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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/108140] tzcnt gives different result in debug vs release
Date: Fri, 16 Dec 2022 08:28:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108140

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> ---
If you call __builtin_ctzll with argument 0, it is undefined behavior, you
can't make any assumptions, regardless what instructions you assume will be
used or are actually used.
If you write your code as x ? __builtin_ctzll (x) : 64 or similar, GCC if it
uses an instruction that guarantees that result for 0 might actually fold it
back into just that instruction unconditionally.=