Missed optimization: xor eax, eax is unnecessary on x86-64

[AZero13]

define i64 @tgt2(i64 %3) {
entry:          
  %4 = icmp eq i64 %3, 0
  %5 = sext i1 %4 to i64
  ret i64 %5
}

compiles to:

tgt2:                                   # @tgt2
        xor     eax, eax
        cmp     rdi, 1
        sbb     rax, rax
        ret

For x86-64.

This has a completely unnecessary xor eax, eax at the start

I assume the function is simply -!arg there

https://godbolt.org/z/zaPY53YT1

The sbb rax, rax renders it so.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Rose (AreaZR)

define i64 @tgt2(i64 %3) { entry: %4 = icmp eq i64 %3, 0 %5 = sext i1 %4 to i64 ret i64 %5 }

compiles to:

tgt2: # @tgt2
xor eax, eax
cmp rdi, 1
sbb rax, rax
ret

For x86-64.

This has a completely unnecessary xor eax, eax at the start

I assume the function is simply -!arg there

https://godbolt.org/z/zaPY53YT1

The sbb rax, rax renders it so.

[topperc]

sbb has a false dependency on the previous value of eax on Intel CPUs. This might be trying to break that dependency. See also #47201

[rilysh]

godbolt testcase: https://godbolt.org/z/bY9f87ofx
MSVC and GCC don't seem to address this yet

[boomanaiden154]

Passing -mtune=znver4 (or any other arch that has SBB64rr marked as a dependency breaking instruction in the scheduling model) gets rid of the extra xor.

Given this is fundamentally a performance optimization, I'm wondering if it's worth preventing the emission of the extra dependency breaking instruction when optimizing for size?