avoid -Warray-bounds checks for vtable assignments (PR 98266)

Similar to the problem reported for -Wstringop-overflow in pr98266
and already fixed, -Warray-bounds is also susceptible to false
positives in assignments and copies involving virtual inheritance.
Because the two warnings don't share code yet (hopefully in GCC 12)
the attached patch adds its own workaround for this problem to
gimple-array-bounds.cc, this one slightly more crude because of
the limited insight the array bounds checking has into the checked
expressions.

Tested on x86_64-linux.

Martin

Ping:
https://gcc.gnu.org/pipermail/gcc-patches/2021-January/563894.html

On 1/19/21 5:56 PM, Martin Sebor wrote:
> Similar to the problem reported for -Wstringop-overflow in pr98266
> and already fixed, -Warray-bounds is also susceptible to false
> positives in assignments and copies involving virtual inheritance.
> Because the two warnings don't share code yet (hopefully in GCC 12)
> the attached patch adds its own workaround for this problem to
> gimple-array-bounds.cc, this one slightly more crude because of
> the limited insight the array bounds checking has into the checked
> expressions.
> 
> Tested on x86_64-linux.
> 
> Martin

Ping 2:
   https://gcc.gnu.org/pipermail/gcc-patches/2021-January/563894.html

On 1/29/21 10:22 AM, Martin Sebor wrote:
> Ping:
> https://gcc.gnu.org/pipermail/gcc-patches/2021-January/563894.html
> 
> On 1/19/21 5:56 PM, Martin Sebor wrote:
>> Similar to the problem reported for -Wstringop-overflow in pr98266
>> and already fixed, -Warray-bounds is also susceptible to false
>> positives in assignments and copies involving virtual inheritance.
>> Because the two warnings don't share code yet (hopefully in GCC 12)
>> the attached patch adds its own workaround for this problem to
>> gimple-array-bounds.cc, this one slightly more crude because of
>> the limited insight the array bounds checking has into the checked
>> expressions.
>>
>> Tested on x86_64-linux.
>>
>> Martin
>

On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
> Similar to the problem reported for -Wstringop-overflow in pr98266
> and already fixed, -Warray-bounds is also susceptible to false
> positives in assignments and copies involving virtual inheritance.
> Because the two warnings don't share code yet (hopefully in GCC 12)
> the attached patch adds its own workaround for this problem to
> gimple-array-bounds.cc, this one slightly more crude because of
> the limited insight the array bounds checking has into the checked
> expressions.
>
> Tested on x86_64-linux.
>
> Martin
>
> gcc-98266.diff
>
> PR middle-end/98266 - bogus array subscript is partly outside array bounds on virtual inheritance
>
> gcc/ChangeLog:
>
> 	PR middle-end/98266
> 	* gimple-array-bounds.cc (array_bounds_checker::check_array_bounds):
> 	Avoid checking references involving artificial members.
>
> gcc/testsuite/ChangeLog:
>
> 	PR middle-end/98266
> 	* g++.dg/warn/Warray-bounds-15.C: New test.
It seems to me that we've got the full statement at some point  and thus
the full expression so at some point couldn't we detect when
TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using TYPE_SIZE_UNIT
rather than DECL_SIZE_UNIT in gimple-array-bounds.cc

Am I missing something?


jeff

On 2/8/21 12:59 PM, Jeff Law wrote:
> 
> 
> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>> Similar to the problem reported for -Wstringop-overflow in pr98266
>> and already fixed, -Warray-bounds is also susceptible to false
>> positives in assignments and copies involving virtual inheritance.
>> Because the two warnings don't share code yet (hopefully in GCC 12)
>> the attached patch adds its own workaround for this problem to
>> gimple-array-bounds.cc, this one slightly more crude because of
>> the limited insight the array bounds checking has into the checked
>> expressions.
>>
>> Tested on x86_64-linux.
>>
>> Martin
>>
>> gcc-98266.diff
>>
>> PR middle-end/98266 - bogus array subscript is partly outside array bounds on virtual inheritance
>>
>> gcc/ChangeLog:
>>
>> 	PR middle-end/98266
>> 	* gimple-array-bounds.cc (array_bounds_checker::check_array_bounds):
>> 	Avoid checking references involving artificial members.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 	PR middle-end/98266
>> 	* g++.dg/warn/Warray-bounds-15.C: New test.
> It seems to me that we've got the full statement at some point  and thus
> the full expression so at some point couldn't we detect when
> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using TYPE_SIZE_UNIT
> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
> 
> Am I missing something?

The expression we're looking at when the false positive is issued
is the MEM_REF in the LHS of:

MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void 
*)&_ZTC1E24_1D + 24B];

TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
DECL_SIZE and TYPE_SIZE.

Martin

On 2/8/21 2:56 PM, Martin Sebor wrote:
> On 2/8/21 12:59 PM, Jeff Law wrote:
>>
>>
>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>> Similar to the problem reported for -Wstringop-overflow in pr98266
>>> and already fixed, -Warray-bounds is also susceptible to false
>>> positives in assignments and copies involving virtual inheritance.
>>> Because the two warnings don't share code yet (hopefully in GCC 12)
>>> the attached patch adds its own workaround for this problem to
>>> gimple-array-bounds.cc, this one slightly more crude because of
>>> the limited insight the array bounds checking has into the checked
>>> expressions.
>>>
>>> Tested on x86_64-linux.
>>>
>>> Martin
>>>
>>> gcc-98266.diff
>>>
>>> PR middle-end/98266 - bogus array subscript is partly outside array
>>> bounds on virtual inheritance
>>>
>>> gcc/ChangeLog:
>>>
>>>     PR middle-end/98266
>>>     * gimple-array-bounds.cc
>>> (array_bounds_checker::check_array_bounds):
>>>     Avoid checking references involving artificial members.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>>     PR middle-end/98266
>>>     * g++.dg/warn/Warray-bounds-15.C: New test.
>> It seems to me that we've got the full statement at some point  and thus
>> the full expression so at some point couldn't we detect when
>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using TYPE_SIZE_UNIT
>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>
>> Am I missing something?
>
> The expression we're looking at when the false positive is issued
> is the MEM_REF in the LHS of:
>
> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
> *)&_ZTC1E24_1D + 24B];
>
> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
> DECL_SIZE and TYPE_SIZE.
So that seems like it's a different issue then, unrelated to 97595.  Right?

Jeff

On 2/8/21 3:26 PM, Jeff Law wrote:
> 
> 
> On 2/8/21 2:56 PM, Martin Sebor wrote:
>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>
>>>
>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>> Similar to the problem reported for -Wstringop-overflow in pr98266
>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>> positives in assignments and copies involving virtual inheritance.
>>>> Because the two warnings don't share code yet (hopefully in GCC 12)
>>>> the attached patch adds its own workaround for this problem to
>>>> gimple-array-bounds.cc, this one slightly more crude because of
>>>> the limited insight the array bounds checking has into the checked
>>>> expressions.
>>>>
>>>> Tested on x86_64-linux.
>>>>
>>>> Martin
>>>>
>>>> gcc-98266.diff
>>>>
>>>> PR middle-end/98266 - bogus array subscript is partly outside array
>>>> bounds on virtual inheritance
>>>>
>>>> gcc/ChangeLog:
>>>>
>>>>      PR middle-end/98266
>>>>      * gimple-array-bounds.cc
>>>> (array_bounds_checker::check_array_bounds):
>>>>      Avoid checking references involving artificial members.
>>>>
>>>> gcc/testsuite/ChangeLog:
>>>>
>>>>      PR middle-end/98266
>>>>      * g++.dg/warn/Warray-bounds-15.C: New test.
>>> It seems to me that we've got the full statement at some point  and thus
>>> the full expression so at some point couldn't we detect when
>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using TYPE_SIZE_UNIT
>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>
>>> Am I missing something?
>>
>> The expression we're looking at when the false positive is issued
>> is the MEM_REF in the LHS of:
>>
>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
>> *)&_ZTC1E24_1D + 24B];
>>
>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>> DECL_SIZE and TYPE_SIZE.
> So that seems like it's a different issue then, unrelated to 97595.  Right?

I think the underlying problem is the same.  We're getting a size
that doesn't correspond to what's actually being accessed, and it
happens because of the virtual inheritance.  In pr97595 Jason
suggested to use the decl/type size inequality to identify this
case but I think we could have just as well used DECL_ARTIFICIAL
instead.  At least the test cases from pr97595 both pass with
this change.

Martin

> 
> Jeff
>

On 2/8/21 3:44 PM, Martin Sebor wrote:
> On 2/8/21 3:26 PM, Jeff Law wrote:
>>
>>
>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>
>>>>
>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>> Similar to the problem reported for -Wstringop-overflow in pr98266
>>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>>> positives in assignments and copies involving virtual inheritance.
>>>>> Because the two warnings don't share code yet (hopefully in GCC 12)
>>>>> the attached patch adds its own workaround for this problem to
>>>>> gimple-array-bounds.cc, this one slightly more crude because of
>>>>> the limited insight the array bounds checking has into the checked
>>>>> expressions.
>>>>>
>>>>> Tested on x86_64-linux.
>>>>>
>>>>> Martin
>>>>>
>>>>> gcc-98266.diff
>>>>>
>>>>> PR middle-end/98266 - bogus array subscript is partly outside array
>>>>> bounds on virtual inheritance
>>>>>
>>>>> gcc/ChangeLog:
>>>>>
>>>>>      PR middle-end/98266
>>>>>      * gimple-array-bounds.cc
>>>>> (array_bounds_checker::check_array_bounds):
>>>>>      Avoid checking references involving artificial members.
>>>>>
>>>>> gcc/testsuite/ChangeLog:
>>>>>
>>>>>      PR middle-end/98266
>>>>>      * g++.dg/warn/Warray-bounds-15.C: New test.
>>>> It seems to me that we've got the full statement at some point  and
>>>> thus
>>>> the full expression so at some point couldn't we detect when
>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using TYPE_SIZE_UNIT
>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>
>>>> Am I missing something?
>>>
>>> The expression we're looking at when the false positive is issued
>>> is the MEM_REF in the LHS of:
>>>
>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
>>> *)&_ZTC1E24_1D + 24B];
>>>
>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>> DECL_SIZE and TYPE_SIZE.
>> So that seems like it's a different issue then, unrelated to 97595. 
>> Right?
>
> I think the underlying problem is the same.  We're getting a size
> that doesn't correspond to what's actually being accessed, and it
> happens because of the virtual inheritance.  In pr97595 Jason
> suggested to use the decl/type size inequality to identify this
> case but I think we could have just as well used DECL_ARTIFICIAL
> instead.  At least the test cases from pr97595 both pass with
> this change.
But in the 98266 case the type and decl sizes are the same.  So to be
true that would mean that the underlying type we're using to access
memory differs from its actual type.  Is that the case in the IL?  And
does this have wider implications for diagnostics or optimizations that
rely on accurate type sizing?

I'm just trying to make sure I understand, not accepting or rejecting
the patch yet.

jeff

On 2/8/21 4:11 PM, Jeff Law wrote:
> 
> 
> On 2/8/21 3:44 PM, Martin Sebor wrote:
>> On 2/8/21 3:26 PM, Jeff Law wrote:
>>>
>>>
>>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>>
>>>>>
>>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>>> Similar to the problem reported for -Wstringop-overflow in pr98266
>>>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>>>> positives in assignments and copies involving virtual inheritance.
>>>>>> Because the two warnings don't share code yet (hopefully in GCC 12)
>>>>>> the attached patch adds its own workaround for this problem to
>>>>>> gimple-array-bounds.cc, this one slightly more crude because of
>>>>>> the limited insight the array bounds checking has into the checked
>>>>>> expressions.
>>>>>>
>>>>>> Tested on x86_64-linux.
>>>>>>
>>>>>> Martin
>>>>>>
>>>>>> gcc-98266.diff
>>>>>>
>>>>>> PR middle-end/98266 - bogus array subscript is partly outside array
>>>>>> bounds on virtual inheritance
>>>>>>
>>>>>> gcc/ChangeLog:
>>>>>>
>>>>>>       PR middle-end/98266
>>>>>>       * gimple-array-bounds.cc
>>>>>> (array_bounds_checker::check_array_bounds):
>>>>>>       Avoid checking references involving artificial members.
>>>>>>
>>>>>> gcc/testsuite/ChangeLog:
>>>>>>
>>>>>>       PR middle-end/98266
>>>>>>       * g++.dg/warn/Warray-bounds-15.C: New test.
>>>>> It seems to me that we've got the full statement at some point  and
>>>>> thus
>>>>> the full expression so at some point couldn't we detect when
>>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using TYPE_SIZE_UNIT
>>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>>
>>>>> Am I missing something?
>>>>
>>>> The expression we're looking at when the false positive is issued
>>>> is the MEM_REF in the LHS of:
>>>>
>>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
>>>> *)&_ZTC1E24_1D + 24B];
>>>>
>>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>>> DECL_SIZE and TYPE_SIZE.
>>> So that seems like it's a different issue then, unrelated to 97595.
>>> Right?
>>
>> I think the underlying problem is the same.  We're getting a size
>> that doesn't correspond to what's actually being accessed, and it
>> happens because of the virtual inheritance.  In pr97595 Jason
>> suggested to use the decl/type size inequality to identify this
>> case but I think we could have just as well used DECL_ARTIFICIAL
>> instead.  At least the test cases from pr97595 both pass with
>> this change.
> But in the 98266 case the type and decl sizes are the same.  So to be
> true that would mean that the underlying type we're using to access
> memory differs from its actual type.  Is that the case in the IL?  And
> does this have wider implications for diagnostics or optimizations that
> rely on accurate type sizing?
> 
> I'm just trying to make sure I understand, not accepting or rejecting
> the patch yet.

The part of the IL with the MEM_REF is this:

void g ()
{
   void * D.2789;
   struct E D.2652;

   <bb 2> [local count: 1073741824]:
   E::E (&D.2652, "");
   f (&D.2652);

   <bb 3> [local count: 1073741824]:
   MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void 
*)&_ZTC1E24_1D + 24B];
   ...

The access here is to the _vptr.D pointer member of D.2652 which is
just past the end of the parent object (as reflected by its SIZE):
it sets sets up the virtual table pointer.

The access in pr97595 is to the member subobject, which, as Jason
explained (and I accordingly documented under DECL_SIZE in tree.h),
is also laid out separately from the parent object.

These cases aren't exactly the same (which is also why the test
I added for -Warray-bounds in pr97595 didn't expose this bug) but
they are closely related.  The one here can be distinguished by
DECL_ARTIFICAL.  The other by the DECL_SIZE != TYPE_SIZE member
inequality.

Might this impact other warnings?  I'd say so if they don't take
these things into account.  I just learned about this in pr97595
which was a -Wstringop-overflow false positive but I also saw
a similar instance of -Warray-bounds with my patch to improve
caching and enhance array bounds checking.  I dealt with that
instance of the warning in that patch but proactively added
a test case to the fix for pr97595.  But the test case is focused
on the subobject access and not on one to the virtual table so
(as I said above) it didn't expose this bug.

Might this also impact optimizations?  I can imagine someone
unaware of this "gotcha" making the same "naive" assumption
I did, but I'd also expect such an invalid assumption to be
found either in code review or quickly cause problems in
testing.

Martin

[CC Jason for any further comments/clarification]

On 2/9/21 10:49 AM, Martin Sebor wrote:
> On 2/8/21 4:11 PM, Jeff Law wrote:
>>
>>
>> On 2/8/21 3:44 PM, Martin Sebor wrote:
>>> On 2/8/21 3:26 PM, Jeff Law wrote:
>>>>
>>>>
>>>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>>>
>>>>>>
>>>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>>>> Similar to the problem reported for -Wstringop-overflow in pr98266
>>>>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>>>>> positives in assignments and copies involving virtual inheritance.
>>>>>>> Because the two warnings don't share code yet (hopefully in GCC 12)
>>>>>>> the attached patch adds its own workaround for this problem to
>>>>>>> gimple-array-bounds.cc, this one slightly more crude because of
>>>>>>> the limited insight the array bounds checking has into the checked
>>>>>>> expressions.
>>>>>>>
>>>>>>> Tested on x86_64-linux.
>>>>>>>
>>>>>>> Martin
>>>>>>>
>>>>>>> gcc-98266.diff
>>>>>>>
>>>>>>> PR middle-end/98266 - bogus array subscript is partly outside array
>>>>>>> bounds on virtual inheritance
>>>>>>>
>>>>>>> gcc/ChangeLog:
>>>>>>>
>>>>>>>       PR middle-end/98266
>>>>>>>       * gimple-array-bounds.cc
>>>>>>> (array_bounds_checker::check_array_bounds):
>>>>>>>       Avoid checking references involving artificial members.
>>>>>>>
>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>
>>>>>>>       PR middle-end/98266
>>>>>>>       * g++.dg/warn/Warray-bounds-15.C: New test.
>>>>>> It seems to me that we've got the full statement at some point  and
>>>>>> thus
>>>>>> the full expression so at some point couldn't we detect when
>>>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using 
>>>>>> TYPE_SIZE_UNIT
>>>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>>>
>>>>>> Am I missing something?
>>>>>
>>>>> The expression we're looking at when the false positive is issued
>>>>> is the MEM_REF in the LHS of:
>>>>>
>>>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
>>>>> *)&_ZTC1E24_1D + 24B];
>>>>>
>>>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>>>> DECL_SIZE and TYPE_SIZE.
>>>> So that seems like it's a different issue then, unrelated to 97595.
>>>> Right?
>>>
>>> I think the underlying problem is the same.  We're getting a size
>>> that doesn't correspond to what's actually being accessed, and it
>>> happens because of the virtual inheritance.  In pr97595 Jason
>>> suggested to use the decl/type size inequality to identify this
>>> case but I think we could have just as well used DECL_ARTIFICIAL
>>> instead.  At least the test cases from pr97595 both pass with
>>> this change.
>> But in the 98266 case the type and decl sizes are the same.  So to be
>> true that would mean that the underlying type we're using to access
>> memory differs from its actual type.  Is that the case in the IL?  And
>> does this have wider implications for diagnostics or optimizations that
>> rely on accurate type sizing?
>>
>> I'm just trying to make sure I understand, not accepting or rejecting
>> the patch yet.
> 
> The part of the IL with the MEM_REF is this:
> 
> void g ()
> {
>    void * D.2789;
>    struct E D.2652;
> 
>    <bb 2> [local count: 1073741824]:
>    E::E (&D.2652, "");
>    f (&D.2652);
> 
>    <bb 3> [local count: 1073741824]:
>    MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void 
> *)&_ZTC1E24_1D + 24B];
>    ...
> 
> The access here is to the _vptr.D pointer member of D.2652 which is
> just past the end of the parent object (as reflected by its SIZE):
> it sets sets up the virtual table pointer.
> 
> The access in pr97595 is to the member subobject, which, as Jason
> explained (and I accordingly documented under DECL_SIZE in tree.h),
> is also laid out separately from the parent object.
> 
> These cases aren't exactly the same (which is also why the test
> I added for -Warray-bounds in pr97595 didn't expose this bug) but
> they are closely related.  The one here can be distinguished by
> DECL_ARTIFICAL.  The other by the DECL_SIZE != TYPE_SIZE member
> inequality.
> 
> Might this impact other warnings?  I'd say so if they don't take
> these things into account.  I just learned about this in pr97595
> which was a -Wstringop-overflow false positive but I also saw
> a similar instance of -Warray-bounds with my patch to improve
> caching and enhance array bounds checking.  I dealt with that
> instance of the warning in that patch but proactively added
> a test case to the fix for pr97595.  But the test case is focused
> on the subobject access and not on one to the virtual table so
> (as I said above) it didn't expose this bug.
> 
> Might this also impact optimizations?  I can imagine someone
> unaware of this "gotcha" making the same "naive" assumption
> I did, but I'd also expect such an invalid assumption to be
> found either in code review or quickly cause problems in
> testing.

Jeff, does this answer your question?

> 
> Martin

On 2/23/21 11:02 AM, Martin Sebor wrote:
> [CC Jason for any further comments/clarification]
> 
> On 2/9/21 10:49 AM, Martin Sebor wrote:
>> On 2/8/21 4:11 PM, Jeff Law wrote:
>>>
>>>
>>> On 2/8/21 3:44 PM, Martin Sebor wrote:
>>>> On 2/8/21 3:26 PM, Jeff Law wrote:
>>>>>
>>>>>
>>>>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>>>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>>>>> Similar to the problem reported for -Wstringop-overflow in pr98266
>>>>>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>>>>>> positives in assignments and copies involving virtual inheritance.
>>>>>>>> Because the two warnings don't share code yet (hopefully in GCC 12)
>>>>>>>> the attached patch adds its own workaround for this problem to
>>>>>>>> gimple-array-bounds.cc, this one slightly more crude because of
>>>>>>>> the limited insight the array bounds checking has into the checked
>>>>>>>> expressions.
>>>>>>>>
>>>>>>>> Tested on x86_64-linux.
>>>>>>>>
>>>>>>>> Martin
>>>>>>>>
>>>>>>>> gcc-98266.diff
>>>>>>>>
>>>>>>>> PR middle-end/98266 - bogus array subscript is partly outside array
>>>>>>>> bounds on virtual inheritance
>>>>>>>>
>>>>>>>> gcc/ChangeLog:
>>>>>>>>
>>>>>>>>       PR middle-end/98266
>>>>>>>>       * gimple-array-bounds.cc
>>>>>>>> (array_bounds_checker::check_array_bounds):
>>>>>>>>       Avoid checking references involving artificial members.
>>>>>>>>
>>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>>
>>>>>>>>       PR middle-end/98266
>>>>>>>>       * g++.dg/warn/Warray-bounds-15.C: New test.
>>>>>>> It seems to me that we've got the full statement at some point  and
>>>>>>> thus
>>>>>>> the full expression so at some point couldn't we detect when
>>>>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using 
>>>>>>> TYPE_SIZE_UNIT
>>>>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>>>>
>>>>>>> Am I missing something?
>>>>>>
>>>>>> The expression we're looking at when the false positive is issued
>>>>>> is the MEM_REF in the LHS of:
>>>>>>
>>>>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
>>>>>> *)&_ZTC1E24_1D + 24B];
>>>>>>
>>>>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>>>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>>>>> DECL_SIZE and TYPE_SIZE.
>>>>> So that seems like it's a different issue then, unrelated to 97595.
>>>>> Right?
>>>>
>>>> I think the underlying problem is the same.  We're getting a size
>>>> that doesn't correspond to what's actually being accessed, and it
>>>> happens because of the virtual inheritance.  In pr97595 Jason
>>>> suggested to use the decl/type size inequality to identify this
>>>> case but I think we could have just as well used DECL_ARTIFICIAL
>>>> instead.  At least the test cases from pr97595 both pass with
>>>> this change.
>>> But in the 98266 case the type and decl sizes are the same.  So to be
>>> true that would mean that the underlying type we're using to access
>>> memory differs from its actual type.  Is that the case in the IL?  And
>>> does this have wider implications for diagnostics or optimizations that
>>> rely on accurate type sizing?
>>>
>>> I'm just trying to make sure I understand, not accepting or rejecting
>>> the patch yet.
>>
>> The part of the IL with the MEM_REF is this:
>>
>> void g ()
>> {
>>    void * D.2789;
>>    struct E D.2652;
>>
>>    <bb 2> [local count: 1073741824]:
>>    E::E (&D.2652, "");
>>    f (&D.2652);
>>
>>    <bb 3> [local count: 1073741824]:
>>    MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void 
>> *)&_ZTC1E24_1D + 24B];
>>    ...
>>
>> The access here is to the _vptr.D pointer member of D.2652 which is
>> just past the end of the parent object (as reflected by its SIZE):
>> it sets sets up the virtual table pointer.
>>
>> The access in pr97595 is to the member subobject, which, as Jason
>> explained (and I accordingly documented under DECL_SIZE in tree.h),
>> is also laid out separately from the parent object.
>>
>> These cases aren't exactly the same (which is also why the test
>> I added for -Warray-bounds in pr97595 didn't expose this bug) but
>> they are closely related.  The one here can be distinguished by
>> DECL_ARTIFICAL.  The other by the DECL_SIZE != TYPE_SIZE member
>> inequality.
>>
>> Might this impact other warnings?  I'd say so if they don't take
>> these things into account.  I just learned about this in pr97595
>> which was a -Wstringop-overflow false positive but I also saw
>> a similar instance of -Warray-bounds with my patch to improve
>> caching and enhance array bounds checking.  I dealt with that
>> instance of the warning in that patch but proactively added
>> a test case to the fix for pr97595.  But the test case is focused
>> on the subobject access and not on one to the virtual table so
>> (as I said above) it didn't expose this bug.
>>
>> Might this also impact optimizations?  I can imagine someone
>> unaware of this "gotcha" making the same "naive" assumption
>> I did, but I'd also expect such an invalid assumption to be
>> found either in code review or quickly cause problems in
>> testing.
> 
> Jeff, does this answer your question?

I don't see how the issue here depends on the artificiality of the vptr; 
I'd expect to see the same problem with a data member.  The problem is 
that a D base subobject is smaller than a complete D object, and in this 
case the base subobject is allocated such that if it were a full D 
object, it would overlap the end of E.  And we're checking the MEM_REF 
as though accessing a full D object, so we get a warning.

The general issue about the confusion between complete and as-base types 
is PR 22488; I have various thoughts about a proper fix, but there have 
always been higher-priority things to work on.

One possible approach to this PR is that we don't need to check an 
intermediate _REF; in

MEM[(struct D *)&D.2651 + 24B]._vptr.D

we aren't loading a whole D, we're only looking at the _vptr, which is 
within the bounds of E.

Jason

On 2/23/21 2:52 PM, Jason Merrill wrote:
> On 2/23/21 11:02 AM, Martin Sebor wrote:
>> [CC Jason for any further comments/clarification]
>>
>> On 2/9/21 10:49 AM, Martin Sebor wrote:
>>> On 2/8/21 4:11 PM, Jeff Law wrote:
>>>>
>>>>
>>>> On 2/8/21 3:44 PM, Martin Sebor wrote:
>>>>> On 2/8/21 3:26 PM, Jeff Law wrote:
>>>>>>
>>>>>>
>>>>>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>>>>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>>>>>> Similar to the problem reported for -Wstringop-overflow in pr98266
>>>>>>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>>>>>>> positives in assignments and copies involving virtual inheritance.
>>>>>>>>> Because the two warnings don't share code yet (hopefully in GCC 
>>>>>>>>> 12)
>>>>>>>>> the attached patch adds its own workaround for this problem to
>>>>>>>>> gimple-array-bounds.cc, this one slightly more crude because of
>>>>>>>>> the limited insight the array bounds checking has into the checked
>>>>>>>>> expressions.
>>>>>>>>>
>>>>>>>>> Tested on x86_64-linux.
>>>>>>>>>
>>>>>>>>> Martin
>>>>>>>>>
>>>>>>>>> gcc-98266.diff
>>>>>>>>>
>>>>>>>>> PR middle-end/98266 - bogus array subscript is partly outside 
>>>>>>>>> array
>>>>>>>>> bounds on virtual inheritance
>>>>>>>>>
>>>>>>>>> gcc/ChangeLog:
>>>>>>>>>
>>>>>>>>>       PR middle-end/98266
>>>>>>>>>       * gimple-array-bounds.cc
>>>>>>>>> (array_bounds_checker::check_array_bounds):
>>>>>>>>>       Avoid checking references involving artificial members.
>>>>>>>>>
>>>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>>>
>>>>>>>>>       PR middle-end/98266
>>>>>>>>>       * g++.dg/warn/Warray-bounds-15.C: New test.
>>>>>>>> It seems to me that we've got the full statement at some point  and
>>>>>>>> thus
>>>>>>>> the full expression so at some point couldn't we detect when
>>>>>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using 
>>>>>>>> TYPE_SIZE_UNIT
>>>>>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>>>>>
>>>>>>>> Am I missing something?
>>>>>>>
>>>>>>> The expression we're looking at when the false positive is issued
>>>>>>> is the MEM_REF in the LHS of:
>>>>>>>
>>>>>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
>>>>>>> *)&_ZTC1E24_1D + 24B];
>>>>>>>
>>>>>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>>>>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>>>>>> DECL_SIZE and TYPE_SIZE.
>>>>>> So that seems like it's a different issue then, unrelated to 97595.
>>>>>> Right?
>>>>>
>>>>> I think the underlying problem is the same.  We're getting a size
>>>>> that doesn't correspond to what's actually being accessed, and it
>>>>> happens because of the virtual inheritance.  In pr97595 Jason
>>>>> suggested to use the decl/type size inequality to identify this
>>>>> case but I think we could have just as well used DECL_ARTIFICIAL
>>>>> instead.  At least the test cases from pr97595 both pass with
>>>>> this change.
>>>> But in the 98266 case the type and decl sizes are the same.  So to be
>>>> true that would mean that the underlying type we're using to access
>>>> memory differs from its actual type.  Is that the case in the IL?  And
>>>> does this have wider implications for diagnostics or optimizations that
>>>> rely on accurate type sizing?
>>>>
>>>> I'm just trying to make sure I understand, not accepting or rejecting
>>>> the patch yet.
>>>
>>> The part of the IL with the MEM_REF is this:
>>>
>>> void g ()
>>> {
>>>    void * D.2789;
>>>    struct E D.2652;
>>>
>>>    <bb 2> [local count: 1073741824]:
>>>    E::E (&D.2652, "");
>>>    f (&D.2652);
>>>
>>>    <bb 3> [local count: 1073741824]:
>>>    MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void 
>>> *)&_ZTC1E24_1D + 24B];
>>>    ...
>>>
>>> The access here is to the _vptr.D pointer member of D.2652 which is
>>> just past the end of the parent object (as reflected by its SIZE):
>>> it sets sets up the virtual table pointer.
>>>
>>> The access in pr97595 is to the member subobject, which, as Jason
>>> explained (and I accordingly documented under DECL_SIZE in tree.h),
>>> is also laid out separately from the parent object.
>>>
>>> These cases aren't exactly the same (which is also why the test
>>> I added for -Warray-bounds in pr97595 didn't expose this bug) but
>>> they are closely related.  The one here can be distinguished by
>>> DECL_ARTIFICAL.  The other by the DECL_SIZE != TYPE_SIZE member
>>> inequality.
>>>
>>> Might this impact other warnings?  I'd say so if they don't take
>>> these things into account.  I just learned about this in pr97595
>>> which was a -Wstringop-overflow false positive but I also saw
>>> a similar instance of -Warray-bounds with my patch to improve
>>> caching and enhance array bounds checking.  I dealt with that
>>> instance of the warning in that patch but proactively added
>>> a test case to the fix for pr97595.  But the test case is focused
>>> on the subobject access and not on one to the virtual table so
>>> (as I said above) it didn't expose this bug.
>>>
>>> Might this also impact optimizations?  I can imagine someone
>>> unaware of this "gotcha" making the same "naive" assumption
>>> I did, but I'd also expect such an invalid assumption to be
>>> found either in code review or quickly cause problems in
>>> testing.
>>
>> Jeff, does this answer your question?
> 
> I don't see how the issue here depends on the artificiality of the vptr; 
> I'd expect to see the same problem with a data member.  The problem is 
> that a D base subobject is smaller than a complete D object, and in this 
> case the base subobject is allocated such that if it were a full D 
> object, it would overlap the end of E.  And we're checking the MEM_REF 
> as though accessing a full D object, so we get a warning.

Thanks for chiming in!

There is no warning for accesses to data members defined in
the virtual bases because their offset isn't constant.  For example,
given the following modified version of the test case from the patch:

struct A
{
   virtual ~A ();
   int a;
};

struct B: virtual A { int b; };
struct C: virtual A { int c; };
struct D: virtual B, virtual C
{
   int d;
};

D d;

void g ()
{
   D *p = &d;
   p->a = p->b = p->c = p->d = 1;
}

we end up with:

void g ()
{
   ...
   <bb 2> [local count: 1073741824]:
   d.d = 1;                           <<< okay
   _1 = d._vptr.D;                    <<< DECL_ARTIFICIAL
   _2 = MEM[(long int *)_1 + -40B];   <<< not checked
   _3 = (sizetype) _2;                <<< not constant...
   _4 = &d + _3;
   MEM[(struct C *)_4].c = 1;         <<< ...no warning
   _5 = MEM[(long int *)_1 + -24B];
   _6 = (sizetype) _5;
   _7 = &d + _6;
   MEM[(struct B *)_7].b = 1;
   _8 = MEM[(long int *)_1 + -32B];
   _9 = (sizetype) _8;
   _10 = &d + _9;
   MEM[(struct A *)_10].a = 1;
   return;

}

> 
> The general issue about the confusion between complete and as-base types 
> is PR 22488; I have various thoughts about a proper fix, but there have 
> always been higher-priority things to work on.

Thanks for the reference!

> 
> One possible approach to this PR is that we don't need to check an 
> intermediate _REF; in
> 
> MEM[(struct D *)&D.2651 + 24B]._vptr.D
> 
> we aren't loading a whole D, we're only looking at the _vptr, which is 
> within the bounds of E.

This is also what the patch does: it keeps us from reaching
the MEM_REF because _vptr.D is artificial.

-Warray-bounds still of course checks other MEM_REFs in order to
detect bugs like:

struct A { int i, j; };

A* f ()
{
   A *p = (A*)new char[4];
   p->i = 1;
   p->j = 2;
   return p;
}

Martin

> 
> Jason
>

On 2/23/21 6:07 PM, Martin Sebor wrote:
> On 2/23/21 2:52 PM, Jason Merrill wrote:
>> On 2/23/21 11:02 AM, Martin Sebor wrote:
>>> [CC Jason for any further comments/clarification]
>>>
>>> On 2/9/21 10:49 AM, Martin Sebor wrote:
>>>> On 2/8/21 4:11 PM, Jeff Law wrote:
>>>>>
>>>>>
>>>>> On 2/8/21 3:44 PM, Martin Sebor wrote:
>>>>>> On 2/8/21 3:26 PM, Jeff Law wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>>>>>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>>>>>>> Similar to the problem reported for -Wstringop-overflow in 
>>>>>>>>>> pr98266
>>>>>>>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>>>>>>>> positives in assignments and copies involving virtual 
>>>>>>>>>> inheritance.
>>>>>>>>>> Because the two warnings don't share code yet (hopefully in 
>>>>>>>>>> GCC 12)
>>>>>>>>>> the attached patch adds its own workaround for this problem to
>>>>>>>>>> gimple-array-bounds.cc, this one slightly more crude because of
>>>>>>>>>> the limited insight the array bounds checking has into the 
>>>>>>>>>> checked
>>>>>>>>>> expressions.
>>>>>>>>>>
>>>>>>>>>> Tested on x86_64-linux.
>>>>>>>>>>
>>>>>>>>>> Martin
>>>>>>>>>>
>>>>>>>>>> gcc-98266.diff
>>>>>>>>>>
>>>>>>>>>> PR middle-end/98266 - bogus array subscript is partly outside 
>>>>>>>>>> array
>>>>>>>>>> bounds on virtual inheritance
>>>>>>>>>>
>>>>>>>>>> gcc/ChangeLog:
>>>>>>>>>>
>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>       * gimple-array-bounds.cc
>>>>>>>>>> (array_bounds_checker::check_array_bounds):
>>>>>>>>>>       Avoid checking references involving artificial members.
>>>>>>>>>>
>>>>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>>>>
>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>       * g++.dg/warn/Warray-bounds-15.C: New test.
>>>>>>>>> It seems to me that we've got the full statement at some point  
>>>>>>>>> and
>>>>>>>>> thus
>>>>>>>>> the full expression so at some point couldn't we detect when
>>>>>>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using 
>>>>>>>>> TYPE_SIZE_UNIT
>>>>>>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>>>>>>
>>>>>>>>> Am I missing something?
>>>>>>>>
>>>>>>>> The expression we're looking at when the false positive is issued
>>>>>>>> is the MEM_REF in the LHS of:
>>>>>>>>
>>>>>>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
>>>>>>>> *)&_ZTC1E24_1D + 24B];
>>>>>>>>
>>>>>>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>>>>>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>>>>>>> DECL_SIZE and TYPE_SIZE.
>>>>>>> So that seems like it's a different issue then, unrelated to 97595.
>>>>>>> Right?
>>>>>>
>>>>>> I think the underlying problem is the same.  We're getting a size
>>>>>> that doesn't correspond to what's actually being accessed, and it
>>>>>> happens because of the virtual inheritance.  In pr97595 Jason
>>>>>> suggested to use the decl/type size inequality to identify this
>>>>>> case but I think we could have just as well used DECL_ARTIFICIAL
>>>>>> instead.  At least the test cases from pr97595 both pass with
>>>>>> this change.
>>>>> But in the 98266 case the type and decl sizes are the same.  So to be
>>>>> true that would mean that the underlying type we're using to access
>>>>> memory differs from its actual type.  Is that the case in the IL?  And
>>>>> does this have wider implications for diagnostics or optimizations 
>>>>> that
>>>>> rely on accurate type sizing?
>>>>>
>>>>> I'm just trying to make sure I understand, not accepting or rejecting
>>>>> the patch yet.
>>>>
>>>> The part of the IL with the MEM_REF is this:
>>>>
>>>> void g ()
>>>> {
>>>>    void * D.2789;
>>>>    struct E D.2652;
>>>>
>>>>    <bb 2> [local count: 1073741824]:
>>>>    E::E (&D.2652, "");
>>>>    f (&D.2652);
>>>>
>>>>    <bb 3> [local count: 1073741824]:
>>>>    MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void 
>>>> *)&_ZTC1E24_1D + 24B];
>>>>    ...
>>>>
>>>> The access here is to the _vptr.D pointer member of D.2652 which is
>>>> just past the end of the parent object (as reflected by its SIZE):
>>>> it sets sets up the virtual table pointer.
>>>>
>>>> The access in pr97595 is to the member subobject, which, as Jason
>>>> explained (and I accordingly documented under DECL_SIZE in tree.h),
>>>> is also laid out separately from the parent object.
>>>>
>>>> These cases aren't exactly the same (which is also why the test
>>>> I added for -Warray-bounds in pr97595 didn't expose this bug) but
>>>> they are closely related.  The one here can be distinguished by
>>>> DECL_ARTIFICAL.  The other by the DECL_SIZE != TYPE_SIZE member
>>>> inequality.
>>>>
>>>> Might this impact other warnings?  I'd say so if they don't take
>>>> these things into account.  I just learned about this in pr97595
>>>> which was a -Wstringop-overflow false positive but I also saw
>>>> a similar instance of -Warray-bounds with my patch to improve
>>>> caching and enhance array bounds checking.  I dealt with that
>>>> instance of the warning in that patch but proactively added
>>>> a test case to the fix for pr97595.  But the test case is focused
>>>> on the subobject access and not on one to the virtual table so
>>>> (as I said above) it didn't expose this bug.
>>>>
>>>> Might this also impact optimizations?  I can imagine someone
>>>> unaware of this "gotcha" making the same "naive" assumption
>>>> I did, but I'd also expect such an invalid assumption to be
>>>> found either in code review or quickly cause problems in
>>>> testing.
>>>
>>> Jeff, does this answer your question?
>>
>> I don't see how the issue here depends on the artificiality of the 
>> vptr; I'd expect to see the same problem with a data member.  The 
>> problem is that a D base subobject is smaller than a complete D 
>> object, and in this case the base subobject is allocated such that if 
>> it were a full D object, it would overlap the end of E.  And we're 
>> checking the MEM_REF as though accessing a full D object, so we get a 
>> warning.
> 
> Thanks for chiming in!
> 
> There is no warning for accesses to data members defined in
> the virtual bases because their offset isn't constant.  For example,
> given the following modified version of the test case from the patch:
> 
> struct A
> {
>    virtual ~A ();
>    int a;
> };
> 
> struct B: virtual A { int b; };
> struct C: virtual A { int c; };
> struct D: virtual B, virtual C
> {
>    int d;
> };
> 
> D d;
> 
> void g ()
> {
>    D *p = &d;
>    p->a = p->b = p->c = p->d = 1;
> }
> 
> we end up with:
> 
> void g ()
> {
>    ...
>    <bb 2> [local count: 1073741824]:
>    d.d = 1;                           <<< okay
>    _1 = d._vptr.D;                    <<< DECL_ARTIFICIAL
>    _2 = MEM[(long int *)_1 + -40B];   <<< not checked
>    _3 = (sizetype) _2;                <<< not constant...
>    _4 = &d + _3;
>    MEM[(struct C *)_4].c = 1;         <<< ...no warning

Interesting.  I do get a warning if I embed the access in the destructor:

struct A
{
   virtual ~A ();
   int a;
};

struct B: virtual A { };
struct C: virtual A {
   int c;
   ~C() { c = 0; } // warns twice, for _vptr and c accesses
};
struct D: virtual B, virtual C
{
   D();
};

void g()
{
   D d;
}

>    _5 = MEM[(long int *)_1 + -24B];
>    _6 = (sizetype) _5;
>    _7 = &d + _6;
>    MEM[(struct B *)_7].b = 1;
>    _8 = MEM[(long int *)_1 + -32B];
>    _9 = (sizetype) _8;
>    _10 = &d + _9;
>    MEM[(struct A *)_10].a = 1;
>    return;
> 
> }
> 
>> The general issue about the confusion between complete and as-base 
>> types is PR 22488; I have various thoughts about a proper fix, but 
>> there have always been higher-priority things to work on.
> 
> Thanks for the reference!
> 
>> One possible approach to this PR is that we don't need to check an 
>> intermediate _REF; in
>>
>> MEM[(struct D *)&D.2651 + 24B]._vptr.D
>>
>> we aren't loading a whole D, we're only looking at the _vptr, which is 
>> within the bounds of E.
> 
> This is also what the patch does: it keeps us from reaching
> the MEM_REF because _vptr.D is artificial.

My point applies equally to the access to c in my testcase above: we 
shouldn't check lvalue refs as though we're loading or storing the whole 
aggregate object.

The full access is (*((D*)&e))._vptr.D.  The MEM_REF represents the *, 
which we shouldn't check by itself, because it doesn't imply a memory 
access of its own.  We should only check the COMPONENT_REF around it 
that is an actual memory access.

> -Warray-bounds still of course checks other MEM_REFs in order to
> detect bugs like:
> 
> struct A { int i, j; };
> 
> A* f ()
> {
>    A *p = (A*)new char[4];
>    p->i = 1;
>    p->j = 2;
>    return p;
> }

Of course.

On 2/8/21 2:56 PM, Martin Sebor wrote:
> On 2/8/21 12:59 PM, Jeff Law wrote:
>>
>>
>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>> Similar to the problem reported for -Wstringop-overflow in pr98266
>>> and already fixed, -Warray-bounds is also susceptible to false
>>> positives in assignments and copies involving virtual inheritance.
>>> Because the two warnings don't share code yet (hopefully in GCC 12)
>>> the attached patch adds its own workaround for this problem to
>>> gimple-array-bounds.cc, this one slightly more crude because of
>>> the limited insight the array bounds checking has into the checked
>>> expressions.
>>>
>>> Tested on x86_64-linux.
>>>
>>> Martin
>>>
>>> gcc-98266.diff
>>>
>>> PR middle-end/98266 - bogus array subscript is partly outside array
>>> bounds on virtual inheritance
>>>
>>> gcc/ChangeLog:
>>>
>>>     PR middle-end/98266
>>>     * gimple-array-bounds.cc
>>> (array_bounds_checker::check_array_bounds):
>>>     Avoid checking references involving artificial members.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>>     PR middle-end/98266
>>>     * g++.dg/warn/Warray-bounds-15.C: New test.
>> It seems to me that we've got the full statement at some point  and thus
>> the full expression so at some point couldn't we detect when
>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using TYPE_SIZE_UNIT
>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>
>> Am I missing something?
>
> The expression we're looking at when the false positive is issued
> is the MEM_REF in the LHS of:
>
> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
> *)&_ZTC1E24_1D + 24B];
>
> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
> DECL_SIZE and TYPE_SIZE.
So it's a related, but not necessary exactly the same as pr97595 where
we do have the discrepancy between DECL_SIZE and TYPE_SIZE.

I think Jason's comment in pr97595 is the key here.  For a base subject
with virtual bases the offset is bounded by the most-derived object size
-- which runs a bit counter to some of the concepts we rely on.

So I think that raises two key questions.

First, can we detect when we've got an access to a base subobject with
virtual bases.  I think that's what you're trying to do with the
DECL_ARTIFICIAL check.  I don't know offhand if that's the best way to
detect these kinds of accesses or not.  That's probably the most
important question here, particularly for gcc-11.

Second, if we can detect those kinds of accesses, do we have any way to
get the most derived object size?  If so, obviously we'd want to use it
(gcc-12 perhaps), if not, we'd punt like your patch does.

Jeff

On 2/8/21 3:44 PM, Martin Sebor wrote:
>
> I think the underlying problem is the same.  We're getting a size
> that doesn't correspond to what's actually being accessed, and it
> happens because of the virtual inheritance.  In pr97595 Jason
> suggested to use the decl/type size inequality to identify this
> case but I think we could have just as well used DECL_ARTIFICIAL
> instead.  At least the test cases from pr97595 both pass with
> this change.
OK.  I guess it's a minor question of semantics between pr97595 and
pr98266  being the same.  But I think we can put that behind us now.

I think the big question is whether or not DECL_ARTIFICIAL is the right
way to detect these cases, or if there simply isn't a way as one message
from Jason seems to imply.

Jeff

On 2/23/21 2:52 PM, Jason Merrill wrote:
>
> I don't see how the issue here depends on the artificiality of the vptr;
That's what I was trying to get at -- is DECL_ARTIFICIAL really a good
way to detect these kinds of cases.  It sounds like it likely isn't, but
it may also be the best we can do.  I'm not sure yet.




> I'd expect to see the same problem with a data member.  The problem is
> that a D base subobject is smaller than a complete D object, and in
> this case the base subobject is allocated such that if it were a full
> D object, it would overlap the end of E.  And we're checking the
> MEM_REF as though accessing a full D object, so we get a warning.
>
> The general issue about the confusion between complete and as-base
> types is PR 22488; I have various thoughts about a proper fix, but
> there have always been higher-priority things to work on.
>
> One possible approach to this PR is that we don't need to check an
> intermediate _REF; in
>
> MEM[(struct D *)&D.2651 + 24B]._vptr.D
>
> we aren't loading a whole D, we're only looking at the _vptr, which is
> within the bounds of E.
That raises a really interesting question.  What's the right bounds to
use with these kinds of constructs.  Should we be looking at D's type
(struct E) or struct D?  I'm not sure there's a one-size-fits-all answer
here and I don't think we have any other context go help guide us.

Jeff

On 2/25/21 6:47 PM, Jeff Law wrote:
> 
> 
> On 2/23/21 2:52 PM, Jason Merrill wrote:
>>
>> I don't see how the issue here depends on the artificiality of the vptr;
> That's what I was trying to get at -- is DECL_ARTIFICIAL really a good
> way to detect these kinds of cases.  It sounds like it likely isn't, but
> it may also be the best we can do.  I'm not sure yet.
> 
> 
> 
> 
>> I'd expect to see the same problem with a data member.  The problem is
>> that a D base subobject is smaller than a complete D object, and in
>> this case the base subobject is allocated such that if it were a full
>> D object, it would overlap the end of E.  And we're checking the
>> MEM_REF as though accessing a full D object, so we get a warning.
>>
>> The general issue about the confusion between complete and as-base
>> types is PR 22488; I have various thoughts about a proper fix, but
>> there have always been higher-priority things to work on.
>>
>> One possible approach to this PR is that we don't need to check an
>> intermediate _REF; in
>>
>> MEM[(struct D *)&D.2651 + 24B]._vptr.D
>>
>> we aren't loading a whole D, we're only looking at the _vptr, which is
>> within the bounds of E.

> That raises a really interesting question.  What's the right bounds to
> use with these kinds of constructs.  Should we be looking at D's type
> (struct E) or struct D?  I'm not sure there's a one-size-fits-all answer
> here and I don't think we have any other context go help guide us.

struct E, definitely; the actual bounds of the struct D object are 
unknown here.  Unless, I suppose, we were to work harder to find the D 
FIELD_DECL in E that corresponds to this offset and get its DECL_SIZE, but:

We know the bounds of the E object, and we can determine that _vptr.D is 
within those bounds; that seems sufficient to decide not to warn.

Jason

On 2/24/21 5:35 PM, Jason Merrill wrote:
> On 2/23/21 6:07 PM, Martin Sebor wrote:
>> On 2/23/21 2:52 PM, Jason Merrill wrote:
>>> On 2/23/21 11:02 AM, Martin Sebor wrote:
>>>> [CC Jason for any further comments/clarification]
>>>>
>>>> On 2/9/21 10:49 AM, Martin Sebor wrote:
>>>>> On 2/8/21 4:11 PM, Jeff Law wrote:
>>>>>>
>>>>>>
>>>>>> On 2/8/21 3:44 PM, Martin Sebor wrote:
>>>>>>> On 2/8/21 3:26 PM, Jeff Law wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>>>>>>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>>>>>>>> Similar to the problem reported for -Wstringop-overflow in 
>>>>>>>>>>> pr98266
>>>>>>>>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>>>>>>>>> positives in assignments and copies involving virtual 
>>>>>>>>>>> inheritance.
>>>>>>>>>>> Because the two warnings don't share code yet (hopefully in 
>>>>>>>>>>> GCC 12)
>>>>>>>>>>> the attached patch adds its own workaround for this problem to
>>>>>>>>>>> gimple-array-bounds.cc, this one slightly more crude because of
>>>>>>>>>>> the limited insight the array bounds checking has into the 
>>>>>>>>>>> checked
>>>>>>>>>>> expressions.
>>>>>>>>>>>
>>>>>>>>>>> Tested on x86_64-linux.
>>>>>>>>>>>
>>>>>>>>>>> Martin
>>>>>>>>>>>
>>>>>>>>>>> gcc-98266.diff
>>>>>>>>>>>
>>>>>>>>>>> PR middle-end/98266 - bogus array subscript is partly outside 
>>>>>>>>>>> array
>>>>>>>>>>> bounds on virtual inheritance
>>>>>>>>>>>
>>>>>>>>>>> gcc/ChangeLog:
>>>>>>>>>>>
>>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>>       * gimple-array-bounds.cc
>>>>>>>>>>> (array_bounds_checker::check_array_bounds):
>>>>>>>>>>>       Avoid checking references involving artificial members.
>>>>>>>>>>>
>>>>>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>>>>>
>>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>>       * g++.dg/warn/Warray-bounds-15.C: New test.
>>>>>>>>>> It seems to me that we've got the full statement at some point 
>>>>>>>>>> and
>>>>>>>>>> thus
>>>>>>>>>> the full expression so at some point couldn't we detect when
>>>>>>>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using 
>>>>>>>>>> TYPE_SIZE_UNIT
>>>>>>>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>>>>>>>
>>>>>>>>>> Am I missing something?
>>>>>>>>>
>>>>>>>>> The expression we're looking at when the false positive is issued
>>>>>>>>> is the MEM_REF in the LHS of:
>>>>>>>>>
>>>>>>>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
>>>>>>>>> *)&_ZTC1E24_1D + 24B];
>>>>>>>>>
>>>>>>>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>>>>>>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>>>>>>>> DECL_SIZE and TYPE_SIZE.
>>>>>>>> So that seems like it's a different issue then, unrelated to 97595.
>>>>>>>> Right?
>>>>>>>
>>>>>>> I think the underlying problem is the same.  We're getting a size
>>>>>>> that doesn't correspond to what's actually being accessed, and it
>>>>>>> happens because of the virtual inheritance.  In pr97595 Jason
>>>>>>> suggested to use the decl/type size inequality to identify this
>>>>>>> case but I think we could have just as well used DECL_ARTIFICIAL
>>>>>>> instead.  At least the test cases from pr97595 both pass with
>>>>>>> this change.
>>>>>> But in the 98266 case the type and decl sizes are the same.  So to be
>>>>>> true that would mean that the underlying type we're using to access
>>>>>> memory differs from its actual type.  Is that the case in the IL?  
>>>>>> And
>>>>>> does this have wider implications for diagnostics or optimizations 
>>>>>> that
>>>>>> rely on accurate type sizing?
>>>>>>
>>>>>> I'm just trying to make sure I understand, not accepting or rejecting
>>>>>> the patch yet.
>>>>>
>>>>> The part of the IL with the MEM_REF is this:
>>>>>
>>>>> void g ()
>>>>> {
>>>>>    void * D.2789;
>>>>>    struct E D.2652;
>>>>>
>>>>>    <bb 2> [local count: 1073741824]:
>>>>>    E::E (&D.2652, "");
>>>>>    f (&D.2652);
>>>>>
>>>>>    <bb 3> [local count: 1073741824]:
>>>>>    MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> 
>>>>> [(void *)&_ZTC1E24_1D + 24B];
>>>>>    ...
>>>>>
>>>>> The access here is to the _vptr.D pointer member of D.2652 which is
>>>>> just past the end of the parent object (as reflected by its SIZE):
>>>>> it sets sets up the virtual table pointer.
>>>>>
>>>>> The access in pr97595 is to the member subobject, which, as Jason
>>>>> explained (and I accordingly documented under DECL_SIZE in tree.h),
>>>>> is also laid out separately from the parent object.
>>>>>
>>>>> These cases aren't exactly the same (which is also why the test
>>>>> I added for -Warray-bounds in pr97595 didn't expose this bug) but
>>>>> they are closely related.  The one here can be distinguished by
>>>>> DECL_ARTIFICAL.  The other by the DECL_SIZE != TYPE_SIZE member
>>>>> inequality.
>>>>>
>>>>> Might this impact other warnings?  I'd say so if they don't take
>>>>> these things into account.  I just learned about this in pr97595
>>>>> which was a -Wstringop-overflow false positive but I also saw
>>>>> a similar instance of -Warray-bounds with my patch to improve
>>>>> caching and enhance array bounds checking.  I dealt with that
>>>>> instance of the warning in that patch but proactively added
>>>>> a test case to the fix for pr97595.  But the test case is focused
>>>>> on the subobject access and not on one to the virtual table so
>>>>> (as I said above) it didn't expose this bug.
>>>>>
>>>>> Might this also impact optimizations?  I can imagine someone
>>>>> unaware of this "gotcha" making the same "naive" assumption
>>>>> I did, but I'd also expect such an invalid assumption to be
>>>>> found either in code review or quickly cause problems in
>>>>> testing.
>>>>
>>>> Jeff, does this answer your question?
>>>
>>> I don't see how the issue here depends on the artificiality of the 
>>> vptr; I'd expect to see the same problem with a data member.  The 
>>> problem is that a D base subobject is smaller than a complete D 
>>> object, and in this case the base subobject is allocated such that if 
>>> it were a full D object, it would overlap the end of E.  And we're 
>>> checking the MEM_REF as though accessing a full D object, so we get a 
>>> warning.
>>
>> Thanks for chiming in!
>>
>> There is no warning for accesses to data members defined in
>> the virtual bases because their offset isn't constant.  For example,
>> given the following modified version of the test case from the patch:
>>
>> struct A
>> {
>>    virtual ~A ();
>>    int a;
>> };
>>
>> struct B: virtual A { int b; };
>> struct C: virtual A { int c; };
>> struct D: virtual B, virtual C
>> {
>>    int d;
>> };
>>
>> D d;
>>
>> void g ()
>> {
>>    D *p = &d;
>>    p->a = p->b = p->c = p->d = 1;
>> }
>>
>> we end up with:
>>
>> void g ()
>> {
>>    ...
>>    <bb 2> [local count: 1073741824]:
>>    d.d = 1;                           <<< okay
>>    _1 = d._vptr.D;                    <<< DECL_ARTIFICIAL
>>    _2 = MEM[(long int *)_1 + -40B];   <<< not checked
>>    _3 = (sizetype) _2;                <<< not constant...
>>    _4 = &d + _3;
>>    MEM[(struct C *)_4].c = 1;         <<< ...no warning
> 
> Interesting.  I do get a warning if I embed the access in the destructor:
> 
> struct A
> {
>    virtual ~A ();
>    int a;
> };
> 
> struct B: virtual A { };
> struct C: virtual A {
>    int c;
>    ~C() { c = 0; } // warns twice, for _vptr and c accesses
> };
> struct D: virtual B, virtual C
> {
>    D();
> };
> 
> void g()
> {
>    D d;
> }

Ah, right.  Unlike in member functions, in a dtor (or in the ctors
of the derived class) the offsets are known/constant so accesses
involving those are checked and we do get a false positive.  Thanks
for the test case!

> 
>>    _5 = MEM[(long int *)_1 + -24B];
>>    _6 = (sizetype) _5;
>>    _7 = &d + _6;
>>    MEM[(struct B *)_7].b = 1;
>>    _8 = MEM[(long int *)_1 + -32B];
>>    _9 = (sizetype) _8;
>>    _10 = &d + _9;
>>    MEM[(struct A *)_10].a = 1;
>>    return;
>>
>> }
>>
>>> The general issue about the confusion between complete and as-base 
>>> types is PR 22488; I have various thoughts about a proper fix, but 
>>> there have always been higher-priority things to work on.
>>
>> Thanks for the reference!
>>
>>> One possible approach to this PR is that we don't need to check an 
>>> intermediate _REF; in
>>>
>>> MEM[(struct D *)&D.2651 + 24B]._vptr.D
>>>
>>> we aren't loading a whole D, we're only looking at the _vptr, which 
>>> is within the bounds of E.
>>
>> This is also what the patch does: it keeps us from reaching
>> the MEM_REF because _vptr.D is artificial.
> 
> My point applies equally to the access to c in my testcase above: we 
> shouldn't check lvalue refs as though we're loading or storing the whole 
> aggregate object.
> 
> The full access is (*((D*)&e))._vptr.D.  The MEM_REF represents the *, 
> which we shouldn't check by itself, because it doesn't imply a memory 
> access of its own.  We should only check the COMPONENT_REF around it 
> that is an actual memory access.

The -Warray-bounds implementation checks every ARRAY_REF and MEM_REF,
regardless of their context(*). It doesn't check COMPONENT_REF except
while checking the other REFs (while walking up the use-def chains),
and the checks don't know the context of the REF.  The hack this patch
put in was to circumvent that when the walk saw a COMPONENT_REF to
an artificial member (which as you showed isn't enough).

I'm not sure if what you suggest (IIUC) is feasible without reworking
how the checking works in general.  What is feasible is working harder
and detecting if a) the type of the accessed object has virtual bases
and b) if the offset of the accessed field is within the bounds of
the enclosing object.  In that case, skipping the MEM_REF check avoids
the warnings for all the cases we have discussed (artificial members,
ctors and dtors).  It still seems hacky but sufficiently narrow in
scope for GCC 11(**).

Please see the attached revision.

Martin

[*] The warning uses walk_gimple_op whose documentation suggests
is the wrong API to use for code in an SSA form.  SSA form code is
supposed to use the standard gimple operand APIs.  I think those
would make it easier consider the context.

[**] In the future I'd like to restructure the whole checker to avoid
this walk altogether (besides also using Ranger and the pointer_query
class).

> 
>> -Warray-bounds still of course checks other MEM_REFs in order to
>> detect bugs like:
>>
>> struct A { int i, j; };
>>
>> A* f ()
>> {
>>    A *p = (A*)new char[4];
>>    p->i = 1;
>>    p->j = 2;
>>    return p;
>> }
> 
> Of course.
>

On 2/25/21 4:40 PM, Jeff Law wrote:
> 
> 
> On 2/8/21 3:44 PM, Martin Sebor wrote:
>>
>> I think the underlying problem is the same.  We're getting a size
>> that doesn't correspond to what's actually being accessed, and it
>> happens because of the virtual inheritance.  In pr97595 Jason
>> suggested to use the decl/type size inequality to identify this
>> case but I think we could have just as well used DECL_ARTIFICIAL
>> instead.  At least the test cases from pr97595 both pass with
>> this change.
> OK.  I guess it's a minor question of semantics between pr97595 and
> pr98266  being the same.  But I think we can put that behind us now.
> 
> I think the big question is whether or not DECL_ARTIFICIAL is the right
> way to detect these cases, or if there simply isn't a way as one message
> from Jason seems to imply.

DECL_ARTIFICIAL happens to work for the vtbl case but it doesn't work
for other accesses to members of virtual bases  that don't involve
artificial members but where the offset is constant.  I didn't think
those came up but as Jason showed, they do: in accesses in destructors
(and also in some in constructors).  So the DECL_ARTIFICIAL test isn't
the right way to detect all those cases.

I have posted an updated patch that handles those cases as well as
direct vtbl accesses by detecting the COMPONENT_REF (MEM_REF (&d), m))
access to member m of object d with virtual bases, that doesn't depend
on DECL_ARTIFICIAL.

Martin

On 3/1/21 6:11 PM, Martin Sebor wrote:
> On 2/24/21 5:35 PM, Jason Merrill wrote:
>> On 2/23/21 6:07 PM, Martin Sebor wrote:
>>> On 2/23/21 2:52 PM, Jason Merrill wrote:
>>>> On 2/23/21 11:02 AM, Martin Sebor wrote:
>>>>> [CC Jason for any further comments/clarification]
>>>>>
>>>>> On 2/9/21 10:49 AM, Martin Sebor wrote:
>>>>>> On 2/8/21 4:11 PM, Jeff Law wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 2/8/21 3:44 PM, Martin Sebor wrote:
>>>>>>>> On 2/8/21 3:26 PM, Jeff Law wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>>>>>>>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>>>>>>>>> Similar to the problem reported for -Wstringop-overflow in 
>>>>>>>>>>>> pr98266
>>>>>>>>>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>>>>>>>>>> positives in assignments and copies involving virtual 
>>>>>>>>>>>> inheritance.
>>>>>>>>>>>> Because the two warnings don't share code yet (hopefully in 
>>>>>>>>>>>> GCC 12)
>>>>>>>>>>>> the attached patch adds its own workaround for this problem to
>>>>>>>>>>>> gimple-array-bounds.cc, this one slightly more crude because of
>>>>>>>>>>>> the limited insight the array bounds checking has into the 
>>>>>>>>>>>> checked
>>>>>>>>>>>> expressions.
>>>>>>>>>>>>
>>>>>>>>>>>> Tested on x86_64-linux.
>>>>>>>>>>>>
>>>>>>>>>>>> Martin
>>>>>>>>>>>>
>>>>>>>>>>>> gcc-98266.diff
>>>>>>>>>>>>
>>>>>>>>>>>> PR middle-end/98266 - bogus array subscript is partly 
>>>>>>>>>>>> outside array
>>>>>>>>>>>> bounds on virtual inheritance
>>>>>>>>>>>>
>>>>>>>>>>>> gcc/ChangeLog:
>>>>>>>>>>>>
>>>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>>>       * gimple-array-bounds.cc
>>>>>>>>>>>> (array_bounds_checker::check_array_bounds):
>>>>>>>>>>>>       Avoid checking references involving artificial members.
>>>>>>>>>>>>
>>>>>>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>>>>>>
>>>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>>>       * g++.dg/warn/Warray-bounds-15.C: New test.
>>>>>>>>>>> It seems to me that we've got the full statement at some 
>>>>>>>>>>> point and
>>>>>>>>>>> thus
>>>>>>>>>>> the full expression so at some point couldn't we detect when
>>>>>>>>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using 
>>>>>>>>>>> TYPE_SIZE_UNIT
>>>>>>>>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>>>>>>>>
>>>>>>>>>>> Am I missing something?
>>>>>>>>>>
>>>>>>>>>> The expression we're looking at when the false positive is issued
>>>>>>>>>> is the MEM_REF in the LHS of:
>>>>>>>>>>
>>>>>>>>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> [(void
>>>>>>>>>> *)&_ZTC1E24_1D + 24B];
>>>>>>>>>>
>>>>>>>>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>>>>>>>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>>>>>>>>> DECL_SIZE and TYPE_SIZE.
>>>>>>>>> So that seems like it's a different issue then, unrelated to 
>>>>>>>>> 97595.
>>>>>>>>> Right?
>>>>>>>>
>>>>>>>> I think the underlying problem is the same.  We're getting a size
>>>>>>>> that doesn't correspond to what's actually being accessed, and it
>>>>>>>> happens because of the virtual inheritance.  In pr97595 Jason
>>>>>>>> suggested to use the decl/type size inequality to identify this
>>>>>>>> case but I think we could have just as well used DECL_ARTIFICIAL
>>>>>>>> instead.  At least the test cases from pr97595 both pass with
>>>>>>>> this change.
>>>>>>> But in the 98266 case the type and decl sizes are the same.  So 
>>>>>>> to be
>>>>>>> true that would mean that the underlying type we're using to access
>>>>>>> memory differs from its actual type.  Is that the case in the IL? 
>>>>>>> And
>>>>>>> does this have wider implications for diagnostics or 
>>>>>>> optimizations that
>>>>>>> rely on accurate type sizing?
>>>>>>>
>>>>>>> I'm just trying to make sure I understand, not accepting or 
>>>>>>> rejecting
>>>>>>> the patch yet.
>>>>>>
>>>>>> The part of the IL with the MEM_REF is this:
>>>>>>
>>>>>> void g ()
>>>>>> {
>>>>>>    void * D.2789;
>>>>>>    struct E D.2652;
>>>>>>
>>>>>>    <bb 2> [local count: 1073741824]:
>>>>>>    E::E (&D.2652, "");
>>>>>>    f (&D.2652);
>>>>>>
>>>>>>    <bb 3> [local count: 1073741824]:
>>>>>>    MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> 
>>>>>> [(void *)&_ZTC1E24_1D + 24B];
>>>>>>    ...
>>>>>>
>>>>>> The access here is to the _vptr.D pointer member of D.2652 which is
>>>>>> just past the end of the parent object (as reflected by its SIZE):
>>>>>> it sets sets up the virtual table pointer.
>>>>>>
>>>>>> The access in pr97595 is to the member subobject, which, as Jason
>>>>>> explained (and I accordingly documented under DECL_SIZE in tree.h),
>>>>>> is also laid out separately from the parent object.
>>>>>>
>>>>>> These cases aren't exactly the same (which is also why the test
>>>>>> I added for -Warray-bounds in pr97595 didn't expose this bug) but
>>>>>> they are closely related.  The one here can be distinguished by
>>>>>> DECL_ARTIFICAL.  The other by the DECL_SIZE != TYPE_SIZE member
>>>>>> inequality.
>>>>>>
>>>>>> Might this impact other warnings?  I'd say so if they don't take
>>>>>> these things into account.  I just learned about this in pr97595
>>>>>> which was a -Wstringop-overflow false positive but I also saw
>>>>>> a similar instance of -Warray-bounds with my patch to improve
>>>>>> caching and enhance array bounds checking.  I dealt with that
>>>>>> instance of the warning in that patch but proactively added
>>>>>> a test case to the fix for pr97595.  But the test case is focused
>>>>>> on the subobject access and not on one to the virtual table so
>>>>>> (as I said above) it didn't expose this bug.
>>>>>>
>>>>>> Might this also impact optimizations?  I can imagine someone
>>>>>> unaware of this "gotcha" making the same "naive" assumption
>>>>>> I did, but I'd also expect such an invalid assumption to be
>>>>>> found either in code review or quickly cause problems in
>>>>>> testing.
>>>>>
>>>>> Jeff, does this answer your question?
>>>>
>>>> I don't see how the issue here depends on the artificiality of the 
>>>> vptr; I'd expect to see the same problem with a data member.  The 
>>>> problem is that a D base subobject is smaller than a complete D 
>>>> object, and in this case the base subobject is allocated such that 
>>>> if it were a full D object, it would overlap the end of E.  And 
>>>> we're checking the MEM_REF as though accessing a full D object, so 
>>>> we get a warning.
>>>
>>> Thanks for chiming in!
>>>
>>> There is no warning for accesses to data members defined in
>>> the virtual bases because their offset isn't constant.  For example,
>>> given the following modified version of the test case from the patch:
>>>
>>> struct A
>>> {
>>>    virtual ~A ();
>>>    int a;
>>> };
>>>
>>> struct B: virtual A { int b; };
>>> struct C: virtual A { int c; };
>>> struct D: virtual B, virtual C
>>> {
>>>    int d;
>>> };
>>>
>>> D d;
>>>
>>> void g ()
>>> {
>>>    D *p = &d;
>>>    p->a = p->b = p->c = p->d = 1;
>>> }
>>>
>>> we end up with:
>>>
>>> void g ()
>>> {
>>>    ...
>>>    <bb 2> [local count: 1073741824]:
>>>    d.d = 1;                           <<< okay
>>>    _1 = d._vptr.D;                    <<< DECL_ARTIFICIAL
>>>    _2 = MEM[(long int *)_1 + -40B];   <<< not checked
>>>    _3 = (sizetype) _2;                <<< not constant...
>>>    _4 = &d + _3;
>>>    MEM[(struct C *)_4].c = 1;         <<< ...no warning
>>
>> Interesting.  I do get a warning if I embed the access in the destructor:
>>
>> struct A
>> {
>>    virtual ~A ();
>>    int a;
>> };
>>
>> struct B: virtual A { };
>> struct C: virtual A {
>>    int c;
>>    ~C() { c = 0; } // warns twice, for _vptr and c accesses
>> };
>> struct D: virtual B, virtual C
>> {
>>    D();
>> };
>>
>> void g()
>> {
>>    D d;
>> }
> 
> Ah, right.  Unlike in member functions, in a dtor (or in the ctors
> of the derived class) the offsets are known/constant so accesses
> involving those are checked and we do get a false positive.  Thanks
> for the test case!
> 
>>
>>>    _5 = MEM[(long int *)_1 + -24B];
>>>    _6 = (sizetype) _5;
>>>    _7 = &d + _6;
>>>    MEM[(struct B *)_7].b = 1;
>>>    _8 = MEM[(long int *)_1 + -32B];
>>>    _9 = (sizetype) _8;
>>>    _10 = &d + _9;
>>>    MEM[(struct A *)_10].a = 1;
>>>    return;
>>>
>>> }
>>>
>>>> The general issue about the confusion between complete and as-base 
>>>> types is PR 22488; I have various thoughts about a proper fix, but 
>>>> there have always been higher-priority things to work on.
>>>
>>> Thanks for the reference!
>>>
>>>> One possible approach to this PR is that we don't need to check an 
>>>> intermediate _REF; in
>>>>
>>>> MEM[(struct D *)&D.2651 + 24B]._vptr.D
>>>>
>>>> we aren't loading a whole D, we're only looking at the _vptr, which 
>>>> is within the bounds of E.
>>>
>>> This is also what the patch does: it keeps us from reaching
>>> the MEM_REF because _vptr.D is artificial.
>>
>> My point applies equally to the access to c in my testcase above: we 
>> shouldn't check lvalue refs as though we're loading or storing the 
>> whole aggregate object.
>>
>> The full access is (*((D*)&e))._vptr.D.  The MEM_REF represents the *, 
>> which we shouldn't check by itself, because it doesn't imply a memory 
>> access of its own.  We should only check the COMPONENT_REF around it 
>> that is an actual memory access.
> 
> The -Warray-bounds implementation checks every ARRAY_REF and MEM_REF,
> regardless of their context(*).

Aha.  That seems like a flaw, but as you say, not one that can be 
addressed easily.

> It doesn't check COMPONENT_REF except
> while checking the other REFs (while walking up the use-def chains),
> and the checks don't know the context of the REF.  The hack this patch
> put in was to circumvent that when the walk saw a COMPONENT_REF to
> an artificial member (which as you showed isn't enough).
> 
> I'm not sure if what you suggest (IIUC) is feasible without reworking
> how the checking works in general.

> What is feasible is working harder
> and detecting if a) the type of the accessed object has virtual bases
> and b) if the offset of the accessed field is within the bounds of
> the enclosing object.  In that case, skipping the MEM_REF check avoids
> the warnings for all the cases we have discussed (artificial members,
> ctors and dtors).  It still seems hacky but sufficiently narrow in
> scope for GCC 11(**).
> 
> Please see the attached revision.

> +  else if (TREE_CODE (t) == COMPONENT_REF
> +	   && TREE_CODE (TREE_OPERAND (t, 0)) == MEM_REF)
> +    {
> +      /* Hack: Skip MEM_REF checks in accesses to a member of a type
> +	 with virtual bases at an offset that's in bounds of the size
> +	 of the enclosing object.  See pr98266 and pr97595.  */
> +      tree ref = TREE_OPERAND (TREE_OPERAND (t, 0), 0);
> +      if (TREE_CODE (ref) == ADDR_EXPR)
> +	ref = TREE_OPERAND (ref, 0);
> +      tree reftype = TREE_TYPE (ref);
> +      if (DECL_P (ref)
> +	  && RECORD_OR_UNION_TYPE_P (reftype)
> +	  && TYPE_BINFO (reftype))
> +	{
> +	  tree fld = TREE_OPERAND (t, 1);
> +	  tree fldpos = byte_position (fld);
> +	  if (tree refsize = DECL_SIZE_UNIT (ref))
> +	    if (TREE_CODE (refsize) == INTEGER_CST
> +		&& tree_int_cst_lt (fldpos, refsize))
> +	      *walk_subtree = false;
> +	}
> +    }

Here fld F is a member of a base class B, and ref is a variable of a 
derived class D, so you're comparing the offset of F in B to the size of 
D without considering the offset of B in D.  I think you just need to 
add the MEM_REF offset to fldpos before comparing.

Jason

On 3/2/21 7:11 AM, Jason Merrill wrote:
> On 3/1/21 6:11 PM, Martin Sebor wrote:
>> On 2/24/21 5:35 PM, Jason Merrill wrote:
>>> On 2/23/21 6:07 PM, Martin Sebor wrote:
>>>> On 2/23/21 2:52 PM, Jason Merrill wrote:
>>>>> On 2/23/21 11:02 AM, Martin Sebor wrote:
>>>>>> [CC Jason for any further comments/clarification]
>>>>>>
>>>>>> On 2/9/21 10:49 AM, Martin Sebor wrote:
>>>>>>> On 2/8/21 4:11 PM, Jeff Law wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 2/8/21 3:44 PM, Martin Sebor wrote:
>>>>>>>>> On 2/8/21 3:26 PM, Jeff Law wrote:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>>>>>>>>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>>>>>>>>>> Similar to the problem reported for -Wstringop-overflow in 
>>>>>>>>>>>>> pr98266
>>>>>>>>>>>>> and already fixed, -Warray-bounds is also susceptible to false
>>>>>>>>>>>>> positives in assignments and copies involving virtual 
>>>>>>>>>>>>> inheritance.
>>>>>>>>>>>>> Because the two warnings don't share code yet (hopefully in 
>>>>>>>>>>>>> GCC 12)
>>>>>>>>>>>>> the attached patch adds its own workaround for this problem to
>>>>>>>>>>>>> gimple-array-bounds.cc, this one slightly more crude 
>>>>>>>>>>>>> because of
>>>>>>>>>>>>> the limited insight the array bounds checking has into the 
>>>>>>>>>>>>> checked
>>>>>>>>>>>>> expressions.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Tested on x86_64-linux.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Martin
>>>>>>>>>>>>>
>>>>>>>>>>>>> gcc-98266.diff
>>>>>>>>>>>>>
>>>>>>>>>>>>> PR middle-end/98266 - bogus array subscript is partly 
>>>>>>>>>>>>> outside array
>>>>>>>>>>>>> bounds on virtual inheritance
>>>>>>>>>>>>>
>>>>>>>>>>>>> gcc/ChangeLog:
>>>>>>>>>>>>>
>>>>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>>>>       * gimple-array-bounds.cc
>>>>>>>>>>>>> (array_bounds_checker::check_array_bounds):
>>>>>>>>>>>>>       Avoid checking references involving artificial members.
>>>>>>>>>>>>>
>>>>>>>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>>>>>>>
>>>>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>>>>       * g++.dg/warn/Warray-bounds-15.C: New test.
>>>>>>>>>>>> It seems to me that we've got the full statement at some 
>>>>>>>>>>>> point and
>>>>>>>>>>>> thus
>>>>>>>>>>>> the full expression so at some point couldn't we detect when
>>>>>>>>>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using 
>>>>>>>>>>>> TYPE_SIZE_UNIT
>>>>>>>>>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>>>>>>>>>
>>>>>>>>>>>> Am I missing something?
>>>>>>>>>>>
>>>>>>>>>>> The expression we're looking at when the false positive is 
>>>>>>>>>>> issued
>>>>>>>>>>> is the MEM_REF in the LHS of:
>>>>>>>>>>>
>>>>>>>>>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> 
>>>>>>>>>>> [(void
>>>>>>>>>>> *)&_ZTC1E24_1D + 24B];
>>>>>>>>>>>
>>>>>>>>>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>>>>>>>>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>>>>>>>>>> DECL_SIZE and TYPE_SIZE.
>>>>>>>>>> So that seems like it's a different issue then, unrelated to 
>>>>>>>>>> 97595.
>>>>>>>>>> Right?
>>>>>>>>>
>>>>>>>>> I think the underlying problem is the same.  We're getting a size
>>>>>>>>> that doesn't correspond to what's actually being accessed, and it
>>>>>>>>> happens because of the virtual inheritance.  In pr97595 Jason
>>>>>>>>> suggested to use the decl/type size inequality to identify this
>>>>>>>>> case but I think we could have just as well used DECL_ARTIFICIAL
>>>>>>>>> instead.  At least the test cases from pr97595 both pass with
>>>>>>>>> this change.
>>>>>>>> But in the 98266 case the type and decl sizes are the same.  So 
>>>>>>>> to be
>>>>>>>> true that would mean that the underlying type we're using to access
>>>>>>>> memory differs from its actual type.  Is that the case in the 
>>>>>>>> IL? And
>>>>>>>> does this have wider implications for diagnostics or 
>>>>>>>> optimizations that
>>>>>>>> rely on accurate type sizing?
>>>>>>>>
>>>>>>>> I'm just trying to make sure I understand, not accepting or 
>>>>>>>> rejecting
>>>>>>>> the patch yet.
>>>>>>>
>>>>>>> The part of the IL with the MEM_REF is this:
>>>>>>>
>>>>>>> void g ()
>>>>>>> {
>>>>>>>    void * D.2789;
>>>>>>>    struct E D.2652;
>>>>>>>
>>>>>>>    <bb 2> [local count: 1073741824]:
>>>>>>>    E::E (&D.2652, "");
>>>>>>>    f (&D.2652);
>>>>>>>
>>>>>>>    <bb 3> [local count: 1073741824]:
>>>>>>>    MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> 
>>>>>>> [(void *)&_ZTC1E24_1D + 24B];
>>>>>>>    ...
>>>>>>>
>>>>>>> The access here is to the _vptr.D pointer member of D.2652 which is
>>>>>>> just past the end of the parent object (as reflected by its SIZE):
>>>>>>> it sets sets up the virtual table pointer.
>>>>>>>
>>>>>>> The access in pr97595 is to the member subobject, which, as Jason
>>>>>>> explained (and I accordingly documented under DECL_SIZE in tree.h),
>>>>>>> is also laid out separately from the parent object.
>>>>>>>
>>>>>>> These cases aren't exactly the same (which is also why the test
>>>>>>> I added for -Warray-bounds in pr97595 didn't expose this bug) but
>>>>>>> they are closely related.  The one here can be distinguished by
>>>>>>> DECL_ARTIFICAL.  The other by the DECL_SIZE != TYPE_SIZE member
>>>>>>> inequality.
>>>>>>>
>>>>>>> Might this impact other warnings?  I'd say so if they don't take
>>>>>>> these things into account.  I just learned about this in pr97595
>>>>>>> which was a -Wstringop-overflow false positive but I also saw
>>>>>>> a similar instance of -Warray-bounds with my patch to improve
>>>>>>> caching and enhance array bounds checking.  I dealt with that
>>>>>>> instance of the warning in that patch but proactively added
>>>>>>> a test case to the fix for pr97595.  But the test case is focused
>>>>>>> on the subobject access and not on one to the virtual table so
>>>>>>> (as I said above) it didn't expose this bug.
>>>>>>>
>>>>>>> Might this also impact optimizations?  I can imagine someone
>>>>>>> unaware of this "gotcha" making the same "naive" assumption
>>>>>>> I did, but I'd also expect such an invalid assumption to be
>>>>>>> found either in code review or quickly cause problems in
>>>>>>> testing.
>>>>>>
>>>>>> Jeff, does this answer your question?
>>>>>
>>>>> I don't see how the issue here depends on the artificiality of the 
>>>>> vptr; I'd expect to see the same problem with a data member.  The 
>>>>> problem is that a D base subobject is smaller than a complete D 
>>>>> object, and in this case the base subobject is allocated such that 
>>>>> if it were a full D object, it would overlap the end of E.  And 
>>>>> we're checking the MEM_REF as though accessing a full D object, so 
>>>>> we get a warning.
>>>>
>>>> Thanks for chiming in!
>>>>
>>>> There is no warning for accesses to data members defined in
>>>> the virtual bases because their offset isn't constant.  For example,
>>>> given the following modified version of the test case from the patch:
>>>>
>>>> struct A
>>>> {
>>>>    virtual ~A ();
>>>>    int a;
>>>> };
>>>>
>>>> struct B: virtual A { int b; };
>>>> struct C: virtual A { int c; };
>>>> struct D: virtual B, virtual C
>>>> {
>>>>    int d;
>>>> };
>>>>
>>>> D d;
>>>>
>>>> void g ()
>>>> {
>>>>    D *p = &d;
>>>>    p->a = p->b = p->c = p->d = 1;
>>>> }
>>>>
>>>> we end up with:
>>>>
>>>> void g ()
>>>> {
>>>>    ...
>>>>    <bb 2> [local count: 1073741824]:
>>>>    d.d = 1;                           <<< okay
>>>>    _1 = d._vptr.D;                    <<< DECL_ARTIFICIAL
>>>>    _2 = MEM[(long int *)_1 + -40B];   <<< not checked
>>>>    _3 = (sizetype) _2;                <<< not constant...
>>>>    _4 = &d + _3;
>>>>    MEM[(struct C *)_4].c = 1;         <<< ...no warning
>>>
>>> Interesting.  I do get a warning if I embed the access in the 
>>> destructor:
>>>
>>> struct A
>>> {
>>>    virtual ~A ();
>>>    int a;
>>> };
>>>
>>> struct B: virtual A { };
>>> struct C: virtual A {
>>>    int c;
>>>    ~C() { c = 0; } // warns twice, for _vptr and c accesses
>>> };
>>> struct D: virtual B, virtual C
>>> {
>>>    D();
>>> };
>>>
>>> void g()
>>> {
>>>    D d;
>>> }
>>
>> Ah, right.  Unlike in member functions, in a dtor (or in the ctors
>> of the derived class) the offsets are known/constant so accesses
>> involving those are checked and we do get a false positive.  Thanks
>> for the test case!
>>
>>>
>>>>    _5 = MEM[(long int *)_1 + -24B];
>>>>    _6 = (sizetype) _5;
>>>>    _7 = &d + _6;
>>>>    MEM[(struct B *)_7].b = 1;
>>>>    _8 = MEM[(long int *)_1 + -32B];
>>>>    _9 = (sizetype) _8;
>>>>    _10 = &d + _9;
>>>>    MEM[(struct A *)_10].a = 1;
>>>>    return;
>>>>
>>>> }
>>>>
>>>>> The general issue about the confusion between complete and as-base 
>>>>> types is PR 22488; I have various thoughts about a proper fix, but 
>>>>> there have always been higher-priority things to work on.
>>>>
>>>> Thanks for the reference!
>>>>
>>>>> One possible approach to this PR is that we don't need to check an 
>>>>> intermediate _REF; in
>>>>>
>>>>> MEM[(struct D *)&D.2651 + 24B]._vptr.D
>>>>>
>>>>> we aren't loading a whole D, we're only looking at the _vptr, which 
>>>>> is within the bounds of E.
>>>>
>>>> This is also what the patch does: it keeps us from reaching
>>>> the MEM_REF because _vptr.D is artificial.
>>>
>>> My point applies equally to the access to c in my testcase above: we 
>>> shouldn't check lvalue refs as though we're loading or storing the 
>>> whole aggregate object.
>>>
>>> The full access is (*((D*)&e))._vptr.D.  The MEM_REF represents the 
>>> *, which we shouldn't check by itself, because it doesn't imply a 
>>> memory access of its own.  We should only check the COMPONENT_REF 
>>> around it that is an actual memory access.
>>
>> The -Warray-bounds implementation checks every ARRAY_REF and MEM_REF,
>> regardless of their context(*).
> 
> Aha.  That seems like a flaw, but as you say, not one that can be 
> addressed easily.
> 
>> It doesn't check COMPONENT_REF except
>> while checking the other REFs (while walking up the use-def chains),
>> and the checks don't know the context of the REF.  The hack this patch
>> put in was to circumvent that when the walk saw a COMPONENT_REF to
>> an artificial member (which as you showed isn't enough).
>>
>> I'm not sure if what you suggest (IIUC) is feasible without reworking
>> how the checking works in general.
> 
>> What is feasible is working harder
>> and detecting if a) the type of the accessed object has virtual bases
>> and b) if the offset of the accessed field is within the bounds of
>> the enclosing object.  In that case, skipping the MEM_REF check avoids
>> the warnings for all the cases we have discussed (artificial members,
>> ctors and dtors).  It still seems hacky but sufficiently narrow in
>> scope for GCC 11(**).
>>
>> Please see the attached revision.
> 
>> +  else if (TREE_CODE (t) == COMPONENT_REF
>> +       && TREE_CODE (TREE_OPERAND (t, 0)) == MEM_REF)
>> +    {
>> +      /* Hack: Skip MEM_REF checks in accesses to a member of a type
>> +     with virtual bases at an offset that's in bounds of the size
>> +     of the enclosing object.  See pr98266 and pr97595.  */
>> +      tree ref = TREE_OPERAND (TREE_OPERAND (t, 0), 0);
>> +      if (TREE_CODE (ref) == ADDR_EXPR)
>> +    ref = TREE_OPERAND (ref, 0);
>> +      tree reftype = TREE_TYPE (ref);
>> +      if (DECL_P (ref)
>> +      && RECORD_OR_UNION_TYPE_P (reftype)
>> +      && TYPE_BINFO (reftype))
>> +    {
>> +      tree fld = TREE_OPERAND (t, 1);
>> +      tree fldpos = byte_position (fld);
>> +      if (tree refsize = DECL_SIZE_UNIT (ref))
>> +        if (TREE_CODE (refsize) == INTEGER_CST
>> +        && tree_int_cst_lt (fldpos, refsize))
>> +          *walk_subtree = false;
>> +    }
>> +    }
> 
> Here fld F is a member of a base class B, and ref is a variable of a 
> derived class D, so you're comparing the offset of F in B to the size of 
> D without considering the offset of B in D.  I think you just need to 
> add the MEM_REF offset to fldpos before comparing.

I see what you mean, thanks, but I can't think of a test case to
trigger a false negative due to the smaller offset.  Any suggestions?

With more testing I also realized that focusing solely on an underlying
DECL like in the above doesn't prevent the warning when an object is
created in dynamically allocated memory or in a backing buffer.  So
the attached revision both adjusts the offset computation upward and
handles all kinds of backing store and moves the logic into a helper
function for better readability.  I've also added more tests.

Retested on x86_64-linux.

Thanks again for your help!

Martin

PS The TYPE_BINFO test isn't as quite as restrictive as I had hoped.
It means we consider all derived class, not just those with virtual
bases.  I tried also requiring BINFO_VIRTUAL_P() to be true but that
doesn't work.  Using BINFO_VTABLE() does work but it still isn't
the same.

> 
> Jason
>

On 3/3/21 6:20 PM, Martin Sebor wrote:
> On 3/2/21 7:11 AM, Jason Merrill wrote:
>> On 3/1/21 6:11 PM, Martin Sebor wrote:
>>> On 2/24/21 5:35 PM, Jason Merrill wrote:
>>>> On 2/23/21 6:07 PM, Martin Sebor wrote:
>>>>> On 2/23/21 2:52 PM, Jason Merrill wrote:
>>>>>> On 2/23/21 11:02 AM, Martin Sebor wrote:
>>>>>>> [CC Jason for any further comments/clarification]
>>>>>>>
>>>>>>> On 2/9/21 10:49 AM, Martin Sebor wrote:
>>>>>>>> On 2/8/21 4:11 PM, Jeff Law wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 2/8/21 3:44 PM, Martin Sebor wrote:
>>>>>>>>>> On 2/8/21 3:26 PM, Jeff Law wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On 2/8/21 2:56 PM, Martin Sebor wrote:
>>>>>>>>>>>> On 2/8/21 12:59 PM, Jeff Law wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> On 1/19/21 5:56 PM, Martin Sebor via Gcc-patches wrote:
>>>>>>>>>>>>>> Similar to the problem reported for -Wstringop-overflow in 
>>>>>>>>>>>>>> pr98266
>>>>>>>>>>>>>> and already fixed, -Warray-bounds is also susceptible to 
>>>>>>>>>>>>>> false
>>>>>>>>>>>>>> positives in assignments and copies involving virtual 
>>>>>>>>>>>>>> inheritance.
>>>>>>>>>>>>>> Because the two warnings don't share code yet (hopefully 
>>>>>>>>>>>>>> in GCC 12)
>>>>>>>>>>>>>> the attached patch adds its own workaround for this 
>>>>>>>>>>>>>> problem to
>>>>>>>>>>>>>> gimple-array-bounds.cc, this one slightly more crude 
>>>>>>>>>>>>>> because of
>>>>>>>>>>>>>> the limited insight the array bounds checking has into the 
>>>>>>>>>>>>>> checked
>>>>>>>>>>>>>> expressions.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Tested on x86_64-linux.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Martin
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> gcc-98266.diff
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> PR middle-end/98266 - bogus array subscript is partly 
>>>>>>>>>>>>>> outside array
>>>>>>>>>>>>>> bounds on virtual inheritance
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> gcc/ChangeLog:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>>>>>       * gimple-array-bounds.cc
>>>>>>>>>>>>>> (array_bounds_checker::check_array_bounds):
>>>>>>>>>>>>>>       Avoid checking references involving artificial members.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>       PR middle-end/98266
>>>>>>>>>>>>>>       * g++.dg/warn/Warray-bounds-15.C: New test.
>>>>>>>>>>>>> It seems to me that we've got the full statement at some 
>>>>>>>>>>>>> point and
>>>>>>>>>>>>> thus
>>>>>>>>>>>>> the full expression so at some point couldn't we detect when
>>>>>>>>>>>>> TYPE_SIZE_UNIT!= DECL_SIZE_UNIT?  Or should we be using 
>>>>>>>>>>>>> TYPE_SIZE_UNIT
>>>>>>>>>>>>> rather than DECL_SIZE_UNIT in gimple-array-bounds.cc
>>>>>>>>>>>>>
>>>>>>>>>>>>> Am I missing something?
>>>>>>>>>>>>
>>>>>>>>>>>> The expression we're looking at when the false positive is 
>>>>>>>>>>>> issued
>>>>>>>>>>>> is the MEM_REF in the LHS of:
>>>>>>>>>>>>
>>>>>>>>>>>> MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> 
>>>>>>>>>>>> [(void
>>>>>>>>>>>> *)&_ZTC1E24_1D + 24B];
>>>>>>>>>>>>
>>>>>>>>>>>> TREE_TYPE(LHS) is D, DECL_SIZE_UNIT (D.2652) is 24, and
>>>>>>>>>>>> TYPE_SIZE_UNIT(D) is also 24, so there's no discrepancy between
>>>>>>>>>>>> DECL_SIZE and TYPE_SIZE.
>>>>>>>>>>> So that seems like it's a different issue then, unrelated to 
>>>>>>>>>>> 97595.
>>>>>>>>>>> Right?
>>>>>>>>>>
>>>>>>>>>> I think the underlying problem is the same.  We're getting a size
>>>>>>>>>> that doesn't correspond to what's actually being accessed, and it
>>>>>>>>>> happens because of the virtual inheritance.  In pr97595 Jason
>>>>>>>>>> suggested to use the decl/type size inequality to identify this
>>>>>>>>>> case but I think we could have just as well used DECL_ARTIFICIAL
>>>>>>>>>> instead.  At least the test cases from pr97595 both pass with
>>>>>>>>>> this change.
>>>>>>>>> But in the 98266 case the type and decl sizes are the same.  So 
>>>>>>>>> to be
>>>>>>>>> true that would mean that the underlying type we're using to 
>>>>>>>>> access
>>>>>>>>> memory differs from its actual type.  Is that the case in the 
>>>>>>>>> IL? And
>>>>>>>>> does this have wider implications for diagnostics or 
>>>>>>>>> optimizations that
>>>>>>>>> rely on accurate type sizing?
>>>>>>>>>
>>>>>>>>> I'm just trying to make sure I understand, not accepting or 
>>>>>>>>> rejecting
>>>>>>>>> the patch yet.
>>>>>>>>
>>>>>>>> The part of the IL with the MEM_REF is this:
>>>>>>>>
>>>>>>>> void g ()
>>>>>>>> {
>>>>>>>>    void * D.2789;
>>>>>>>>    struct E D.2652;
>>>>>>>>
>>>>>>>>    <bb 2> [local count: 1073741824]:
>>>>>>>>    E::E (&D.2652, "");
>>>>>>>>    f (&D.2652);
>>>>>>>>
>>>>>>>>    <bb 3> [local count: 1073741824]:
>>>>>>>>    MEM[(struct D *)&D.2652 + 24B]._vptr.D = &MEM <int (*) ()> 
>>>>>>>> [(void *)&_ZTC1E24_1D + 24B];
>>>>>>>>    ...
>>>>>>>>
>>>>>>>> The access here is to the _vptr.D pointer member of D.2652 which is
>>>>>>>> just past the end of the parent object (as reflected by its SIZE):
>>>>>>>> it sets sets up the virtual table pointer.
>>>>>>>>
>>>>>>>> The access in pr97595 is to the member subobject, which, as Jason
>>>>>>>> explained (and I accordingly documented under DECL_SIZE in tree.h),
>>>>>>>> is also laid out separately from the parent object.
>>>>>>>>
>>>>>>>> These cases aren't exactly the same (which is also why the test
>>>>>>>> I added for -Warray-bounds in pr97595 didn't expose this bug) but
>>>>>>>> they are closely related.  The one here can be distinguished by
>>>>>>>> DECL_ARTIFICAL.  The other by the DECL_SIZE != TYPE_SIZE member
>>>>>>>> inequality.
>>>>>>>>
>>>>>>>> Might this impact other warnings?  I'd say so if they don't take
>>>>>>>> these things into account.  I just learned about this in pr97595
>>>>>>>> which was a -Wstringop-overflow false positive but I also saw
>>>>>>>> a similar instance of -Warray-bounds with my patch to improve
>>>>>>>> caching and enhance array bounds checking.  I dealt with that
>>>>>>>> instance of the warning in that patch but proactively added
>>>>>>>> a test case to the fix for pr97595.  But the test case is focused
>>>>>>>> on the subobject access and not on one to the virtual table so
>>>>>>>> (as I said above) it didn't expose this bug.
>>>>>>>>
>>>>>>>> Might this also impact optimizations?  I can imagine someone
>>>>>>>> unaware of this "gotcha" making the same "naive" assumption
>>>>>>>> I did, but I'd also expect such an invalid assumption to be
>>>>>>>> found either in code review or quickly cause problems in
>>>>>>>> testing.
>>>>>>>
>>>>>>> Jeff, does this answer your question?
>>>>>>
>>>>>> I don't see how the issue here depends on the artificiality of the 
>>>>>> vptr; I'd expect to see the same problem with a data member.  The 
>>>>>> problem is that a D base subobject is smaller than a complete D 
>>>>>> object, and in this case the base subobject is allocated such that 
>>>>>> if it were a full D object, it would overlap the end of E.  And 
>>>>>> we're checking the MEM_REF as though accessing a full D object, so 
>>>>>> we get a warning.
>>>>>
>>>>> Thanks for chiming in!
>>>>>
>>>>> There is no warning for accesses to data members defined in
>>>>> the virtual bases because their offset isn't constant.  For example,
>>>>> given the following modified version of the test case from the patch:
>>>>>
>>>>> struct A
>>>>> {
>>>>>    virtual ~A ();
>>>>>    int a;
>>>>> };
>>>>>
>>>>> struct B: virtual A { int b; };
>>>>> struct C: virtual A { int c; };
>>>>> struct D: virtual B, virtual C
>>>>> {
>>>>>    int d;
>>>>> };
>>>>>
>>>>> D d;
>>>>>
>>>>> void g ()
>>>>> {
>>>>>    D *p = &d;
>>>>>    p->a = p->b = p->c = p->d = 1;
>>>>> }
>>>>>
>>>>> we end up with:
>>>>>
>>>>> void g ()
>>>>> {
>>>>>    ...
>>>>>    <bb 2> [local count: 1073741824]:
>>>>>    d.d = 1;                           <<< okay
>>>>>    _1 = d._vptr.D;                    <<< DECL_ARTIFICIAL
>>>>>    _2 = MEM[(long int *)_1 + -40B];   <<< not checked
>>>>>    _3 = (sizetype) _2;                <<< not constant...
>>>>>    _4 = &d + _3;
>>>>>    MEM[(struct C *)_4].c = 1;         <<< ...no warning
>>>>
>>>> Interesting.  I do get a warning if I embed the access in the 
>>>> destructor:
>>>>
>>>> struct A
>>>> {
>>>>    virtual ~A ();
>>>>    int a;
>>>> };
>>>>
>>>> struct B: virtual A { };
>>>> struct C: virtual A {
>>>>    int c;
>>>>    ~C() { c = 0; } // warns twice, for _vptr and c accesses
>>>> };
>>>> struct D: virtual B, virtual C
>>>> {
>>>>    D();
>>>> };
>>>>
>>>> void g()
>>>> {
>>>>    D d;
>>>> }
>>>
>>> Ah, right.  Unlike in member functions, in a dtor (or in the ctors
>>> of the derived class) the offsets are known/constant so accesses
>>> involving those are checked and we do get a false positive.  Thanks
>>> for the test case!
>>>
>>>>
>>>>>    _5 = MEM[(long int *)_1 + -24B];
>>>>>    _6 = (sizetype) _5;
>>>>>    _7 = &d + _6;
>>>>>    MEM[(struct B *)_7].b = 1;
>>>>>    _8 = MEM[(long int *)_1 + -32B];
>>>>>    _9 = (sizetype) _8;
>>>>>    _10 = &d + _9;
>>>>>    MEM[(struct A *)_10].a = 1;
>>>>>    return;
>>>>>
>>>>> }
>>>>>
>>>>>> The general issue about the confusion between complete and as-base 
>>>>>> types is PR 22488; I have various thoughts about a proper fix, but 
>>>>>> there have always been higher-priority things to work on.
>>>>>
>>>>> Thanks for the reference!
>>>>>
>>>>>> One possible approach to this PR is that we don't need to check an 
>>>>>> intermediate _REF; in
>>>>>>
>>>>>> MEM[(struct D *)&D.2651 + 24B]._vptr.D
>>>>>>
>>>>>> we aren't loading a whole D, we're only looking at the _vptr, 
>>>>>> which is within the bounds of E.
>>>>>
>>>>> This is also what the patch does: it keeps us from reaching
>>>>> the MEM_REF because _vptr.D is artificial.
>>>>
>>>> My point applies equally to the access to c in my testcase above: we 
>>>> shouldn't check lvalue refs as though we're loading or storing the 
>>>> whole aggregate object.
>>>>
>>>> The full access is (*((D*)&e))._vptr.D.  The MEM_REF represents the 
>>>> *, which we shouldn't check by itself, because it doesn't imply a 
>>>> memory access of its own.  We should only check the COMPONENT_REF 
>>>> around it that is an actual memory access.
>>>
>>> The -Warray-bounds implementation checks every ARRAY_REF and MEM_REF,
>>> regardless of their context(*).
>>
>> Aha.  That seems like a flaw, but as you say, not one that can be 
>> addressed easily.
>>
>>> It doesn't check COMPONENT_REF except
>>> while checking the other REFs (while walking up the use-def chains),
>>> and the checks don't know the context of the REF.  The hack this patch
>>> put in was to circumvent that when the walk saw a COMPONENT_REF to
>>> an artificial member (which as you showed isn't enough).
>>>
>>> I'm not sure if what you suggest (IIUC) is feasible without reworking
>>> how the checking works in general.
>>
>>> What is feasible is working harder
>>> and detecting if a) the type of the accessed object has virtual bases
>>> and b) if the offset of the accessed field is within the bounds of
>>> the enclosing object.  In that case, skipping the MEM_REF check avoids
>>> the warnings for all the cases we have discussed (artificial members,
>>> ctors and dtors).  It still seems hacky but sufficiently narrow in
>>> scope for GCC 11(**).
>>>
>>> Please see the attached revision.
>>
>>> +  else if (TREE_CODE (t) == COMPONENT_REF
>>> +       && TREE_CODE (TREE_OPERAND (t, 0)) == MEM_REF)
>>> +    {
>>> +      /* Hack: Skip MEM_REF checks in accesses to a member of a type
>>> +     with virtual bases at an offset that's in bounds of the size
>>> +     of the enclosing object.  See pr98266 and pr97595.  */
>>> +      tree ref = TREE_OPERAND (TREE_OPERAND (t, 0), 0);
>>> +      if (TREE_CODE (ref) == ADDR_EXPR)
>>> +    ref = TREE_OPERAND (ref, 0);
>>> +      tree reftype = TREE_TYPE (ref);
>>> +      if (DECL_P (ref)
>>> +      && RECORD_OR_UNION_TYPE_P (reftype)
>>> +      && TYPE_BINFO (reftype))
>>> +    {
>>> +      tree fld = TREE_OPERAND (t, 1);
>>> +      tree fldpos = byte_position (fld);
>>> +      if (tree refsize = DECL_SIZE_UNIT (ref))
>>> +        if (TREE_CODE (refsize) == INTEGER_CST
>>> +        && tree_int_cst_lt (fldpos, refsize))
>>> +          *walk_subtree = false;
>>> +    }
>>> +    }
>>
>> Here fld F is a member of a base class B, and ref is a variable of a 
>> derived class D, so you're comparing the offset of F in B to the size 
>> of D without considering the offset of B in D.  I think you just need 
>> to add the MEM_REF offset to fldpos before comparing.
> 
> I see what you mean, thanks, but I can't think of a test case to
> trigger a false negative due to the smaller offset.  Any suggestions?

My only ideas involve undefined behavior, casting the address to a 
pointer to an unrelated too-large class.  I don't know if that would 
show up as a MEM_REF of this pattern.

> With more testing I also realized that focusing solely on an underlying
> DECL like in the above doesn't prevent the warning when an object is
> created in dynamically allocated memory or in a backing buffer.  So
> the attached revision both adjusts the offset computation upward and
> handles all kinds of backing store and moves the logic into a helper
> function for better readability.  I've also added more tests.
> 
> Retested on x86_64-linux.
> 
> Thanks again for your help!
> 
> Martin
> 
> PS The TYPE_BINFO test isn't as quite as restrictive as I had hoped.
> It means we consider all derived class, not just those with virtual
> bases.

It's even less restrictive than that: all C++ classes have TYPE_BINFO.

> I tried also requiring BINFO_VIRTUAL_P() to be true but that
> doesn't work.

Right, BINFO_VIRTUAL_P is true for the binfo representing the virtual 
base in the inheritance hierarchy, not on the binfo for the derived class.

> Using BINFO_VTABLE() does work but it still isn't the same.

Indeed, virtual functions and virtual bases both cause a class to have a 
vtable.  You could also check BINFO_N_BASE_BINFOS.

Or not bother trying to restrict this to classes with virtual bases 
(i.e. leave the patch as it is), since the result is just as correct for 
other classes.

Jason

On 3/3/21 10:33 PM, Jason Merrill wrote:
> On 3/3/21 6:20 PM, Martin Sebor wrote:
...
>> I see what you mean, thanks, but I can't think of a test case to
>> trigger a false negative due to the smaller offset.  Any suggestions?
> 
> My only ideas involve undefined behavior, casting the address to a 
> pointer to an unrelated too-large class.  I don't know if that would 
> show up as a MEM_REF of this pattern.

Okay, let me see if I can come up with something based on that.

> 
>> With more testing I also realized that focusing solely on an underlying
>> DECL like in the above doesn't prevent the warning when an object is
>> created in dynamically allocated memory or in a backing buffer.  So
>> the attached revision both adjusts the offset computation upward and
>> handles all kinds of backing store and moves the logic into a helper
>> function for better readability.  I've also added more tests.
>>
>> Retested on x86_64-linux.
>>
>> Thanks again for your help!
>>
>> Martin
>>
>> PS The TYPE_BINFO test isn't as quite as restrictive as I had hoped.
>> It means we consider all derived class, not just those with virtual
>> bases.
> 
> It's even less restrictive than that: all C++ classes have TYPE_BINFO.
> 
>> I tried also requiring BINFO_VIRTUAL_P() to be true but that
>> doesn't work.
> 
> Right, BINFO_VIRTUAL_P is true for the binfo representing the virtual 
> base in the inheritance hierarchy, not on the binfo for the derived class.
> 
>> Using BINFO_VTABLE() does work but it still isn't the same.
> 
> Indeed, virtual functions and virtual bases both cause a class to have a 
> vtable.  You could also check BINFO_N_BASE_BINFOS.
> 
> Or not bother trying to restrict this to classes with virtual bases 
> (i.e. leave the patch as it is), since the result is just as correct for 
> other classes.
> 
> Jason
> 

I think I'll just go with this last version if you're okay with it
as is.  Okay to commit?

Martin

PS Let me propose a separate patch to add some text to the BINFO
comments in tree.h to clarify the macros per the above.

On 3/4/21 12:44 PM, Martin Sebor wrote:
> On 3/3/21 10:33 PM, Jason Merrill wrote:
>> On 3/3/21 6:20 PM, Martin Sebor wrote:
> ...
>>> I see what you mean, thanks, but I can't think of a test case to
>>> trigger a false negative due to the smaller offset.  Any suggestions?
>>
>> My only ideas involve undefined behavior, casting the address to a 
>> pointer to an unrelated too-large class.  I don't know if that would 
>> show up as a MEM_REF of this pattern.
> 
> Okay, let me see if I can come up with something based on that.
> 
>>
>>> With more testing I also realized that focusing solely on an underlying
>>> DECL like in the above doesn't prevent the warning when an object is
>>> created in dynamically allocated memory or in a backing buffer.  So
>>> the attached revision both adjusts the offset computation upward and
>>> handles all kinds of backing store and moves the logic into a helper
>>> function for better readability.  I've also added more tests.
>>>
>>> Retested on x86_64-linux.
>>>
>>> Thanks again for your help!
>>>
>>> Martin
>>>
>>> PS The TYPE_BINFO test isn't as quite as restrictive as I had hoped.
>>> It means we consider all derived class, not just those with virtual
>>> bases.
>>
>> It's even less restrictive than that: all C++ classes have TYPE_BINFO.
>>
>>> I tried also requiring BINFO_VIRTUAL_P() to be true but that
>>> doesn't work.
>>
>> Right, BINFO_VIRTUAL_P is true for the binfo representing the virtual 
>> base in the inheritance hierarchy, not on the binfo for the derived 
>> class.
>>
>>> Using BINFO_VTABLE() does work but it still isn't the same.
>>
>> Indeed, virtual functions and virtual bases both cause a class to have 
>> a vtable.  You could also check BINFO_N_BASE_BINFOS.
>>
>> Or not bother trying to restrict this to classes with virtual bases 
>> (i.e. leave the patch as it is), since the result is just as correct 
>> for other classes.
>>
>> Jason
>>
> 
> I think I'll just go with this last version if you're okay with it
> as is.  Okay to commit?

OK.

> Martin
> 
> PS Let me propose a separate patch to add some text to the BINFO
> comments in tree.h to clarify the macros per the above.
>