[PATCH] Optimize unrolled reductions in LoopStrengthReduce

[Quentin Colombet]

On Feb 10, 2015, at 1:22 PM, Eric Christopher <echristo at gmail.com> wrote:

> Cautious is fine, but adding more register pressure heuristics to the middle end just gets us to a point where everything is poorly done via heuristics.

What do you think would be the right metric to control this optimization?
My inclination right now was to use the same, if possible, heuristic as the vectorizer, which IIRC, uses some register pressure heuristic.

I am not saying this is good, I am saying that like every unrolling technic we need to find a good balance between code size explosion and performance improvements. Exposing too much ILP is probably not a good idea.

-Quentin
> 
> -eric
> 
> On Tue Feb 10 2015 at 11:02:04 AM Quentin Colombet <qcolombet at apple.com> wrote:
> > FWIW I'm not such a huge fan of using register pressure heuristics in the middle end
> 
> 
> FWIW, I agree, but this is not what LSR currently does and since the backends do not expect that yet, I would prefer moving with cautious.
> 
> Q.
> 
> 
> http://reviews.llvm.org/D7128
> 
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