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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/114032] ifcvt may introduce UB calls to
 __builtin_clz(0)
Date: Wed, 21 Feb 2024 14:37:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114032

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Indeed, this feels like a bug, though unlikely to actually trigger anything
wrong.
I wouldn't worry that much about ifcvt IL, but about what makes it out of t=
he
vectorizer.

void
foo (unsigned *p)
{
  for (int i =3D 0; i < 1024; ++i)
    p[i] =3D p[i] ? __builtin_clz (p[i]) : 42;
}

void
bar (unsigned *p)
{
  for (int i =3D 0; i < 1024; ++i)
    p[i] =3D p[i] ? __builtin_clz (p[i]) : 32;
}

with -O3 -mavx512{cd,bw,vl,dq} -mlzcnt -mbmi -mbmi2 -fvect-cost-model=3Dunl=
imited
Before ifcvt we have conditional __builtin_clz in foo and .CLZ (_4, 32); in
bar,
and out of the vectorizer get the same .CLZ (vect__4.10_37, 32); in both ca=
ses,
that looks ok.  But without the -mlzcnt -mbmi -mbmi2 options, we have
conditional __builtin_clz in both cases, and vectorizer results in .CLZ
(vect__4.10_37) in both cases.  We don't have value ranges on vectors thoug=
h,
so not really sure what could misbehave during the optimizations later on a=
nd I
bet all the targets which support vector .CLZ/.CTZ actually have some well
defined value for zero.
Maybe just the backends even for cases like -mlzcnt -mbmi -mbmi2 should
announce C?Z_VALUE_DEFINED_AT_ZERO for vector modes if it supports vector c=
?z
optabs? even
if it isn't defined for scalar?=