llvm/llvm-project

~llvm.fsh[rl] patterns not recognized

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#173.132 aberto em 20 de dez. de 2025

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good first issuellvm:instcombinellvm:optimizationsmissed-optimization

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Description

Zig Godbolt LLVM Godbolt

Example 1:

export fn rotate(x: u64, y: u64) u64 {
    return x >> @truncate(~y) | x << @truncate(y + 1);
}

Should be:

rotate:
        lea     ecx, [rsi + 1]
        mov     rax, rdi
        rol     rax, cl
        ret

Example 2:

export fn rotate2(x: u64, y: u64) u64 {
    return x >> @truncate(y + 1) | x << @truncate(~y);
}

Should be:

rotate2:
        lea     ecx, [rsi + 1]
        mov     rax, rdi
        ror     rax, cl
        ret

Might not be useful in most situations, but these are also valid transformations:

rotate:
        mov     rcx, rsi
        not     cl
        mov     rax, rdi
        ror     rax, cl
        ret

rotate2:
        mov     rcx, rsi
        not     cl
        mov     rax, rdi
        rol     rax, cl
        ret

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