[Bug middle-end/126625] hwint.cc reflect_hwi () should use __builtin_bitreverse64 if supported.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126625

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Kael Andrew Franco <[email protected]>:

https://gcc.gnu.org/g:c73b71d6bc7e106714bec94d13f877c6027d9531

commit r17-3165-gc73b71d6bc7e106714bec94d13f877c6027d9531
Author: Kael Andrew Alonzo Franco <[email protected]>
Date:   Sat Aug 8 21:59:24 2026 -0400

    middle-end: If supported, use __builtin_*. [PR126625]

    hwint.cc reflect_hwi uses a naive for loop approach to
    emulate __builtin_bitreverse64 ().
    This is slow compared to using __builtin_bitreverse64 () plus bitshift.
    This is useful for bootstrapping GCC since r17-523.
    Also add assert on BITWIDTH <= 64.

    For reflect_hwi and ira-color.cc, add STAGE0_CXX_HAS_BUILTIN
    in system.h to test for specific builtins.

    Bootstrapped and regtested on x86_64-pc-linux-gnu.

            PR middle-end/126625

    gcc/ChangeLog:

            * hwint.cc (reflect_hwi): Use STAGE0_CXX_HAS_BUILTIN
            for __builtin_bitreverse64.
            * ira-color.cc (HAS_SMUL_OVERFLOW): Remove
            and use STAGE0_CXX_HAS_BUILTIN instead.
            * system.h (STAGE0_CXX_HAS_BUILTIN): New macro.

    Signed-off-by: Kael Andrew Franco <[email protected]>