Re: [PATCH][PR 59521] Respect probabilities when expanding switch statement

[Martin Liška <mliska at suse dot cz>]

On 08/02/2017 11:53 AM, Jan Hubicka wrote:
> Hello,
> sorry for not responding for a while.  Martin Liska has patch to move switch
> expansion to gimple level that will likely simplify the code combinatoin.

Hello.

Yep, will land today to gcc-patches mailing list.

> 
>>
>> combine_predictions_for_bb calculates final probability for edges of
>> if-else or switch statements.
>>
>> For if-elses this is done by combining values computed by different
>> predictors using Dempster-Shafer theory.  For switch statement DS is
>> not used, mainly because we do not have heuristics for predicting
>> which case will be taken (paper by Larus concluded that using if-else
>> heuristics does not give good results).
>>
>> So until this patch we just used set_even_probabilities. The name of
>> this function is misleading, in addition to setting even probabilities
>> it can also understand that some edges are very unlikely and set
>> unlikely probs for those.  With patch it now also understands that one
>> edge is very likely.
> 
> I am not sure that the conclusion of Ball&Larus paper applies to us here.
> In addition to usual if-then-else heuristics we have those based on walk
> of CFG (such as ones predicting paths to unlikely calls) and those should
> work well on switch statements. 
> 
> We discussed adding predictor combining code for BBs with more than 2
> successors. Martin, do you have some code for that?

This has been discussed and we decided to reject that as we're unable to
apply DS theory as we can't evaluate what probability has a predictor for
edges different from the edge which it can evaluate. Note that with 2 edges
and probability x, one can calculate probability of the second edge
simply by 1 - x. That's not doable if one has > 2 edges. That was reason
why I decided to use DF theory for such situations and wrote just simple
handling of very {un,}likely probabilities.

Maybe I overlooked something in understanding of DF theory?

Martin 

> 
> I guess teaching even propbabilities about likely edges also works, but
> perhaps doing more general prediction combining would be cleaner...
> 
> Honza
>