[LLVMdev] On large vectors

[Renato Golin]

On 6 February 2013 17:03, Nadav Rotem <nrotem at apple.com> 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.

cheers,
--renato
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