[LLVMdev] On large vectors

[David Given]

Renato Golin wrote:
[...]
>     I can see why freakishly large vectors would produce bad code.  The
>     type <50 x float> would be widened to the next power of two, and
>     then split over and over again until it fits into registers.  So,
>     any <50 x float> would take 16 XMM registers, that will be spilled.
>     The situation with integer types is even worse because you can
>     truncate or extend from one type to another.
> 
> In that sense, an inner loop with sequential access would be vectorized
> into much better code than having a <50 x float>. 
> 
> Whether this is something LLVM could do with <50 x float> or should
> always be up to the front-end developer, I don't know. It doesn't seem
> particularly hard to do it  in the vectorizer, but it's also probably
> won't be high on the TODO list for a while.

I have actually been reading up on the vectorizer. I'm using LLVM 3.2,
so the vectorizer isn't turned on by default. Would it be feasible to
explicitly *not* use vectors --- switching to aggregates instead --- and
then rely on the vectorizer to autovectorize the code where appropriate?

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