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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/94793] Failure to optimize clz idiom
Date: Thu, 07 May 2020 17:06:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94793

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

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

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
You mean like PR82479 ?  Though, __builtin_clz is UB at zero, so it would n=
eed
to be v >>=3D 1; r =3D v ? 32 - __builtin_clz(v) : 0; v =3D 0; or so, excep=
t that on
aarch64 (or where we CLZ_DEFINED_VALUE_AT_ZERO as 2, i.e. guarantee if CLZ =
ifn
or builtins to have the behavior defined at GIMPLE rather than say just RTL=
 or
never).  And on other targets that have CLZ_DEFINED_VALUE_AT_ZERO 1 with va=
lue
of precision, perhaps convert that conditional into just CLZ at RTL time.=