Ancestors

Written by Marc B. Reynolds on 2024-09-28 at 11:42

Noob compiler question. type-punning (via cast) a primitive array (say 64-bit integers) to a SIMD array is technically illegal (strict aliasing) but it's also (seemingly) ubiquitous. Do any of the main compilers (clang/gcc/vc) have a stance on this?

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Toot

Written by Harold Aptroot on 2024-09-28 at 12:31

@mbr direct access through the casted pointer, or shoving it into a load/store intrinsic?

A compiler that breaks the load/store intrinsics is straight up trolling.

Direct access.. who knows. Not very interesting IMO since it has another big problem: you cannot portably mark direct access as unaligned.

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Descendants

Written by Alexander Monakov on 2024-09-28 at 12:35

@harold @mbr you can make a typedef of a vector type with reduced alignment: https://godbolt.org/z/9fh5Wqh6h

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Written by Harold Aptroot on 2024-09-28 at 12:36

@amonakov @mbr yes but not portably

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Written by Alexander Monakov on 2024-09-28 at 12:48

@harold @mbr I'm not sure what you're implying? We do not have portable vector types in the first place. If this is about immintrin.h intrinsics on MSVC, from what I see you cannot even write vec1 + vec2, and the point of using C-style access when you need to write out all computations via intrinsics escapes me.

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Written by Harold Aptroot on 2024-09-28 at 12:57

@amonakov @mbr Isn't the thing we're talking about here, writing *(__m128i*)&something (direct access) or _mm_load(u)_si128((__m128i*)&something)?

Sure, doing it the first way isn't particularly of any use, since you could just do it the second way. But that first is somewhat used in practice too. And at least it can be talked about in a hypothetical sense

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Written by Alexander Monakov on 2024-09-28 at 13:12

@harold @mbr ah, fair — thanks

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