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Fix "simd_support" feature #1056

Merged
merged 3 commits into from
Oct 15, 2020
Merged

Fix "simd_support" feature #1056

merged 3 commits into from
Oct 15, 2020

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vks
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@vks vks commented Oct 8, 2020

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This is necessary, because support for `__m64` was removed from nighly
Rust [1].

Fixes rust-random#1050.

[1] rust-lang/stdarch#823
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Looks good to me

src/distributions/utils.rs Outdated Show resolved Hide resolved
The specialized instructions are already available with `sse2`.
#[cfg(not(target_feature = "sse4.2"))]
#[cfg(not(target_feature = "sse2"))]
wmul_impl! { (u16x8, u32x8),, 16 }
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IIUC this is a 256-bit op and thus requires AVX?

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I think packed_simd provides a fallback implementation?

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Does it?

Can you at least clarify on what basis you decided to make this change?

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without 128-bit XMM multiplications, packed_simd falls back to a scalar implementation.

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In that case, why do we bother with these cfg selectors at all?

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I think it is to provide a specialized implementation that uses less instructions.

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So you're saying the use of a SSE4.2 target originally was a mistake?

Maybe it was, since the insrtuctions appear to be supported on SSE2: https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mm_mullo_epi16&expand=3967,3990

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I appears to be a mistake. I guess that was my bad

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vks commented Oct 15, 2020

Assuming @dhardy is satisfied, I'm going ahead and merge this.

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4 participants