[Bug target/101175] builtin_clz generates wrong bsr instruction

Thu Jun 24 06:17:30 GMT 2021

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101175

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Uros Bizjak <uros@gcc.gnu.org>:

https://gcc.gnu.org/g:ab383ecb4a45413fcc0012bc1791c094fe7fed29

commit r10-9955-gab383ecb4a45413fcc0012bc1791c094fe7fed29
Author: Uros Bizjak <ubizjak@gmail.com>
Date:   Wed Jun 23 12:50:53 2021 +0200

    i386: Prevent unwanted combine from LZCNT to BSR [PR101175]

    The current RTX pattern for BSR allows combine pass to convert LZCNT insn
    to BSR. Note that the LZCNT has a defined behavior to return the operand
    size when operand is zero, where BSR has not.

    Add a BSR specific setting of zero-flag to RTX pattern of BSR insn
    in order to avoid matching unwanted combinations.

    2021-06-23  UroÅ¡ Bizjak  <ubizjak@gmail.com>

    gcc/
            PR target/101175
            * config/i386/i386.md (bsr_rex64): Add zero-flag setting RTX.
            (bsr): Ditto.
            (*bsrhi): Remove.
            (clz<mode>2): Update RTX pattern for additions.

    gcc/testsuite/

            PR target/101175
            * gcc.target/i386/pr101175.c: New test.

    (cherry picked from commit 1e16f2b472c7d253d564556a048dc4ae16119c00)