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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/115073] RISC-V: Gimple fold not honor
 C[LT]Z_DEFINED_VALUE_AT_ZERO
Date: Mon, 13 May 2024 15:09:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115073

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> ---
That is misunderstanding of what C[LT]Z_DEFINED_VALUE_AT_ZERO is.
The builtin has always UB on zero.
If you want to define it for zero, use x ? __builtin_clz (x) : 32 (and the
like) or
__builtin_clzg (x, 32) in the source.
C[LT]Z_DEFINED_VALUE_AT_ZERO returning 2 then makes sure the compiler can f=
old
those conditionals or builtins into .CLZ internal calls with 2 arguments wh=
ich
makes it well defined for all inputs.=