Re: targetm.vectorize.builtin_vec_perm

[Ira Rosen <IRAR at il dot ibm dot com>]

> > I can imagine having some sort of target hook that computed a cost
> > metric for a given constant permutation pattern.  For instance, I'd
> > imagine that the interleave patterns are half as expensive as a full
> > permute for altivec, due to not having to load a mask.  This hook would
> > be fairly complicated for x86, given all of the permuting insns that
> > were incrementally added in various ISA revisions, but such is life.
>
> There should be some way to account for the difference between the cost
> in straight-line code, where a mask load is a hard cost, a large loop,
> where the load can be hoisted at the cost of some target-dependent
> register pressure (e.g. being able to use inverted masks might save half
> of the cost), and a tight loop, where the constant load can be easily
> amortized over the entire loop.

Vectorizer cost model already does that. AFAIU, vectorizer cost model will
call the cost model hook to get a cost of a permute, and then incorporate
that cost into the general loop/basic block vectorization cost.

Ira