[LLVMdev] On large vectors

[David Given]

I have a simple expression-evaluation language using LLVM (it's at
https://cowlark.com/calculon, if anyone's interested). It has pretty
primitive support for 3-vectors, which I'm representing as a <3 x float>.

One of my users has asked for proper n-vector support, and I agree with
him so I'm adding that. However, he wants to use quite large vectors.
He's mentioned 30 elements, and I think he would like 50.

At what point do vectors stop being useful?

I've experimented with llc and, while large vectors *work*, the code
that's produced looks pretty scary (and I don't know enough about SSE
and AVX instructions to evaluate whether it's good or not). I can think
of various things I could do: passing input parameters by reference
instead of value, ditto with output parameters, falling back to
aggregates or plain arrays... but all this adds complexity to the code
generator, and I don't know if it's worth it (or at what point it
*becomes* worth it).

My JITted code consists of a single module with a single entry point,
and is all compiled up front with aggressive inlining. Vector parameters
are only ever used internally. I've noticed that LLVM does an excellent
job of optimising. Can I just use the naive approach and trust LLVM to
get on with it and make it work?

-- 
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