[Bug target/123792] New: aarch64: stdc_bit_ceil generates inefficient code

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123792

            Bug ID: 123792
           Summary: aarch64: stdc_bit_ceil generates inefficient code
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wilco at gcc dot gnu.org
  Target Milestone: ---

This:

#include <stdbit.h>

long f (unsigned long n)
{
  return stdc_bit_ceil (n);
}

generates:

        sub     x1, x0, #1
        mov     w2, 63
        clz     x1, x1
        cmp     x0, 1
        sub     w2, w2, w1
        mov     x1, 2
        lsl     x1, x1, x2
        csinc   x0, x1, xzr, hi
        ret

The MOV/SUB can be replaced by MVN. The SUB/CMP can be combined into SUBS:

        subs    x1, x0, #1
        clz     x1, x1
        mvn     w2, w1
        mov     x1, 2
        lsl     x1, x1, x2
        csinc   x0, x1, xzr, hi
        ret