Add gimple-compat.h (was Re: [gimple-classes, committed 4/6] tree-ssa-tail-merge.c: Use gassign)

On Thu, 2014-11-13 at 11:45 +0100, Richard Biener wrote:
> On Thu, Nov 13, 2014 at 2:41 AM, David Malcolm <dmalcolm@redhat.com> wrote:
> > On Tue, 2014-11-11 at 11:43 +0100, Richard Biener wrote:
> >> On Tue, Nov 11, 2014 at 8:26 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> >> > On Mon, Nov 10, 2014 at 05:27:50PM -0500, David Malcolm wrote:
> >> >> On Sat, 2014-11-08 at 14:56 +0100, Jakub Jelinek wrote:
> >> >> > On Sat, Nov 08, 2014 at 01:07:28PM +0100, Richard Biener wrote:
> >> >> > > To be constructive here - the above case is from within a
> >> >> > > GIMPLE_ASSIGN case label
> >> >> > > and thus I'd have expected
> >> >> > >
> >> >> > >     case GIMPLE_ASSIGN:
> >> >> > >       {
> >> >> > >         gassign *a1 = as_a <gassign *> (s1);
> >> >> > >         gassign *a2 = as_a <gassign *> (s2);
> >> >> > >       lhs1 = gimple_assign_lhs (a1);
> >> >> > >       lhs2 = gimple_assign_lhs (a2);
> >> >> > >       if (TREE_CODE (lhs1) != SSA_NAME
> >> >> > >           && TREE_CODE (lhs2) != SSA_NAME)
> >> >> > >         return (operand_equal_p (lhs1, lhs2, 0)
> >> >> > >                 && gimple_operand_equal_value_p (gimple_assign_rhs1 (a1),
> >> >> > >                                                  gimple_assign_rhs1 (a2)));
> >> >> > >       else if (TREE_CODE (lhs1) == SSA_NAME
> >> >> > >                && TREE_CODE (lhs2) == SSA_NAME)
> >> >> > >         return vn_valueize (lhs1) == vn_valueize (lhs2);
> >> >> > >       return false;
> >> >> > >       }
> >> >> > >
> >> >> > > instead.  That's the kind of changes I have expected and have approved of.
> >> >> >
> >> >> > But even that looks like just adding extra work for all developers, with no
> >> >> > gain.  You only have to add extra code and extra temporaries, in switches
> >> >> > typically also have to add {} because of the temporaries and thus extra
> >> >> > indentation level, and it doesn't simplify anything in the code.
> >> >>
> >> >> The branch attempts to use the C++ typesystem to capture information
> >> >> about the kinds of gimple statement we expect, both:
> >> >>   (A) so that the compiler can detect type errors, and
> >> >>   (B) as a comprehension aid to the human reader of the code
> >> >>
> >> >> The ideal here is when function params and struct field can be
> >> >> strengthened from "gimple" to a subclass ptr.  This captures the
> >> >> knowledge that every use of a function or within a struct has a given
> >> >> gimple code.
> >> >
> >> > I just don't like all the as_a/is_a stuff enforced everywhere,
> >> > it means more typing, more temporaries, more indentation.
> >> > So, as I view it, instead of the checks being done cheaply (yes, I think
> >> > the gimple checking as we have right now is very cheap) under the
> >> > hood by the accessors (gimple_assign_{lhs,rhs1} etc.), those changes
> >> > put the burden on the developers, who has to check that manually through
> >> > the as_a/is_a stuff everywhere, more typing and uglier syntax.
> >> > I just don't see that as a step forward, instead a huge step backwards.
> >> > But perhaps I'm alone with this.
> >> > Can you e.g. compare the size of - lines in your patchset combined, and
> >> > size of + lines in your patchset?  As in, if your changes lead to less
> >> > typing or more.
> >>
> >> I see two ways out here.  One is to add overloads to all the functions
> >> taking the special types like
> >>
> >> tree
> >> gimple_assign_rhs1 (gimple *);
> >>
> >> or simply add
> >>
> >> gassign *operator ()(gimple *g) { return as_a <gassign *> (g); }
> >>
> >> into a gimple-compat.h header which you include in places that
> >> are not converted "nicely".
> >
> > Thanks for the suggestions.
> >
> > Am I missing something, or is the gimple-compat.h idea above not valid C
> > ++?
> >
> > Note that "gimple" is still a typedef to
> >   gimple_statement_base *
> > (as noted before, the gimple -> gimple * change would break everyone
> > else's patches, so we talked about that as a followup patch for early
> > stage3).
> >
> > Given that, if I try to create an "operator ()" outside of a class, I
> > get this error:
> >
> > ‘gassign* operator()(gimple)’ must be a nonstatic member function
> >
> > which is emitted from cp/decl.c's grok_op_properties:
> >       /* An operator function must either be a non-static member function
> >          or have at least one parameter of a class, a reference to a class,
> >          an enumeration, or a reference to an enumeration.  13.4.0.6 */
> >
> > I tried making it a member function of gimple_statement_base, but that
> > doesn't work either: we want a conversion
> >   from a gimple_statement_base * to a gassign *, not
> >   from a gimple_statement_base   to a gassign *.
> >
> > Is there some syntactic trick here that I'm missing?  Sorry if I'm being
> > dumb (I can imagine there's a way of doing it by making "gimple" become
> > some kind of wrapped ptr class, but that way lies madness, surely).
> 
> Hmm.
> 
> struct assign;
> struct base {
>   operator assign *() const { return (assign *)this; }
> };
> struct assign : base {
> };
> 
> void foo (assign *);
> void bar (base *b)
> {
>   foo (b);
> }
> 
> doesn't work, but
> 
> void bar (base &b)
> {
>   foo (b);
> }
> 
> does.  Indeed C++ doesn't seem to provide what is necessary
> for the compat trick :(
> 
> So the gimple-compat.h header would need to provide
> additional overloads for the affected functions like
> 
> inline tree
> gimple_assign_rhs1 (gimple *g)
> {
>   return gimple_assign_rhs1 (as_a <gassign *> (g));
> }
> 
> that would work for me as well.
> 
> >> Both avoid manually making the compiler happy (which the
> >> explicit as_a<> stuff is!  It doesn't add any "checking" - it's
> >> just placing the as_a<> at the callers and thus make the
> >> runtine ICE fire there).
> >>
> >> As much as I don't like "global" conversion operators I don't
> >> like adding overloads to all of the accessor functions even more.
> >
> > (nods)
> >
> > Some other options:
> >
> > Option 3: only convert the "easy" accessors: the ones I already did in
> > the /89 patch kit, as reviewed by Jeff, and rebased by me recently,
> > which is this 92-patch kit:
> >   "[gimple-classes, committed 00/92] Initial slew of commits":
> >      https://gcc.gnu.org/ml/gcc-patches/2014-10/msg02791.html
> > Doing so converts about half of the gimple_foo_ accessors to taking a
> > gfoo *, giving a mixture of GIMPLE_CHECK vs subclass use.   I believe
> > the quality of those patches was higher than the later ones on the
> > branch: I was doing the places that didn't require the invasive/verbose
> > changes seen in the later patches.  Shelve the remaining ~80
> > increasingly ugly patches, starting a new branch to contain just the
> > good ones.
> >
> >
> > Option 4: don't convert any accessors, but instead focus on fields of
> > structs (e.g. "call_stmt" within a cgraph_edge), and on params of other
> > functions (e.g. phi-manipulation code).  That way we'd avoid the
> > inconsistency of some accessors using GIMPLE_CHECK and some using
> > subclasses - all would continue to consistently use GIMPLE_CHECK, but
> > there would be some extra type-checking and self-documentation of the
> > expected statement kinds in the code.
> >
> >
> >
> > FWIW, option 3 is my preferred approach (I've already done the bulk of
> > the work and it's already been reviewed; it would need an update merger
> > from trunk, and there's the big gimple to gimple * fixup you wanted).
> 
> Works for me as well.  The compat solution looks somewhat appealing
> as we can then incrementally fix up things rather than requiring to
> mass-convert everything.
> 
> Thanks,
> Richard.

I still prefer option 3 above (i.e. go with the patches Jeff reviewed,
with the fixups discussed earlier, with a merger from trunk), but I had
a go at implementing your gimple-compat.h idea.

Attached is a patch against e74b2ecd78d21fc7bcc2f50dacc92819e0e69e6b on
the gimple-classes branch (the last commit before I went overboard with
the as_a changes), which converts the remaining gimple_assign_ accessors
to take a gassign *, adding a gimple-compat.h to provide overloaded
accessors taking just a gimple for those source files that needed them.
It also strengthens some places I spotted where converting params,
locals or fields from gimple to gassign * was simple and obvious.

Successfully bootstrapped&regrtested on x86_64-unknown-linux-gnu,
comparing the branch+patch against master r216746 (2014-10-27).

This only covers gimple_assign_; it would still be a hybrid of is-a and
GIMPLE_CHECK unless I extended the patch to cover all the other
accessors.

Do you want me to continue converting things for gimple-compat.h to see
what a set of patches for that approach looks like?

If so, would it be better to split things out and have a
gimple-compat-assign.h, gimple-compat-call.h etc rather than one big
gimple-compat.h?

That said, I still prefer option 3.

> >> Whether you enable them generally or just for selected files
> >> via a gimple-compat.h will be up to you (but I'd rather get
> >> rid of them at some point).
> >>
> >> Note this allows seamless transform of "random" functions
> >> taking a gimple now but really only expecting a single kind.
> >>
> >> Note that we don't absolutely have to rush this all in for GCC 5.
> >> Being the very first for GCC 6 stage1 is another possibility.
> >> We just should get it right.
> >
> > Thanks
> > Dave
> >

Add gimple-compat.h (was Re: [gimple-classes, committed 4/6] tree-ssa-tail-merge.c: Use gassign)

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