A 94x speed improvement demonstrated using handwritten assembly

Re:A 94x speed improvement demonstrated using handwritten as... by account42 on Monday November 04, 2024 @08:56AM (#42041546)

> This is a slightly misleading comparison, because they're comparing naive scalar C code to hand-vectorized assembly, skipping over hand-vectorized C with vendor intrinsics. Assembly of course always wins if enough effort is put into tuning it, but intrinsics can usually get extremely close with much less effort.

It's a perfectly valid comparision between straightforward C and the best hand optimization you can get.

> For reasons unknown to me the FFmpeg team has a weird vendetta against intrinsics, they require all platform-specific code to be written in assembly even if C with intrinsics would perform exactly the same.

Wanting to standardize on a single language for low level code is absolutely reasonable. This way contributors only need to know standard C as well as assembly instead of standar C, assembly and also intel intrisics which are a fusion of the two but also not the same as either and have their own gotchas.

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Re:A 94x speed improvement demonstrated using handwritten as... by mort96 on Monday November 04, 2024 @09:00AM (#42041580)

I mean currently, contributors who want to work on vectorized algorithms need to know C, amd64 assembly with SSE/AVX instructions, aarch64 assembly with NEON instructions and aarch64 assembly with SVE instructions (and presumably soon, or maybe already, risc-v assembly with the vector extension). I wouldn't say it's simpler than needing to know C, C with SSE/AVX instricsics, C with NEON intrinsics, C with SVE intrinsics and C with risc-v vector intrinsics.

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Re:A 94x speed improvement demonstrated using handwritten as... by astrange on Monday November 04, 2024 @03:48PM (#42045835)

You don't need to submit all of those, but generally these vectorized DSP functions are parts of decoding straight out of the spec, so there isn't a new way to do it.

Sometimes there is of course. In that case you can get someone else to help, or one platform just runs ahead of the others.

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Re:A 94x speed improvement demonstrated using handwritten as... by mort96 on Monday November 04, 2024 @04:07PM (#42045985)

Right, but you see my point, don't you? For every set of intrinsics you'd need to know as a contributor if the vectorized code was in C, there is an assembly language and a set of intrinsics you need to know as a contributor today. Moving to C wouldn't increase the set of things a contributor needs to know.

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