[Bug middle-end/91680] New: Integer promotion quirk prevents efficient power of 2 division

[kim.nilsson at effnet dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91680

            Bug ID: 91680
           Summary: Integer promotion quirk prevents efficient power of 2
                    division
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kim.nilsson at effnet dot com
  Target Milestone: ---

Created attachment 46841
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46841&action=edit
Preprocessed source

For the function,

uint_fast32_t foo_a(uint_fast8_t x) {
    const uint_fast32_t q = 1 << x;
    return 256 / q;
}

GCC-{8.*,9.*} generates inefficient division instructions for both x86_64 and
ARM. Since 'q' is a power of 2, the result should be a single right shift.
Indeed, if the function is changed to,

uint_fast32_t foo_b(uint_fast8_t x) {
    return 256 / (1 << x); // This is, of course, equivalent to (256 >> x)
},

the expected instructions are generated. Counter-intuitively, if the original
function is instead changed slightly to,

uint_fast32_t foo_c(uint_fast8_t x) {
    const uint_fast32_t q = (uint_fast32_t)1 << x;
    return 256 / q;
},

the expected instructions are once again generated.

See https://godbolt.org/z/37K0Ig for examples.