Attached is the patchset 2. It takes the max to two hitrates then does
the incremental.
On Fri, Oct 11, 2013 at 2:01 PM, Rong Xu <x...@google.com> wrote:
> Hi,
>
> An earlier patch (r203167) changed the default probability for
> builtin_expect to 90%.
> It does not work properly for the following case:
> while (__builin_expect (expr, 1)) { ....}
>
> W/o builtin_expect, the exit probability is 9% while w/
> builtin_expect, the exit probability is 10%.
> It seems to be wrong as we should estimate this loop has more
> iterations than w/o the annotation.
>
> This attached patch bump the probability for this particular case.
>
> Tested with bootstrap.
>
> Thanks,
>
> -Rong
2013-10-11 Rong Xu <x...@google.com>
* predict.c (tree_predict_by_opcode): Bump the
estimiated probability if builtin_expect expression is
in loop exit test.
Index: predict.c
===================================================================
--- predict.c (revision 203463)
+++ predict.c (working copy)
@@ -2001,11 +2001,39 @@ tree_predict_by_opcode (basic_block bb)
if (val)
{
int percent = PARAM_VALUE (BUILTIN_EXPECT_PROBABILITY);
+ void **preds;
+ int hitrate;
gcc_assert (percent >= 0 && percent <= 100);
+ /* This handles the cases like
+ while (__builtin_expect (exp, 1)) { ... }
+ W/o builtin_expect, the default HITRATE is 91%.
+ It does not make sense to estimate a lower probability of 90%
+ (current default for builtin_expect) with the annotation.
+ So here, we bump the probability by a small amount. */
+ preds = pointer_map_contains (bb_predictions, bb);
+ hitrate = HITRATE (percent);
+ if (preds)
+ {
+ struct edge_prediction *pred;
+ int exit_hitrate = predictor_info [(int) PRED_LOOP_EXIT].hitrate;
+
+ for (pred = (struct edge_prediction *) *preds; pred;
+ pred = pred->ep_next)
+ {
+ if (pred->ep_predictor == PRED_LOOP_EXIT
+ && exit_hitrate > hitrate)
+ {
+ hitrate = exit_hitrate + HITRATE (4);
+ if (hitrate > REG_BR_PROB_BASE)
+ hitrate = REG_BR_PROB_BASE;
+ break;
+ }
+ }
+ }
if (integer_zerop (val))
- percent = 100 - percent;
- predict_edge (then_edge, PRED_BUILTIN_EXPECT, HITRATE (percent));
+ hitrate = REG_BR_PROB_BASE - hitrate;
+ predict_edge (then_edge, PRED_BUILTIN_EXPECT, hitrate);
}
/* Try "pointer heuristic."
A comparison ptr == 0 is predicted as false.
