[Bug tree-optimization/126810] New: simplify a ctz of a bit reversal

"ktkachov at gcc dot gnu.org via Gcc-bugs" <[email protected]> Wed, 12 Aug 2026 07:23:47 +0000

Newsgroups	gmane.comp.gcc.bugs
Message-ID	<[email protected]/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126810

            Bug ID: 126810
           Summary: simplify a ctz of a bit reversal
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
  Target Milestone: ---

int f (unsigned x) { return __builtin_ctz (__builtin_bitreverse32 (x)); }

should be a single clz on aarch64 but GCC generates:
f:
        rbit    w0, w0
        rbit    w0, w0
        clz     w0, w0
        ret