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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/96565] Failure to optimize out VLA even
 though it is left unused
Date: Tue, 25 Aug 2020 09:21:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96565

--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> I guess the "usual" way of dealing with this would be to have
> CLOBBERs for all VLAs before the __builtin_stack_restore.

That looks like a good idea.

I didn't try to follow in a debugger why DSE fails to remove the write when
those 2 builtins are present while it manages if I call
__builtin_alloca_with_align directly, but I don't immediately see a reason =
for
that difference, even in the absence of clobbers. Or maybe that's just the
usual limitations of DSE (there is a branch after all...).

I first thought that __builtin_stack_save/restore might need some extra
attributes (advertising for instance that they do not read/write memory or =
let
anything escape, without weakening them to the point where the compiler wou=
ld
move them around too much or remove them), but since the call to the opaque=
 g
does not seem to prevent DSE from removing the write, that's probably not t=
he
issue.=