[llvm-bugs] [Bug 50187] New: LLVM prefer lzcnt too strongly - extra computation required

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50187

            Bug ID: 50187
           Summary: LLVM prefer lzcnt too strongly - extra computation
                    required
           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: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, pengfei.wang at intel.com,
                    spatel+llvm at rotateright.com

long long clz64(long long num) {
  return 63LL - __builtin_clzll(num | 1);
}

int clz32(unsigned num) {
  return 31 - __builtin_clz(num | 1);
}

int clz32_a(unsigned num) {
  return 31 - __builtin_clz(num);
}


LLVM -march=haswell -O3 

clz64(long long):                              # @clz64(long long)
        or      rdi, 1
        lzcnt   rax, rdi
        xor     rax, 63
        ret
clz32(unsigned int):                              # @clz32(unsigned int)
        or      edi, 1
        lzcnt   eax, edi
        xor     eax, 31
        ret
clz32_a(unsigned int):                            # @clz32_a(unsigned int)
        lzcnt   eax, edi
        xor     eax, 31
        ret

GCC -march=haswell -O3 
clz64(long long):
        or      rdi, 1
        bsr     rax, rdi
        ret
clz32(unsigned int):
        or      edi, 1
        bsr     eax, edi
        ret
clz32_a(unsigned int):
        bsr     eax, edi
        ret


https://gcc.godbolt.org/z/r4bYfocKG

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