[Bug target/126973] __builtin_clzg could be improved by using bsrw rather than bsrl or using a shift rather than an and/sub

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126973

Drea Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Worse X86 codegen for       |__builtin_clzg  could be
                   |arbitrary                   |improved by using bsrw
                   |__builtin_clzg/__builtin_ct |rather than bsrl or using a
                   |zg compared to clang.       |shift rather than an
                   |                            |and/sub
           Severity|normal                      |enhancement
         Resolution|DUPLICATE                   |---
           Keywords|ABI                         |
   Last reconfirmed|                            |2026-08-21
             Status|RESOLVED                    |NEW
     Ever confirmed|0                           |1

--- Comment #3 from Drea Pinski <pinskia at gcc dot gnu.org> ---
Well __builtin_clzg   could be improved slightly by using bsrw rather than
bsrl.

You could do also a shift for __builtin_clzg  too.

But that is not always better:
https://godbolt.org/z/7hbcnGq9E