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It would be very easy to support 512-bit vectors everywhere, and just emulate them on most systems with a small number of smaller vectors. It's easy for a compiler to generate good code for this. Clang does it well if you use its built-in vector types (which can be any length). Variable-length vectors, on the other hand, are a very challenging problem for compiler devs. You tend to get worse code out than if you just statically picked a size, even if it's not the native size.

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jandrewrogers 20 hours ago | parent | next [-]

The risk of 512-bit vectors everywhere is that many algorithms will spill the registers pretty badly if implemented in e.g. 128-bit vectors under the hood. In such cases you may be better off with a completely different algorithm implementation.

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Someone 7 hours ago | parent | prev | next [-]

> It would be very easy to support 512-bit vectors everywhere, and just emulate them on most systems with a small number of smaller vectors. It's easy for a compiler to generate good code for this

Wouldn’t that be suboptimal if/when CPUs that support 1024-bit vectors come along?

> Variable-length vectors, on the other hand, are a very challenging problem for compiler devs. You tend to get worse code out than if you just statically picked a size, even if it's not the native size.

Why would it be challenging? You could statically pick a size on a system with variable-length vectors, too. How would that be worse code?

	▲	kevingadd 43 minutes ago \| parent [-]
		Optimal performance in a vector algorithm typically requires optimizing around things like the number of available registers, whether the registers in use are volatile (mandating stack spills when calling other functions like a comparer), and sizes of sequences. If you know you're engineering for 16-byte vectors you can 'just' align all your data to 16 bytes. And if you know you have 8 vector registers where 4 of them are non-volatile you can design around that too. But without information like that you have to be defensive, like aligning all your data to 128 bytes instead Just In Case (heaven forbid native vectors get bigger than that), minimizing the number of registers you use to try and avoid stack spills, etc. (I mention this because WASM also doesn't expose any of this information.) It's true that you could just design for a static size on a system with variable-length vectors. I suspect you'd see a lot of people do that, and potentially under-utilize the hardware's capabilities. Better than nothing, at least!

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codedokode 12 hours ago | parent | prev [-]

Variable length vectors seem to be made for closed-source manually-written assembly (nobody wants to unroll the loop manually and nobody will rewrite it for new register width).