Use LLVM intrinsics for madd intrinsics
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Contributor
|
Can you add something like this /// This is a trick used in the adler32 algorithm to get a widening addition. The
/// multiplication by 1 is trivial, but must not be optimized out because then the vpmaddwd
/// instruction is no longer selected.
#[target_feature(enable = "avx2")]
#[cfg_attr(test, assert_instr(vpmaddwd))]
unsafe fn _mm256_madd_epi16_mul_one(mad: __m256i) -> __m256i {
let one_v = _mm256_set1_epi16(1);
_mm256_madd_epi16(mad, one_v)
}So that we remember in the future why we can't just use that generic implementation? |
Contributor
Author
|
I have added the following comments: // It's a trick used in the Adler-32 algorithm to perform a widening addition.
//
// ```rust
// #[target_feature(enable = "sse2")]
// unsafe fn widening_add(mad: __m128i) -> __m128i {
// _mm_madd_epi16(mad, _mm_set1_epi16(1))
// }
// ```
//
// If we implement this using generic vector intrinsics, the optimizer
// will eliminate this pattern, and `pmaddwd` will no longer be emitted.
// For this reason, we use x86 intrinsics. |
folkertdev
reviewed
Jan 1, 2026
Comment on lines
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| #[rustc_const_unstable(feature = "stdarch_const_x86", issue = "149298")] | ||
| pub const fn _mm_madd_epi16(a: __m128i, b: __m128i) -> __m128i { | ||
| unsafe { | ||
| let r: i32x8 = simd_mul(simd_cast(a.as_i16x8()), simd_cast(b.as_i16x8())); | ||
| let even: i32x4 = simd_shuffle!(r, r, [0, 2, 4, 6]); | ||
| let odd: i32x4 = simd_shuffle!(r, r, [1, 3, 5, 7]); | ||
| simd_add(even, odd).as_m128i() | ||
| } | ||
| pub fn _mm_madd_epi16(a: __m128i, b: __m128i) -> __m128i { | ||
| // It's a trick used in the Adler-32 algorithm to perform a widening addition. | ||
| // | ||
| // ```rust | ||
| // #[target_feature(enable = "sse2")] | ||
| // unsafe fn widening_add(mad: __m128i) -> __m128i { | ||
| // _mm_madd_epi16(mad, _mm_set1_epi16(1)) | ||
| // } | ||
| // ``` | ||
| // | ||
| // If we implement this using generic vector intrinsics, the optimizer | ||
| // will eliminate this pattern, and `pmaddwd` will no longer be emitted. | ||
| // For this reason, we use x86 intrinsics. | ||
| unsafe { transmute(pmaddwd(a.as_i16x8(), b.as_i16x8())) } | ||
| } |
Contributor
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we could consider using https://doc.rust-lang.org/std/intrinsics/fn.const_eval_select.html so we don't loose all of the const stuff. Up to @sayantn though, I don't have full context on what we'd like to be const fn currently.
Contributor
Author
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#[target_feature] functions do not implement the Fn traits, while const_eval_select restricts FnOnce. So this does not seem feasible.
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rust-lang/rust#150560
r? @sayantn