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Add comment about rustc-constant-info to blend function

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Henry de Valence 2018-06-14 14:22:28 -07:00
parent 28f10bc183
commit cc8728b2a6
1 changed files with 20 additions and 0 deletions
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20
src/backend/avx2/field.rs
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@ -44,6 +44,26 @@ fn blend_lanes(x: u32x8, y: u32x8, control: Lanes) -> u32x8 {
unsafe { unsafe {
use core::arch::x86_64::_mm256_blend_epi32; use core::arch::x86_64::_mm256_blend_epi32;
// This would be much cleaner if we could factor out the match
// statement on the control. Unfortunately, rustc forgets
// constant-info very quickly, so we can't even write
// ```
// match control {
// Lanes::C => {
// let imm = C_LANES as i32;
// _mm256_blend_epi32(..., imm)
// ```
// let alone
// ```
// let imm = match control {
// Lanes::C => C_LANES as i32,
// }
// _mm256_blend_epi32(..., imm)
// ```
// even though both of these would be constant-folded by LLVM
// at a lower level (as happens in the shuffle implementation,
// which does not require a shuffle immediate but *is* lowered
// to immediate shuffles anyways).
match control { match control {
Lanes::C => { Lanes::C => {
_mm256_blend_epi32(x.into_bits(), y.into_bits(), C_LANES as i32).into_bits() _mm256_blend_epi32(x.into_bits(), y.into_bits(), C_LANES as i32).into_bits()