[Bug target/115755] New: mulx (with -mbmi2) does not show up with constant multiply

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115755

            Bug ID: 115755
           Summary: mulx (with -mbmi2) does not show up with constant
                    multiply
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: x86_64-linux-gnu

Take:
```
unsigned long func(unsigned long x)
{
  __uint128_t t = x;
  return (t * 123)>>64;
}


__uint128_t func1(unsigned long x, unsigned long y)
{
  __uint128_t t = x;
  return (t * 123);
}
__uint128_t func2(unsigned long x, unsigned long y)
{
  __uint128_t t = x;
  return (t * y);
}
```

func2 does use mulx but func and func1 does not.