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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/94802] Failure to recognize identities with
 __builtin_clz
Date: Mon, 27 Apr 2020 15:22:45 +0000
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X-Bugzilla-Component: tree-optimization
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X-Bugzilla-Keywords: easyhack, missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94802

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |easyhack,
                   |                            |missed-optimization
   Last reconfirmed|                            |2020-04-27
                 CC|                            |jakub at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
For the first one, guess __builtin_clz (x) =3D=3D 0 can be transformed into=
 (int) x
< 0 and similarly __builtin_clz (x) !=3D 0 to (int) x >=3D 0.
Not sure if we should check CLZ_DEFINED_VALUE_AT_ZERO if it isn't 0 (or
precision-1) for these transformations, while the documentation says that t=
he
builtin is undefined at zero, e.g. if CLZ_DEFINED_VALUE_AT_ZERO returns 2 t=
hen
even at GIMPLE the =3D=3D 0 guard for it could have been optimized away alr=
eady.=