[Bug rtl-optimization/105119] New: the division in x / (1 << y) is optimized away when x has unsigned type, but not when it's signed

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105119

            Bug ID: 105119
           Summary: the division in x / (1 << y) is optimized away when x
                    has unsigned type, but not when it's signed
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

For the following on x86_64 with -O2

int f(int x, int y) {
  return x / (1 << y);
}

GCC generates an idiv instruction:

        movl    %esi, %ecx
        movl    $1, %edx
        movl    %edi, %eax
        sall    %cl, %edx
        movl    %edx, %ecx
        cltd
        idivl   %ecx
        ret

But I believe this is equivalent to the division-less

int g(int x, int y) {
  return (x + (x < 0) * ((1 << y) - 1)) >> y;
}

(which basically generalizes the existing x / (1 << y) -> x >> y transformation
that we perform for unsigned x).  For this latter function, we generate

        movl    %esi, %ecx
        movl    $1, %eax
        xorl    %edx, %edx
        sall    %cl, %eax
        subl    $1, %eax
        testl   %edi, %edi
        cmovns  %edx, %eax
        addl    %edi, %eax
        sarl    %cl, %eax
        ret

which seems to be significantly faster according to some rough benchmarks.