[PATCH, rs6000] Add expansions for min/max vector reductions

Richard Biener rguenther@suse.de
Fri Sep 18 08:39:00 GMT 2015

On Thu, 17 Sep 2015, Segher Boessenkool wrote:

> On Thu, Sep 17, 2015 at 09:18:42AM -0500, Bill Schmidt wrote:
> > On Thu, 2015-09-17 at 09:39 +0200, Richard Biener wrote:
> > > So just to clarify - you need to reduce the vector with max to a scalar
> > > but want the (same) result in all vector elements?
> > 
> > Yes.  Alan Hayward's cond-reduction patch is set up to perform a
> > reduction to scalar, followed by a scalar broadcast to get the value
> > into all positions.  It happens that our most efficient expansion to
> > reduce to scalar will naturally produce the value in all positions.
> It also is many insns after expand, so relying on combine to combine
> all that plus the following splat (as Richard suggests below) is not
> really going to work.
> If there also are targets where the _scal version is cheaper, maybe
> we should keep both, and have expand expand to whatever the target
> supports?

Wait .. so you don't actually have an instruction to do, say,
REDUC_MAX_EXPR (neither to scalar nor to vector)?  Then it's better
to _not_ define such pattern and let the vectorizer generate
its fallback code.  If the fallback code isn't "best" then better
think of a way to make it choose the best variant out of its
available ones (and maybe add another).  I think it tests
availability of the building blocks for the variants and simply
picks the first that works without checking the cost model.


> Segher

Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)

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