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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/112296] __builtin_constant_p doesn't
 propagate through member functions
Date: Mon, 06 Nov 2023 07:38:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112296

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
           Keywords|documentation               |
                 CC|                            |jakub at gcc dot gnu.org
           Assignee|rguenth at gcc dot gnu.org         |unassigned at gcc d=
ot gnu.org

--- Comment #15 from Richard Biener <rguenth at gcc dot gnu.org> ---
I clarified the documentation.  While in the particular case 'span.size ()'
doesn't have side-effects (well, I'm not 100% sure) in general function cal=
ls
are difficult to handle "delayed" while still allowing to discard side-effe=
cts.

Maybe the infrastructure we put in place for [[assume]] could help here by
treating __builtin_constant_p (expr) as

 [["assume" (guard)]]
 {
   tem =3D expr;
   guard =3D __builtin_constant_p (tem);
 }
 guard;

and lower it to

 .ASSUME (&artificial_fn, args...);

but now with a return value that would become zero at the point we elide
.ASSUME unless we end up with a [1,1] range for the result?


That said, it's best to avoid relying on the side-effect removal, that is,
avoid having arguments with possible side-effects in __builtin_constant_p.=