RFC: Modeling horizontal vector reductions

[Arnold Schwaighofer]

On Sep 11, 2013, at 5:30 PM, Chandler Carruth <chandlerc at google.com> wrote:

> 
> On Wed, Sep 11, 2013 at 3:17 PM, Arnold Schwaighofer <aschwaighofer at apple.com> wrote:
> Therefore, I would like to model horizontal reductions as either versions depending on which is deemed cheaper by the cost model.
> 
> What would make the first pattern cheaper? I'd like to better understand why we don't just all ways do the second form…

Less shuffles (because shuffle vec, <0,1, undef, undef> is free) so when you don’t have pairwise vector operations the first pattern is preferable.


> It is a bit unfortunate to not have one canonical form but I don’t think this justifies adding fast-math flags to isel (which will eventually go away).
> 
> I don't really understand this part.
> 
> We have some reason at the IR level to know that we can choose either association and get equivalent results. Why isn't the correct answer to pick a canonical form, but preserve that information long enough to reassociate when it is needed?

We want to pick the right form at ISel time - which is too late to reassociate.

At some point - before ISel - we have to reasscociate because at ISel time we don’t have the fast-math flags that would tell us that it is legal to reassociate.

So, we could for example reassociate in CodeGen prepare. We would still need an interface to tell us when to do so.