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.
