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Re: [PATCH][RFC] Move IVOPTs closer to RTL expansion


On Mon, Sep 9, 2013 at 10:01 AM, Richard Biener wrote:
>> >> First, the loop passes that at the moment preceede IVOPTs leave
>> >> around IL that is in desparate need of basic re-optimization
>> >> like CSE, constant propagation and DCE.  That puts extra load
>> >> on IVOPTs and its cost model, increasing compile-time and
>> >> possibly confusing it.

So why not just run DCE and DOM just before IVOPTs?


>> >> Second, IVOPTs introduces lowered memory accesses that it
>> >> expects to stay as is, likewise it produces auto-inc/dec
>> >> sequences that it expects to stay as is until RTL expansion.
>> >> Passes such as DOM can break this expectation and make the
>> >> work done by IVOPTs a waste.

But IVOPTs leaves its own messy code behind (see below).


>> >> I remember doing this excercise in the GCC 4.3 timeframe where
>> >> benchmarking on x86_64 showed no gains or losses (but x86_64
>> >> isn't very sensitive to IV choices).
>> >>
>> >> Any help with benchmarking this on targets other than x86_64
>> >> is appreciated (I'll re-do x86_64).

Targets like POWER and ARM would be interesting to test on.


> We already run LIM twice, moving the one that is currently after
> IVOPTs as well should be easy.  But of course as you note IVOPTs
> may introduce loop invariant code it also may introduce full
> redundancies in the way it re-writes IVs.  And for both people may
> claim that we have both CSE and LIM on the RTL level, too.

I would claim that relying on RTL (G)CSE and RTL LIM is a step in the
wrong direction. You end up creating a lot of garbage RTL, and many
transformations that are easy on GIMPLE cannot be performed anymore on
RTL.

Is it possible to make IVOPTs clean up after itself? It should be easy
for IVOPTs to notice that it creates loop-invariant code, and position
it on the loop pre-header edge. I suppose full redundancies are
harder, but I would expect that to happen less frequently (the only
situation I can think of right now is where a loop is rewritten with
two IVs where the two IVs share a common sub-expression).

Ciao!
Steven


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