llvm/llvm-project
GitHub で見る[x86] Eliminate `movzx` when upper part of register is irrelevant or overwritten
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#168,828 opened on 2025年11月20日
backend:X86good first issuellvm:SelectionDAGmissed-optimization
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説明
export fn foo(x: u8) u32 {
return @ctz(x >> 1);
}
foo:
shr dil
movzx eax, dil
or eax, 256
tzcnt eax, eax
ret
Could be:
foo:
shr dil
or edi, 256
tzcnt eax, edi
ret
The compiler could also look for things like or eax, -256 and eliminate movzx eax, al since the relevant bits are overwritten anyway.