[llvm-commits] Reassociating for vectors

[Hal Finkel]

On Sat, 26 May 2012 18:18:53 +0200
Duncan Sands <baldrick at free.fr> wrote:

> Hi Hal,
> 
> >> it would be easy enough to have reassociate always write things
> >> out as some kind of balanced tree rather than linearly,
> >
> > Duncan,
> >
> > Can you sketch out how this would be done? It looks like you'd just
> > want to change the recursion used by Reassociate::RewriteExprTree,
> > is that right?
> 
> yes.
> 
> >> though I'm not sure
> >> what the best output order is.  For example in a+b+c+d+e+f+g+h with
> >> these being ever more complicated (i.e. ordered by increasing
> >> 'rank' as computed by reassociate), how should they be grouped?
> >> Should it be
> >>     ((a+b)+(c+d))+((e+f)+(g+h))
> >> ?  That would put high rank elements together (eg g+h).  Or should
> >> it be something like
> >>     ((a+e)+(c+g))+((d+h)+(b+f))
> >> which tries to spread low rank around.  Or something else?
> >
> > I'm not sure there is a best answer here, we'd probably want some
> > kind of relative-disbalance metric to decide. It probably also
> > depends on what else is going on (if there is other work to be
> > scheduled concurrently, then spreading the low rank around would
> > probably be good because it will leave plenty of interspersed spare
> > cycles in which to put other things). Does instruction scheduling
> > also understand about associative instructions? If it does, then
> > the choice may not matter in the same way.
> >
> > If it is easy enough to choose, we could implement both and then
> > experiment.
> >
> > Also, do we need to be careful about changing the default ordering
> > as it is used now because doing so might hurt CSE?
> 
> It should be easy enough to experiment with different versions of
> RewriteExprTree, I may try to whip something up.

Great, thanks!

> 
> Also, if the needs of the vectorizer turn out to not match those of
> reassociate, it would be easy to factorize LinearizeExprTree out of
> reassociate so it could be used both by reassociate and the
> vectorizer.

Sounds good.

 -Hal

> 
> Ciao, Duncan.



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory