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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/113764] [X86] __builtin_clz generates lzcnt when bsr is
 sufficient
Date: Sun, 11 Feb 2024 11:34:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113764

Roger Sayle <roger at nextmovesoftware dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[X86] Generates lzcnt when  |[X86] __builtin_clz
                   |bsr is sufficient           |generates lzcnt when bsr is
                   |                            |sufficient

--- Comment #4 from Roger Sayle <roger at nextmovesoftware dot com> ---
Yep, CLZ_DEFINED_VALUE_AT_ZERO really complicates things.  With a single
"global" macro it's currently impossible for a backend to support two diffe=
rent
CLZ instructions; one with defined behavior at zero, and the other with
undefined behavior at zero.

It might just be possible to do something encoding LZCNT patterns in RTL us=
ing:
(if_then_else:SI (ne:SI (reg:SI x) (const_int 0))
                 (clz:SI (reg:SI x))
                 (const_int VALUE))

Additionally on x86_64, the BSR instruction sets the zero flag if it's inpu=
t is
zero, when the destination register becomes undefined, which can be useful =
with
CMOV, i.e. it's possible to get defined behavior without an additional test=
 and
branch.  But for Pawel's original tescase, __builtin_clz is undefined at ze=
ro,
so this really is a missed optimization, with either -Os or a modern -march
such as cascadelake or znver4.

I agree with Jakub, this is a can of worms; potentially a lot of effort for=
 a
marginal improvement.=