Comment by camel-cdr | Hacker News Reader

Comment by camel-cdr | Hacker News Reader

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The SLP vectorizer is a good point, but I think it's, in comparison with x86, more a problem of the float and vector register files not being shared (in SVE and RVV). You don't need to reconfigure the vector length; just use it at the full width.

> Something like abseil's hash table

If I remember this correctly, the abseil lookup does scale with vector length, as long as you use the native data path width. (albeit with small gains) There is a problem with vector length agnostic handling of abseil, which is the iterator API. With a different API, or compilers that could eliminate redundant predicated load/stores, this would be easier.

> good for SIMT-like codes

Certainly, but I've also seen/written a lot of vector length agnostic code using shuffles, which don't fit into the SIMT paradigm, which means that the scope is larger than just SIMT.

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As a general comparison, take AVX10/128, AVX10/256 and AVX10/512, overlap their instruction encodings, remove the few instructions that don't make sense anymore, and add a cheap instruction to query the vector length. (probably also instructions like vid and viota, for easier shuffle synthesization) Now you have a variable-length SIMD ISA that feels familiar.

The above is basically what SVE is.

(For other readers:) This is what our Highway library does - wrapper functions around intrinsics, plus a (constexpr if possible) Lanes() function to query the length.

For very many cases, writing the code once for an 'unknown to the programmer' vector length indeed works.

One example that doesn't work so well is a sorting network; its size depends on the vector length. (I see you mention this below.)

I quite like highway.

As mentioned, last time I tried vqsort for RVV it was surprisingly slow.

I tried to replicate it yesterday, but noticed that vqsort is now disabled for RVV: https://github.com/google/highway/blob/400fbf20f2e40b984be12...

Does highway support sorting networks for non-128-bit vector registers?

When I tried to compile it for AVX512, the BaseCase seems to only use xmm registers: https://godbolt.org/z/qr9xoTGKn

:) Yes, vqsort recently tickled a bug in clang. I've seen a steady stream of issues, many caused by SLP or the seeming absence of CI. You might try re-enabling it on GCC.

Yes, the issue with the sorting network is that it is limited to 16x16 to reduce code explosion. With uint16_t, XMM are sufficient for the 8-column case; your Godbolt link does have some YMM for the 16-column case. When changing the type to sort to uint32_t, we see ZMM as expected.

Btw, here is a VLA vector register sort: https://godbolt.org/z/Env64961q

It has a few more instructions then the VLS version, but the critical dependency chain is the same.

It's also slightly less optimal on x86, because it alway uses lane crossing permutes. For AVX512 that is 5 out of 15 permutations that are vperm, but could've been vshuf. (if the loop isn't unrolled and optimized by the compiler)

I wasn't able to figure out how to implement the multi vector register sort in a VLA way.

Nice work :) Clang x86 indeed unrolls, which is good. But setting the CC and AA mask constants looks fairly expensive compared to fixed-pattern shuffles.

Yes, the 2D aspect of the sorting network complicates things. Transposing is already harder to make VLA and fusing it with the other shuffles certainly doesn't help.

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