[Bug tree-optimization/94787] New: Failure to detect single bit popcount pattern

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94787

            Bug ID: 94787
           Summary: Failure to detect single bit popcount pattern
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

bool f(unsigned v)
{
    return v && !(v & (v - 1));
}

Depending on the supported architecture, we may want to optimize this to
`__builtin_popcount(v) == 1;` (when a fast hardware instruction is available).
For example, on x86, this should make programs faster when `popcnt` is
available. LLVM already does this.

Comparison with x86 `-mpopcnt` here : https://godbolt.org/z/M6f28d