Indexing with `bool` generates suboptimal assembly

[alion02]

I tried this code:

fn f(a: &[u32; 2], c: bool) -> u32 {
    a[c as usize]
}

I expected to see this assembly:

f:
        mov     eax, dword ptr [rdi + 4*rsi]
        ret

Instead, I got this assembly:

f:
        mov     eax, esi ; zero-extend esi (bool param)
        mov     eax, dword ptr [rdi + 4*rax]
        ret

godbolt

I believe the ABI does not allow the upper 32 bits of rsi to be dirty when passing arguments smaller than 8 bytes, but I could be mistaken or otherwise misunderstanding the problem.

I could not find a workaround other than changing the type of c to usize and adding an unreachable_unchecked.

Meta

rustc --version --verbose:

rustc 1.78.0-nightly (d18480b84 2024-03-04)
binary: rustc
commit-hash: d18480b84fdbf1efc34f62070951334aa833d761
commit-date: 2024-03-04
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0
Compiler returned: 0

[alion02]

@rustbot modify labels: +A-codegen +I-slow +O-x86_64

[workingjubilee]

I believe the ABI does not allow the upper 32 bits of rsi to be dirty when passing arguments smaller than 8 bytes, but I could be mistaken or otherwise misunderstanding the problem.

Well, this ABI concern does not actually matter for an extern "Rust" fn, which is not required to adhere to the C ABI anyways, and thus we could use the fact that we don't have to worry about e.g. gcc and LLVM disagreeing on how to pass bools.

[stepantubanov]

I believe the ABI does not allow the upper 32 bits of rsi to be dirty when passing arguments smaller than 8 bytes, but I could be mistaken or otherwise misunderstanding the problem.

In x86-64 SysV ABI garbage is allowed in higher bits. But LLVM assumes there's no garbage up to 32 bits. But GCC for example compiles similar C++ code to movzx ..., sil (assumes no garbage in bool arg only up to 8 bits).

[lolbinarycat]

triage: still an issue in the latest nightly