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Comment by ribit | Hacker News Reader

> Perhaps, though on VQSort it was more like 50% the performance.

I looked at the paper and my interpretation is that the performance delta between M1 (Neon) and the Xeon (AVX2) can be fully explained by the difference in clock (3.7 vs 3.3 Ghz) and the difference in L1D bandwidth (48byes/cycle vs. 128bytes/cycle). I don't see any evidence here that narrow SIMD is less efficient.

The AVX-512 is much faster, but that is because it has hardware features (most importantly, compact) that are central to the algorithm. On AVX2 and Neon these are emulated with slower sequences.

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Note that compact/compress are not actually the key enablers: also with AVX-512 we use table lookups for u64 keys, because this allows us to actually partition a vector and write it both to the left and write sides, as opposed to compressing twice and writing those individually.

Isn't the L1d bandwidth tied to the SIMD width, i.e., it would be unachievable on Skylake if also only using 128-bit vectors there?

> Note that compact/compress are not actually the key enablers: also with AVX-512 we use table lookups for u64 keys, because this allows us to actually partition a vector and write it both to the left and write sides, as opposed to compressing twice and writing those individually.

That is interesting! So do I understand you correctly that the 512b vectors allow you to implement the algorithm more efficiently? That would indeed be a nice argument for longer SIMD

> Isn't the L1d bandwidth tied to the SIMD width, i.e., it would be unachievable on Skylake if also only using 128-bit vectors there?

It's a hardware detail. Intel does tie it to SIMD width, but it doesn't have to be the case. For example, Apple has 4x128b units but can only load up to 48 bytes (I am not sure about the granularity of the loads) per cycle.

Right, longer vectors let us write more elements at a time.

I agree that the number of L1 load ports (or issue width) is also a parameter: that times the SIMD width gives us the bandwidth. It will be interesting to see what AMD Zen5 brings to the table here.

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