[Bug tree-optimization/94802] Failure to recognize identities with __builtin_clz

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94802

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) == 0 can be transformed into (int) x
< 0 and similarly __builtin_clz (x) != 0 to (int) x >= 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 the
builtin is undefined at zero, e.g. if CLZ_DEFINED_VALUE_AT_ZERO returns 2 then
even at GIMPLE the == 0 guard for it could have been optimized away already.