Unnecessary stack usage?

1

I have the following simple SIMD-powered function which tests whether points are inside one of bounding boxes:

pub unsafe fn foo(
    x: &[__m256i; N],
    y: &[__m256i; N],
    z: &[__m256i; N],
    bboxes: &[[__m256i; 6]],
) -> [__m256i; N] {
    let mut res = [_mm256_setzero_si256(); N];
    for bbox in bboxes {
        for i in 0..N {
            let tx = _mm256_and_si256(
                _mm256_cmpgt_epi32(x[i], bbox[0]),
                _mm256_cmpgt_epi32(bbox[1], x[i]),
            );
            let ty = _mm256_and_si256(
                _mm256_cmpgt_epi32(y[i], bbox[2]),
                _mm256_cmpgt_epi32(bbox[3], y[i]),
            );
            let t = _mm256_and_si256(tx, ty);
            let tz = _mm256_and_si256(
                _mm256_cmpgt_epi32(z[i], bbox[4]),
                _mm256_cmpgt_epi32(bbox[5], z[i]),
            );
            let t = _mm256_and_si256(t, tz);
            res[i] = _mm256_or_si256(res[i], t);
        }
    }
    res
}

By inspecting the generated assembly we can see that for some reason it caches coordinates to stack and reads them from it each iteration instead of using the input pointers. The same behavior can be observed for a function which processes coordinate slices. This caching looks quite redundant to me, especially considering that noalias is enabled (i.e. compiler should know that memory at which coordinates are stored can not change during function execution). Is there a reason for this behavior which I don't see or is it simply an LLVM quirk which produces sub-optimal results?

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2

For those who are interested I've created this issue in the Rust repo: Unnecessary stack usage · Issue #88930 · rust-lang/rust · GitHub

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