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simd sse2: si128/pd casts emitting conversions #55249

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kazcw opened this issue Oct 21, 2018 · 2 comments · Fixed by rust-lang/stdarch#581
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simd sse2: si128/pd casts emitting conversions #55249

kazcw opened this issue Oct 21, 2018 · 2 comments · Fixed by rust-lang/stdarch#581
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A-SIMD Area: SIMD (Single Instruction Multiple Data)

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@kazcw
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kazcw commented Oct 21, 2018

_mm_castsi128_pd and _mm_castpd_si128 should be no-op transmutes (as the corresponding intrinsics are in C), but they're emitting some kind of conversion involving cvtsi2sdq and cvttsd2si (respectively) that gives the wrong result. As a workaround I'm currently using mem::transmute to cast between __m128i and __m128d.

Documentation that these casts shouldn't do any conversion: https://msdn.microsoft.com/en-us/library/bb531385(v=vs.120).aspx

Demo of output with conversions: https://rust.godbolt.org/z/p1zG4C

(pinging simd tracker: #27731).
@Centril Centril added the A-SIMD Area: SIMD (Single Instruction Multiple Data) label Oct 22, 2018
@alexcrichton
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alexcrichton commented Oct 22, 2018 •
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Thanks for the report! On inspection I wonder if the implementation in stdsimd is simply not right for these intrinsics? It should likely be a simple fix for them in theory!

@kazcw kazcw mentioned this issue Oct 23, 2018
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@kazcw
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kazcw commented Oct 23, 2018

My first libstd PR!

@kazcw kazcw mentioned this issue Oct 23, 2018
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pietroalbini added a commit to pietroalbini/rust that referenced this issue Oct 23, 2018
@pietroalbini
Rollup merge of rust-lang#55291 - kazcw:master, r=nikomatsakis
b51c215 
Update stdsimd submodule

Fixes a SSE2 bug. (rust-lang#55249)
kennytm added a commit to kennytm/rust that referenced this issue Oct 25, 2018
@kennytm
Rollup merge of rust-lang#55291 - kazcw:master, r=nikomatsakis
163ffa0 
Update stdsimd submodule

Fixes a SSE2 bug. (rust-lang#55249)
pietroalbini added a commit to pietroalbini/rust that referenced this issue Oct 25, 2018
@pietroalbini
Rollup merge of rust-lang#55291 - kazcw:master, r=nikomatsakis
c4723b3 
Update stdsimd submodule

Fixes a SSE2 bug. (rust-lang#55249)
marysaka pushed a commit to sunriseos/rust that referenced this issue Oct 20, 2019
@kazcw @alexcrichton
fix _mm_castsi128_pd and _mm_castpd_si128 impls (rust-lang#581)
431766a 
* fix _mm_castsi128_pd and _mm_castpd_si128 impls

The _mm_castX_Y SSE intrinsics are "reinterpreting" casts; LLVM's
simd_cast is a "converting" cast. Replace simd_cast with mem::transmute.
Fixes rust-lang#55249

* Temporarily pin CI

* Fix i686 segfaults

* Fix wasm CI

Output of `wasm2wat` has changed!

* Fix AppVeyor with an older nightly
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fix _mm_castsi128_pd and _mm_castpd_si128 impls kazcw/stdsimd
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@alexcrichton @Centril @kazcw