> Hi all,
>
> Currently all cases in switch statement are treated as having equal
> probabilities which causes suboptimal code as demonstrated in
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59521 . This patch
> modifies expander to select pivot point for decision tree so that
> probabilities of cases on the left are roughly equal to probabilities
> on the right.
>
> Patch survives bootstrap and regtesting on x64 but has some issues:
> * tests are fragile but I'm not sure how to make them better
> * I haven't done any performance measurements - would these be needed?
> I don't have access to SPEC these days, any other suggestions?
I think we could just check if daily testers shows some regressions after
patch is comitted. It seems right think to do.
>
> Patch is jointly authored with Martin.
2017-06-29 Yury Gribov <tetra2...@gmail.com>
Martin Liska <mar...@gcc.gnu.org>
PR middle-end/59521
gcc/
* predict.c (set_even_probabilities): Handle case of a single
likely edge.
I have made some changes to this fuction to fix other PR. So you may need
to update the patch. What exactly is set_even_probabilities and
combine_predictions_for_bb shooting for?
@@ -2451,7 +2484,30 @@ tree_predict_by_opcode (basic_block bb)
edge_iterator ei;
enum br_predictor predictor;
- if (!stmt || gimple_code (stmt) != GIMPLE_COND)
+ if (!stmt)
+ return;
+
+ if (gswitch *sw = dyn_cast <gswitch *> (stmt))
+ {
+ tree index = gimple_switch_index (sw);
+ tree val = expr_expected_value (index, auto_bitmap (),
+ &predictor);
+ if (val && TREE_CODE (val) == INTEGER_CST)
+ {
+ edge e = find_taken_edge_switch_expr (sw, bb, val);
+ if (predictor == PRED_BUILTIN_EXPECT)
+ {
+ int percent = PARAM_VALUE (BUILTIN_EXPECT_PROBABILITY);
+ gcc_assert (percent >= 0 && percent <= 100);
+ predict_edge (e, PRED_BUILTIN_EXPECT,
+ HITRATE (percent));
+ }
+ else
+ predict_edge_def (e, predictor, TAKEN);
+ }
+ }
+
+ if (gimple_code (stmt) != GIMPLE_COND)
I think this change can go in separately and is OK
(along with a testcase that checks that tree profile is right).
I will look into the RTL bits next.
Honza
