Re: [PATCH 0/4 GCC11] IVOPTs consider step cost for different forms when unrolling

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Mon, Feb 10, 2020 at 02:17:04PM +0800, Kewen.Lin wrote:
> on 2020/1/20 下午8:33, Segher Boessenkool wrote:
> > On Thu, Jan 16, 2020 at 05:36:52PM +0800, Kewen.Lin wrote:
> >> As we discussed in the thread
> >> https://gcc.gnu.org/ml/gcc-patches/2020-01/msg00196.html
> >> Original: https://gcc.gnu.org/ml/gcc-patches/2020-01/msg00104.html,
> >> I'm working to teach IVOPTs to consider D-form group access during unrolling.
> >> The difference on D-form and other forms during unrolling is we can put the
> >> stride into displacement field to avoid additional step increment. eg:
> > 
> > <snip>
> > 
> >> Imagining that if the loop get unrolled by 8 times, then 3 step updates with
> >> D-form vs. 8 step updates with X-form. Here we only need to check stride
> >> meet D-form field requirement, since if OFF doesn't meet, we can construct
> >> baseA' with baseA + OFF.
> > 
> > So why doesn't the existing code do this already?  Why does it make all
> > the extra induction variables?  Is the existing cost model bad, are our
> > target costs bad, or something like that?
> > 
> 
> I think the main cause is IVOPTs runs before RTL unroll, when it's determining
> the IV sets, it can only take the normal step cost into account, since its
> input isn't unrolled yet.  After unrolling, the x-form indexing register has to
> play with more UF-1 times update, but we can probably hide them in d-form
> displacement field.  The way I proposed here is to adjust IV cost with
> additional cost_step according to estimated unroll.  It doesn't introduce new
> IV cand but can affect the final optimal set.

Yes, we should decide how often we want to unroll things somewhere before
ivopts already, and just use that info here.

Or are there advantage to doing it *in* ivopts?  It sounds like doing
it there is probably expensive, but maybe not, and we need to do similar
analysis there anyway.


Segher