 +0100, Richard Biener wrote:
>>> On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
>>>> On 2/4/20 2:31 PM, Jeff Law wrote:
>>>>> On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
>>>>>> On 2/4/20 12:15 PM, Richard Biener wrote:
>>>>>>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
>>>>>>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
>>>>>>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
>>>>>>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
>>>>>>>> call
>>>>>>>>>> to strcpy that carefully guards against self-copying.  This is
>>>>>>>> caused
>>>>>>>>>> by the caller's arguments substituted into the call during inlining
>>>>>>>> and
>>>>>>>>>> before dead code elimination.
>>>>>>>>>>
>>>>>>>>>> The attached patch avoids this by removing -Wrestrict from the
>>>>>>>> folder
>>>>>>>>>> and deferring folding perfectly overlapping (and so undefined)
>>>>>>>> calls
>>>>>>>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
>>>>>>>>>> perfectly overlapping calls to memcpy are still folded early.
>>>>>>>>>
>>>>>>>>> Why do we bother to warn at all for this case?  Just DWIM here.
>>>>>>>> Warnings like
>>>>>>>>> this can be emitted from the analyzer?
>>>>>>>> They potentially can, but the analyzer is and will almost always
>>>>>>>> certainly be considerably slower.  I would not expect it to be used
>>>>>>>> nearly as much as the core compiler.
>>>>>>>>
>>>>>>>> WHether or not a particular warning makes sense in the core compiler or
>>>>>>>> analyzer would seem to me to depend on whether or not we can reasonably
>>>>>>>> issue warnings without interprocedural analysis.  double-free
>>>>>>>> realistically requires interprocedural analysis to be effective.  I'm
>>>>>>>> not sure Wrestrict really does.
>>>>>>>>
>>>>>>>>
>>>>>>>>> That is, I suggest to simply remove the bogus warning code from
>>>>>>>> folding
>>>>>>>>> (and _not_ fail the folding).
>>>>>>>> I haven't looked at the patch, but if we can get the warning out of the
>>>>>>>> folder that's certainly preferable.  And we could investigate deferring
>>>>>>>> self-copy removal.
>>>>>>>
>>>>>>> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
>>>>>>
>>>>>> In this instance the code is reachable (or isn't obviously unreachable).
>>>>>> GCC doesn't remove it, but provides benign (and reasonable) semantics
>>>>>> for it(*).  To me, that's one aspect of quality.  Letting the user know
>>>>>> that the code is buggy is another.  I view that as at least as important
>>>>>> as folding the ill-effects away because it makes it possible to fix
>>>>>> the problem so the code works correctly even with compilers that don't
>>>>>> provide these benign semantics.
>>>>> If you look at the guts of what happens at the point where we issue the
>>>>> warning from within gimple_fold_builtin_strcpy we have:
>>>>>
>>>>>> DCH_to_char (char * in, char * out, int collid)
>>>>>> {
>>>>>>     int type;
>>>>>>     char * D.2148;
>>>>>>     char * dest;
>>>>>>     char * num;
>>>>>>     long unsigned int _4;
>>>>>>     char * _5;
>>>>>>
>>>>>> ;;   basic block 2, loop depth 0
>>>>>> ;;    pred:       ENTRY
>>>>>> ;;    succ:       4
>>>>>>
>>>>>> ;;   basic block 4, loop depth 0
>>>>>> ;;    pred:       2
>>>>>> ;;    succ:       5
>>>>>>
>>>>>> ;;   basic block 5, loop depth 0
>>>>>> ;;    pred:       4
>>>>>> ;;    succ:       6
>>>>>>
>>>>>> ;;   basic block 6, loop depth 0
>>>>>> ;;    pred:       5
>>>>>>     if (0 != 0)
>>>>>>       goto <bb 7>; [53.47%]
>>>>>>     else
>>>>>>       goto <bb 8>; [46.53%]
>>>>>> ;;    succ:       7
>>>>>> ;;                8
>>>>>>
>>>>>> ;;   basic block 7, loop depth 0
>>>>>> ;;    pred:       6
>>>>>>     strcpy (out_1(D), out_1(D));
>>>>>> ;;    succ:       8
>>>>>>
>>>>>> ;;   basic block 8, loop depth 0
>>>>>> ;;    pred:       6
>>>>>> ;;                7
>>>>>>     _4 = __builtin_strlen (out_1(D));
>>>>>>     _5 = out_1(D) + _4;
>>>>>>     __builtin_memcpy (_5, "foo", 4);
>>>>>> ;;    succ:       3
>>>>>>
>>>>>> ;;   basic block 3, loop depth 0
>>>>>> ;;    pred:       8
>>>>>>     return;
>>>>>> ;;    succ:       EXIT
>>>>>>
>>>>>> }
>>>>>>
>>>>>
>>>>> Which shows the code is obviously unreachable in the case we're warning
>>>>> about.  You can't see this in the dumps because it's exposed by
>>>>> inlining, then cleaned up before writing the dump file.
>>>>
>>>> In the specific case of the bug the code is of course eliminated
>>>> because it's guarded by the if (s != d).  I was referring to
>>>> the general (unguarded) case of:
>>>>
>>>>     char *s = "", *p;
>>>>
>>>>     int main (void)
>>>>     {
>>>>       p = strcpy (s, s);
>>>>       puts (p);
>>>>     }
>>>>
>>>> where GCC folds the assignment 'p = strcpy(s, s);' to effectively
>>>> p = s;  That's perfectly reasonable but it could equally as well
>>>> leave the call alone, as it does when s is null, for instance.
>>>>
>>>> I think folding it away is not only reasonable but preferable to
>>>> making the invalid call, but it's done only rarely.  Most of
>>>> the time GCC does emit the undefined access (it does that with
>>>> calls to library functions as well as with direct stores and
>>>> reads).  (I am hoping we can change that in the future so that
>>>> these kinds of problems are handled with some consistency.)
>>>>
>>>>> ISTM this would be a case we could handle with the __builtin_warning
>>>>> stuff.
>>>>>
>>>>> I think the question is do we want to do anything about it this cycle?
>>>>>
>>>>>
>>>>> If so, I think Martin's approach is quite reasonable.  It disables
>>>>> folding away the self-copies from gimple-fold and moves the warning
>>>>> into the expander.  So if there's such a call in the IL at expansion
>>>>> time we get a warning (-O0).
>>>>>
>>>>> I'd hazard a guess that the diagnostic was added to the strlen pass to
>>>>> capture the missed warning when we're optimizing and the self-copy has
>>>>> survived until that point. There's a couple issues that raises though.
>>>>>
>>>>> First, it's insufficient.  DSE (for example) can do self-copy removal,
>>>>> so it needs the same handling.  There may be others places too.
>>>>>
>>>>> Second, if the code becomes unreachable after strlen, then we've got
>>>>> new false positive issues.
>>>>>
>>>>> It's the classic problems we have with all middle end based warnings.
>>>>>
>>>>> But I could live with those if we can show that using __builtin_warning
>>>>> to handle this stuff in gcc-11 works...  ISTM we emit the
>>>>> __builtin_warning call before any self-copy like that, whenever we
>>>>> happen to spot them.  They'll naturally get removed if the path becomes
>>>>> unreachable.  We'd warn during expansion for calls to
>>>>> __builtin_warning.  We could even optionally warn when removing a call
>>>>> to __bulitin_warning.
>>>>>
>>>>> Thoughts?
>>>>
>>>> The patch has pretty much the same effect as emitting __builtin_warning
>>>> from the folder would.  It defers the folding until much later, and if
>>>> the code isn't eliminated, it issues a warning and folds the call away.
>>>
>>> But it affects subsequent optimizations - the call is more expensive
>>> in any size heuristics, it posses an (alias-set zero) memory write
>>> barrier (unless you start to optimize no-op copies in the alias oracle),
>>> it is a _call_ - passes like the vectorizer are not happy about a call.
>>> It prevents SRA of the accessed object, ...
>> Yes, it does affect subsequent optimizations, but realistically how
>> often do we have self-assignments?  And how often to we have them via
>> the str* functions.
>>
>> Additionally these are undefined except for memmove.  I find it hard to
>> believe these are appearing it performance sensitive code.
>>
>> So I'd claim it's equivalent from a practical standpoint.
> 
> But if the situation is so rare why does emitting the diagnostic matter then...

Because by their very nature bugs are rare, and often lurk in rare,
convoluted, tricky, or just plain stupid code.  Buggy code may be
worth transforming into harmless code as a defensive strategy but
it's not worth optimizing.  In my mind, it is always worth pointing
out and fixing, even if it's (in all likelihood) benign like exactly
overlapping copies.

To get some data I instrumented GCC to report instances of exactly
overlapping calls to strcpy and memcpy/mempcpy and built a bunch
of code with it.  It found none in any of it, including GCC itself,
Glibc, Binutils/GDB, the Linux kernel, and a handful of others.

Does that mean it never happens and so isn't worth detecting?  I'd
say not, because -Wrestrict has uncovered 16 instances of overlapping
sprintf calls in the kernel (where the destination is also an argument
to the first %s directive).

> 
> Also it's not only optimization but eventually further diagnostic that only
> gets uncovered by optimizing such self-assignments.

I can't think of any examples where folding exactly overlapping copies
might expose other problems down the line but I'm all for folding them
away earlier rather than later (it's benign) as long as they're also
diagnosed.  Replacing them with __builtin_warning might be one way to
do that (though no one looked at the patch when I posted it).

> 
>> I guess I could do a Fedora build with a diagnostic to tell us how
>> often this happens.  Would you be willing to reconsider if the data
>> shows this just doesn't happen often enough to matter?
> 
>  From my side its more on principle.

What principle is that?  (Sincerely.)

There are many examples where GCC intentionally emits invalid code
even though folding it away (or replacing it with something like
a trap or unreachable) would be far a better, safer, or friendlier
choice.  Calling library functions with invalid arguments (e.g.,
null pointers) is a prime but not the only example.  It's pretty
much a rule that they don't get touched.  I think it's a decision
worth revisiting throughout the compiler, not just in the folder,
while providing users with a choice of how to consistently respond
to such cases, e.g., via a command line option.

Martin

> 
> But see my two proposed patches to address the actual testcase
> (second one doing a PRE walk for fold_marked_stmts is the one I prefer).
> 
> Richard.
> 
>> Jeff
>>>
>>

On Fri, Feb 7, 2020 at 12:08 AM Martin Sebor <msebor@gmail.com> wrote:
>
> On 2/6/20 6:16 AM, Richard Biener wrote:
> > On Thu, Feb 6, 2020 at 2:00 PM Jeff Law <law@redhat.com> wrote:
> >>
> >> On Wed, 2020-02-05 at 09:19 +0100, Richard Biener wrote:
> >>> On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
> >>>> On 2/4/20 2:31 PM, Jeff Law wrote:
> >>>>> On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> >>>>>> On 2/4/20 12:15 PM, Richard Biener wrote:
> >>>>>>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> >>>>>>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> >>>>>>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> >>>>>>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
> >>>>>>>> call
> >>>>>>>>>> to strcpy that carefully guards against self-copying.  This is
> >>>>>>>> caused
> >>>>>>>>>> by the caller's arguments substituted into the call during inlining
> >>>>>>>> and
> >>>>>>>>>> before dead code elimination.
> >>>>>>>>>>
> >>>>>>>>>> The attached patch avoids this by removing -Wrestrict from the
> >>>>>>>> folder
> >>>>>>>>>> and deferring folding perfectly overlapping (and so undefined)
> >>>>>>>> calls
> >>>>>>>>>> to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> >>>>>>>>>> perfectly overlapping calls to memcpy are still folded early.
> >>>>>>>>>
> >>>>>>>>> Why do we bother to warn at all for this case?  Just DWIM here.
> >>>>>>>> Warnings like
> >>>>>>>>> this can be emitted from the analyzer?
> >>>>>>>> They potentially can, but the analyzer is and will almost always
> >>>>>>>> certainly be considerably slower.  I would not expect it to be used
> >>>>>>>> nearly as much as the core compiler.
> >>>>>>>>
> >>>>>>>> WHether or not a particular warning makes sense in the core compiler or
> >>>>>>>> analyzer would seem to me to depend on whether or not we can reasonably
> >>>>>>>> issue warnings without interprocedural analysis.  double-free
> >>>>>>>> realistically requires interprocedural analysis to be effective.  I'm
> >>>>>>>> not sure Wrestrict really does.
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>> That is, I suggest to simply remove the bogus warning code from
> >>>>>>>> folding
> >>>>>>>>> (and _not_ fail the folding).
> >>>>>>>> I haven't looked at the patch, but if we can get the warning out of the
> >>>>>>>> folder that's certainly preferable.  And we could investigate deferring
> >>>>>>>> self-copy removal.
> >>>>>>>
> >>>>>>> I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> >>>>>>
> >>>>>> In this instance the code is reachable (or isn't obviously unreachable).
> >>>>>> GCC doesn't remove it, but provides benign (and reasonable) semantics
> >>>>>> for it(*).  To me, that's one aspect of quality.  Letting the user know
> >>>>>> that the code is buggy is another.  I view that as at least as important
> >>>>>> as folding the ill-effects away because it makes it possible to fix
> >>>>>> the problem so the code works correctly even with compilers that don't
> >>>>>> provide these benign semantics.
> >>>>> If you look at the guts of what happens at the point where we issue the
> >>>>> warning from within gimple_fold_builtin_strcpy we have:
> >>>>>
> >>>>>> DCH_to_char (char * in, char * out, int collid)
> >>>>>> {
> >>>>>>     int type;
> >>>>>>     char * D.2148;
> >>>>>>     char * dest;
> >>>>>>     char * num;
> >>>>>>     long unsigned int _4;
> >>>>>>     char * _5;
> >>>>>>
> >>>>>> ;;   basic block 2, loop depth 0
> >>>>>> ;;    pred:       ENTRY
> >>>>>> ;;    succ:       4
> >>>>>>
> >>>>>> ;;   basic block 4, loop depth 0
> >>>>>> ;;    pred:       2
> >>>>>> ;;    succ:       5
> >>>>>>
> >>>>>> ;;   basic block 5, loop depth 0
> >>>>>> ;;    pred:       4
> >>>>>> ;;    succ:       6
> >>>>>>
> >>>>>> ;;   basic block 6, loop depth 0
> >>>>>> ;;    pred:       5
> >>>>>>     if (0 != 0)
> >>>>>>       goto <bb 7>; [53.47%]
> >>>>>>     else
> >>>>>>       goto <bb 8>; [46.53%]
> >>>>>> ;;    succ:       7
> >>>>>> ;;                8
> >>>>>>
> >>>>>> ;;   basic block 7, loop depth 0
> >>>>>> ;;    pred:       6
> >>>>>>     strcpy (out_1(D), out_1(D));
> >>>>>> ;;    succ:       8
> >>>>>>
> >>>>>> ;;   basic block 8, loop depth 0
> >>>>>> ;;    pred:       6
> >>>>>> ;;                7
> >>>>>>     _4 = __builtin_strlen (out_1(D));
> >>>>>>     _5 = out_1(D) + _4;
> >>>>>>     __builtin_memcpy (_5, "foo", 4);
> >>>>>> ;;    succ:       3
> >>>>>>
> >>>>>> ;;   basic block 3, loop depth 0
> >>>>>> ;;    pred:       8
> >>>>>>     return;
> >>>>>> ;;    succ:       EXIT
> >>>>>>
> >>>>>> }
> >>>>>>
> >>>>>
> >>>>> Which shows the code is obviously unreachable in the case we're warning
> >>>>> about.  You can't see this in the dumps because it's exposed by
> >>>>> inlining, then cleaned up before writing the dump file.
> >>>>
> >>>> In the specific case of the bug the code is of course eliminated
> >>>> because it's guarded by the if (s != d).  I was referring to
> >>>> the general (unguarded) case of:
> >>>>
> >>>>     char *s = "", *p;
> >>>>
> >>>>     int main (void)
> >>>>     {
> >>>>       p = strcpy (s, s);
> >>>>       puts (p);
> >>>>     }
> >>>>
> >>>> where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> >>>> p = s;  That's perfectly reasonable but it could equally as well
> >>>> leave the call alone, as it does when s is null, for instance.
> >>>>
> >>>> I think folding it away is not only reasonable but preferable to
> >>>> making the invalid call, but it's done only rarely.  Most of
> >>>> the time GCC does emit the undefined access (it does that with
> >>>> calls to library functions as well as with direct stores and
> >>>> reads).  (I am hoping we can change that in the future so that
> >>>> these kinds of problems are handled with some consistency.)
> >>>>
> >>>>> ISTM this would be a case we could handle with the __builtin_warning
> >>>>> stuff.
> >>>>>
> >>>>> I think the question is do we want to do anything about it this cycle?
> >>>>>
> >>>>>
> >>>>> If so, I think Martin's approach is quite reasonable.  It disables
> >>>>> folding away the self-copies from gimple-fold and moves the warning
> >>>>> into the expander.  So if there's such a call in the IL at expansion
> >>>>> time we get a warning (-O0).
> >>>>>
> >>>>> I'd hazard a guess that the diagnostic was added to the strlen pass to
> >>>>> capture the missed warning when we're optimizing and the self-copy has
> >>>>> survived until that point. There's a couple issues that raises though.
> >>>>>
> >>>>> First, it's insufficient.  DSE (for example) can do self-copy removal,
> >>>>> so it needs the same handling.  There may be others places too.
> >>>>>
> >>>>> Second, if the code becomes unreachable after strlen, then we've got
> >>>>> new false positive issues.
> >>>>>
> >>>>> It's the classic problems we have with all middle end based warnings.
> >>>>>
> >>>>> But I could live with those if we can show that using __builtin_warning
> >>>>> to handle this stuff in gcc-11 works...  ISTM we emit the
> >>>>> __builtin_warning call before any self-copy like that, whenever we
> >>>>> happen to spot them.  They'll naturally get removed if the path becomes
> >>>>> unreachable.  We'd warn during expansion for calls to
> >>>>> __builtin_warning.  We could even optionally warn when removing a call
> >>>>> to __bulitin_warning.
> >>>>>
> >>>>> Thoughts?
> >>>>
> >>>> The patch has pretty much the same effect as emitting __builtin_warning
> >>>> from the folder would.  It defers the folding until much later, and if
> >>>> the code isn't eliminated, it issues a warning and folds the call away.
> >>>
> >>> But it affects subsequent optimizations - the call is more expensive
> >>> in any size heuristics, it posses an (alias-set zero) memory write
> >>> barrier (unless you start to optimize no-op copies in the alias oracle),
> >>> it is a _call_ - passes like the vectorizer are not happy about a call.
> >>> It prevents SRA of the accessed object, ...
> >> Yes, it does affect subsequent optimizations, but realistically how
> >> often do we have self-assignments?  And how often to we have them via
> >> the str* functions.
> >>
> >> Additionally these are undefined except for memmove.  I find it hard to
> >> believe these are appearing it performance sensitive code.
> >>
> >> So I'd claim it's equivalent from a practical standpoint.
> >
> > But if the situation is so rare why does emitting the diagnostic matter then...
>
> Because by their very nature bugs are rare, and often lurk in rare,
> convoluted, tricky, or just plain stupid code.  Buggy code may be
> worth transforming into harmless code as a defensive strategy but
> it's not worth optimizing.  In my mind, it is always worth pointing
> out and fixing, even if it's (in all likelihood) benign like exactly
> overlapping copies.
>
> To get some data I instrumented GCC to report instances of exactly
> overlapping calls to strcpy and memcpy/mempcpy and built a bunch
> of code with it.  It found none in any of it, including GCC itself,
> Glibc, Binutils/GDB, the Linux kernel, and a handful of others.
>
> Does that mean it never happens and so isn't worth detecting?  I'd
> say not, because -Wrestrict has uncovered 16 instances of overlapping
> sprintf calls in the kernel (where the destination is also an argument
> to the first %s directive).
>
> >
> > Also it's not only optimization but eventually further diagnostic that only
> > gets uncovered by optimizing such self-assignments.
>
> I can't think of any examples where folding exactly overlapping copies
> might expose other problems down the line but I'm all for folding them
> away earlier rather than later (it's benign) as long as they're also
> diagnosed.  Replacing them with __builtin_warning might be one way to
> do that (though no one looked at the patch when I posted it).
>
> >
> >> I guess I could do a Fedora build with a diagnostic to tell us how
> >> often this happens.  Would you be willing to reconsider if the data
> >> shows this just doesn't happen often enough to matter?
> >
> >  From my side its more on principle.
>
> What principle is that?  (Sincerely.)

The principle that GCC is an optimizing compiler not a static analyzer.

I'm actually fine with diagnosing things (and if the implementation
constraints require that even from folders), but not doing or delaying
optimizations just because of diagnostics starts to cross the line
to a static analysis tool.

> There are many examples where GCC intentionally emits invalid code
> even though folding it away (or replacing it with something like
> a trap or unreachable) would be far a better, safer, or friendlier
> choice.  Calling library functions with invalid arguments (e.g.,
> null pointers) is a prime but not the only example.  It's pretty
> much a rule that they don't get touched.  I think it's a decision
> worth revisiting throughout the compiler, not just in the folder,
> while providing users with a choice of how to consistently respond
> to such cases, e.g., via a command line option.

To me it's a QOI question that depends on the actual case.
Turning memcpy (p, p, N) into a no-op is the correct thing,
even though with (too) large N it might trap.  Folding
a read from outside of an object to zero might be OK
(it's undefined), but the choice of zero is arbitrary and it's
clearly not what the writer of the code intended.  Folding
it do a NaT and do further optimizations based on that
would be even worse.  Currently we leave the undefined
code in place but IMHO another sensible choice would
be to replace it with an explicit trap or unreachable
(and the choice between those could be dependent on
a command-line option).

Richard.

> Martin
>
> >
> > But see my two proposed patches to address the actual testcase
> > (second one doing a PRE walk for fold_marked_stmts is the one I prefer).
> >
> > Richard.
> >
> >> Jeff
> >>>
> >>
>

On Thu, 2020-02-06 at 14:16 +0100, Richard Biener wrote:
> On Thu, Feb 6, 2020 at 2:00 PM Jeff Law <law@redhat.com> wrote:
> > On Wed, 2020-02-05 at 09:19 +0100, Richard Biener wrote:
> > > On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <msebor@gmail.com> wrote:
> > > > On 2/4/20 2:31 PM, Jeff Law wrote:
> > > > > On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> > > > > > On 2/4/20 12:15 PM, Richard Biener wrote:
> > > > > > > On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
> > > > > > > > On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> > > > > > > > > On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <msebor@gmail.com> wrote:
> > > > > > > > > > PR 93519 reports a false positive -Wrestrict issued for an inlined
> > > > > > > > call
> > > > > > > > > > to strcpy that carefully guards against self-copying.  This is
> > > > > > > > caused
> > > > > > > > > > by the caller's arguments substituted into the call during inlining
> > > > > > > > and
> > > > > > > > > > before dead code elimination.
> > > > > > > > > > 
> > > > > > > > > > The attached patch avoids this by removing -Wrestrict from the
> > > > > > > > folder
> > > > > > > > > > and deferring folding perfectly overlapping (and so undefined)
> > > > > > > > calls
> > > > > > > > > > to strcpy (and mempcpy, but not memcpy) until much later.  Calls to
> > > > > > > > > > perfectly overlapping calls to memcpy are still folded early.
> > > > > > > > > 
> > > > > > > > > Why do we bother to warn at all for this case?  Just DWIM here.
> > > > > > > > Warnings like
> > > > > > > > > this can be emitted from the analyzer?
> > > > > > > > They potentially can, but the analyzer is and will almost always
> > > > > > > > certainly be considerably slower.  I would not expect it to be used
> > > > > > > > nearly as much as the core compiler.
> > > > > > > > 
> > > > > > > > WHether or not a particular warning makes sense in the core compiler or
> > > > > > > > analyzer would seem to me to depend on whether or not we can reasonably
> > > > > > > > issue warnings without interprocedural analysis.  double-free
> > > > > > > > realistically requires interprocedural analysis to be effective.  I'm
> > > > > > > > not sure Wrestrict really does.
> > > > > > > > 
> > > > > > > > 
> > > > > > > > > That is, I suggest to simply remove the bogus warning code from
> > > > > > > > folding
> > > > > > > > > (and _not_ fail the folding).
> > > > > > > > I haven't looked at the patch, but if we can get the warning out of the
> > > > > > > > folder that's certainly preferable.  And we could investigate deferring
> > > > > > > > self-copy removal.
> > > > > > > 
> > > > > > > I think the issue is as usual, warning for code we'll later remove as dead. Warning at folding is almost always premature.
> > > > > > 
> > > > > > In this instance the code is reachable (or isn't obviously unreachable).
> > > > > > GCC doesn't remove it, but provides benign (and reasonable) semantics
> > > > > > for it(*).  To me, that's one aspect of quality.  Letting the user know
> > > > > > that the code is buggy is another.  I view that as at least as important
> > > > > > as folding the ill-effects away because it makes it possible to fix
> > > > > > the problem so the code works correctly even with compilers that don't
> > > > > > provide these benign semantics.
> > > > > If you look at the guts of what happens at the point where we issue the
> > > > > warning from within gimple_fold_builtin_strcpy we have:
> > > > > 
> > > > > > DCH_to_char (char * in, char * out, int collid)
> > > > > > {
> > > > > >    int type;
> > > > > >    char * D.2148;
> > > > > >    char * dest;
> > > > > >    char * num;
> > > > > >    long unsigned int _4;
> > > > > >    char * _5;
> > > > > > 
> > > > > > ;;   basic block 2, loop depth 0
> > > > > > ;;    pred:       ENTRY
> > > > > > ;;    succ:       4
> > > > > > 
> > > > > > ;;   basic block 4, loop depth 0
> > > > > > ;;    pred:       2
> > > > > > ;;    succ:       5
> > > > > > 
> > > > > > ;;   basic block 5, loop depth 0
> > > > > > ;;    pred:       4
> > > > > > ;;    succ:       6
> > > > > > 
> > > > > > ;;   basic block 6, loop depth 0
> > > > > > ;;    pred:       5
> > > > > >    if (0 != 0)
> > > > > >      goto <bb 7>; [53.47%]
> > > > > >    else
> > > > > >      goto <bb 8>; [46.53%]
> > > > > > ;;    succ:       7
> > > > > > ;;                8
> > > > > > 
> > > > > > ;;   basic block 7, loop depth 0
> > > > > > ;;    pred:       6
> > > > > >    strcpy (out_1(D), out_1(D));
> > > > > > ;;    succ:       8
> > > > > > 
> > > > > > ;;   basic block 8, loop depth 0
> > > > > > ;;    pred:       6
> > > > > > ;;                7
> > > > > >    _4 = __builtin_strlen (out_1(D));
> > > > > >    _5 = out_1(D) + _4;
> > > > > >    __builtin_memcpy (_5, "foo", 4);
> > > > > > ;;    succ:       3
> > > > > > 
> > > > > > ;;   basic block 3, loop depth 0
> > > > > > ;;    pred:       8
> > > > > >    return;
> > > > > > ;;    succ:       EXIT
> > > > > > 
> > > > > > }
> > > > > > 
> > > > > 
> > > > > Which shows the code is obviously unreachable in the case we're warning
> > > > > about.  You can't see this in the dumps because it's exposed by
> > > > > inlining, then cleaned up before writing the dump file.
> > > > 
> > > > In the specific case of the bug the code is of course eliminated
> > > > because it's guarded by the if (s != d).  I was referring to
> > > > the general (unguarded) case of:
> > > > 
> > > >    char *s = "", *p;
> > > > 
> > > >    int main (void)
> > > >    {
> > > >      p = strcpy (s, s);
> > > >      puts (p);
> > > >    }
> > > > 
> > > > where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> > > > p = s;  That's perfectly reasonable but it could equally as well
> > > > leave the call alone, as it does when s is null, for instance.
> > > > 
> > > > I think folding it away is not only reasonable but preferable to
> > > > making the invalid call, but it's done only rarely.  Most of
> > > > the time GCC does emit the undefined access (it does that with
> > > > calls to library functions as well as with direct stores and
> > > > reads).  (I am hoping we can change that in the future so that
> > > > these kinds of problems are handled with some consistency.)
> > > > 
> > > > > ISTM this would be a case we could handle with the __builtin_warning
> > > > > stuff.
> > > > > 
> > > > > I think the question is do we want to do anything about it this cycle?
> > > > > 
> > > > > 
> > > > > If so, I think Martin's approach is quite reasonable.  It disables
> > > > > folding away the self-copies from gimple-fold and moves the warning
> > > > > into the expander.  So if there's such a call in the IL at expansion
> > > > > time we get a warning (-O0).
> > > > > 
> > > > > I'd hazard a guess that the diagnostic was added to the strlen pass to
> > > > > capture the missed warning when we're optimizing and the self-copy has
> > > > > survived until that point. There's a couple issues that raises though.
> > > > > 
> > > > > First, it's insufficient.  DSE (for example) can do self-copy removal,
> > > > > so it needs the same handling.  There may be others places too.
> > > > > 
> > > > > Second, if the code becomes unreachable after strlen, then we've got
> > > > > new false positive issues.
> > > > > 
> > > > > It's the classic problems we have with all middle end based warnings.
> > > > > 
> > > > > But I could live with those if we can show that using __builtin_warning
> > > > > to handle this stuff in gcc-11 works...  ISTM we emit the
> > > > > __builtin_warning call before any self-copy like that, whenever we
> > > > > happen to spot them.  They'll naturally get removed if the path becomes
> > > > > unreachable.  We'd warn during expansion for calls to
> > > > > __builtin_warning.  We could even optionally warn when removing a call
> > > > > to __bulitin_warning.
> > > > > 
> > > > > Thoughts?
> > > > 
> > > > The patch has pretty much the same effect as emitting __builtin_warning
> > > > from the folder would.  It defers the folding until much later, and if
> > > > the code isn't eliminated, it issues a warning and folds the call away.
> > > 
> > > But it affects subsequent optimizations - the call is more expensive
> > > in any size heuristics, it posses an (alias-set zero) memory write
> > > barrier (unless you start to optimize no-op copies in the alias oracle),
> > > it is a _call_ - passes like the vectorizer are not happy about a call.
> > > It prevents SRA of the accessed object, ...
> > Yes, it does affect subsequent optimizations, but realistically how
> > often do we have self-assignments?  And how often to we have them via
> > the str* functions.
> > 
> > Additionally these are undefined except for memmove.  I find it hard to
> > believe these are appearing it performance sensitive code.
> > 
> > So I'd claim it's equivalent from a practical standpoint.
> 
> But if the situation is so rare why does emitting the diagnostic matter then...
Vulnerabilities are often on uncommon paths.

In the case of a self-assignment I'm not terribly worried because it's
a nop and the potential for exploitation is minimal -- so eliminating
it at a reasonable point and potentially losing the diagnostic seems
reasonable.

A non-perfect overlap is much more of a concern and definitely worth a
diagnostic IMHO.

> 
> Also it's not only optimization but eventually further diagnostic that only
> gets uncovered by optimizing such self-assignments.
Absolutely.  As I' fond of saying there's a natural tension here. 
There are no easy answers and most of the time the choices involve
trading off one against another.

> 
> > I guess I could do a Fedora build with a diagnostic to tell us how
> > often this happens.  Would you be willing to reconsider if the data
> > shows this just doesn't happen often enough to matter?
> 
> From my side its more on principle.
> 
> But see my two proposed patches to address the actual testcase
> (second one doing a PRE walk for fold_marked_stmts is the one I prefer).
They crossed in the ether.  Outgoing messages on my laptop had been
queued up due to a network issue.

jeff

On Fri, 7 Feb 2020, Richard Biener wrote:

> To me it's a QOI question that depends on the actual case.
> Turning memcpy (p, p, N) into a no-op is the correct thing,
> even though with (too) large N it might trap.  Folding
> a read from outside of an object to zero might be OK
> (it's undefined), but the choice of zero is arbitrary and it's
> clearly not what the writer of the code intended.  Folding
> it do a NaT and do further optimizations based on that
> would be even worse.  Currently we leave the undefined
> code in place but IMHO another sensible choice would
> be to replace it with an explicit trap or unreachable
> (and the choice between those could be dependent on
> a command-line option).

Calling va_arg with a type changed by the default argument promotions 
(float, char, etc.) is another case where there would be a reasonably safe 
fallback (making gimplify_va_arg_expr adjust the type itself to the one it 
mentions in the warning message) as an alternative to generating a trap, 
possibly depending on command-line options to say how the user wishes to 
handle such cases.