[llvm-bugs] [Bug 43238] New: Integer promotion quirk prevents efficient power of 2 division

via llvm-bugs llvm-bugs at lists.llvm.org
Fri Sep 6 05:17:37 PDT 2019


            Bug ID: 43238
           Summary: Integer promotion quirk prevents efficient power of 2
           Product: clang
           Version: 8.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: kim.nilsson at effnet.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Created attachment 22476
  --> https://bugs.llvm.org/attachment.cgi?id=22476&action=edit
Example LLVM IR

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/X-It3b for examples.

(FWIW, GCC also has this behavior and I've reported the same thing there

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