Re: Fix merging of value predictors

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Jan 17, 2024 at 01:45:18PM +0100, Jan Hubicka wrote:
> Hi,
> expr_expected_value is doing some guesswork when it is merging two or more
> independent value predictions either in PHI node or in binary operation.
> Since we do not know how the predictions interact with each other, we can
> not really merge the values precisely.
> 
> The previous logic merged the prediciton and picked the later predictor
> (since predict.def is sorted by reliability). This however leads to troubles
> with __builtin_expect_with_probability since it is special cased as a predictor
> with custom probabilities.  If this predictor is downgraded to something else,
> we ICE since we have prediction given by predictor that is not expected
> to have customprobability.
> 
> This patch fixies it by inventing new predictors PRED_COMBINED_VALUE_PREDICTIONS
> and PRED_COMBINED_VALUE_PREDICTIONS_PHI which also allows custom values but
> are considered less reliable then __builtin_expect_with_probability (they
> are combined by ds theory rather then by first match).  This is less likely
> going to lead to very stupid decisions if combining does not work as expected.
> 
> I also updated the code to be bit more careful about merging values and do not
> downgrade the precision when unnecesary (as tested by new testcases).
> 
> Bootstrapped/regtested x86_64-linux, will commit it tomorrow if there are
> no complains.
> 
> 2024-01-17  Jan Hubicka <jh@suse.cz>
> 	    Jakub Jelinek <jakub@redhat.com>

2 spaces before < rather than 1.
> 
> 	PR tree-optimization/110852
> 
> gcc/ChangeLog:
> 
> 	* predict.cc (expr_expected_value_1):
> 	(get_predictor_value):
> 	* predict.def (PRED_COMBINED_VALUE_PREDICTIONS):
> 	(PRED_COMBINED_VALUE_PREDICTIONS_PHI):
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.dg/predict-18.c:

Please fill in what has changed, both for predict-18.c and predict.{cc,def}
changes.

> @@ -2613,24 +2658,40 @@ expr_expected_value_1 (tree type, tree op0, enum tree_code code,
>  	  if (!nop1)
>  	    nop1 = op1;
>  	 }
> +      /* We already checked if folding one of arguments to constant is good
> +	 enough.  Consequently failing to fold both means that we will not
> +	 succeed determinging the value.  */

s/determinging/determining/

Otherwise LGTM.

	Jakub