[llvm-bugs] [Bug 37938] New: Missed opportunity for BTR instruction

via llvm-bugs llvm-bugs at lists.llvm.org
Tue Jun 26 04:06:31 PDT 2018


            Bug ID: 37938
           Summary: Missed opportunity for BTR instruction
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org


uint64_t btr_maybe(uint64_t x, unsigned bit) {
    return x & ~(1ULL << bit);

Clang -03 gives:
btr_maybe: # @btr_maybe
  mov rax, -2
  mov ecx, esi
  rol rax, cl
  and rax, rdi

But BTR instruction can be used (and GCC 8+ emits it):
btr_maybe(unsigned long, unsigned int):
  mov rax, rdi
  btr rax, rsi

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