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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/94802] Failure to recognize identities with
 __builtin_clz
Date: Sat, 02 Jan 2021 08:44:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94802

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Even when the optimization is restricted to GIMPLE only, the single_use in
there doesn't work really well before fre1 optimizes it, because the above
mentioned testcase doesn't reuse the same expression, but has a different
subtraction that just needs to be CSEd first.
An alternative would be to treat (signed undefined overflow) x >=3D y and x=
 - y
interchangeably by SCCVN, i.e. hash them the same and if we find both forms=
 (in
either order), transform the x >=3D y back into (x - y) >=3D 0 (similar for=
 other
non-equality comparisons).  I guess that would fix both that testcase and
PR94798 which also suffers from that:
int
foo (int a, int b)
{
  return (b >=3D a) ? (b - a) : 0;
}

int
bar (int a, int b)
{
  return ((b - a) >=3D 0) ? (b - a) : 0;
}

bar is optimized more than foo - but in this case it is a user that wrote i=
t in
two different forms rather than one.

Another option is to enable the x - y op 0 to x op y canonicalization only
after SCCVN got a chance to CSE stuff after inlining, while that would fix =
the
por45397.c testcase, it wouldn't the above PR94798 one.

Richi, any thoughts on this?=