[llvm-bugs] [Bug 37974] New: Opportunity to use BTS instruction instead of cmpxchg

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37974

            Bug ID: 37974
           Summary: Opportunity to use BTS instruction instead of cmpxchg
           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

Hello,

Code:
bool foo(int *n, int bit) {
  unsigned mask = (1u << bit);

  return (__sync_fetch_and_or (n, mask) & mask) != 0;
}

Clang -O3:
foo(int*, int):
  mov edx, 1
  mov ecx, esi
  shl edx, cl
  mov eax, dword ptr [rdi]
.LBB0_1: 
  mov ecx, eax
  or ecx, edx
  lock cmpxchg dword ptr [rdi], ecx
  jne .LBB0_1
  test eax, edx
  setne al
  ret

GCC 7+ generates code using BTS instruction:
foo(int*, int):
  lock bts DWORD PTR [rdi], esi
  setc al
  ret


For more code examples see link:
https://godbolt.org/g/28nkRu

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