These predictors are in my opinion not reliable and thus I decided to remove
them:
1) PRED_NEGATIVE_RETURN: probability is ~51%
2) PRED_RECURSIVE_CALL: there are 2 dominant edges that influence value to 63%;
w/o these edges it goes down to 52%
3) PRED_POLYMORPHIC_CALL: having very low coverage, probability is ~51%
4) PRED_INDIR_CALL: likewise
Question here is whether we want to remove them, or to predict them with a
'ignored'
flag? Doing that, we can measure statistics of the predictor in the future?
Martin
>From afbc86cb72eab37bcf6325954d0bf306b301f76e Mon Sep 17 00:00:00 2001
From: marxin <mli...@suse.cz>
Date: Thu, 28 Dec 2017 10:23:48 +0100
Subject: [PATCH 4/5] Remove predictors that are unrealiable.
gcc/ChangeLog:
2017-12-28 Martin Liska <mli...@suse.cz>
* predict.c (return_prediction): Do not predict
PRED_NEGATIVE_RETURN.
(tree_bb_level_predictions): Do not predict PRED_RECURSIVE_CALL.
(tree_estimate_probability_bb): Do not predict
PRED_POLYMORPHIC_CALL and PRED_INDIR_CALL.
* predict.def (PRED_INDIR_CALL): Remove unused predictors.
(PRED_POLYMORPHIC_CALL): Likewise.
(PRED_RECURSIVE_CALL): Likewise.
(PRED_NEGATIVE_RETURN): Likewise.
---
gcc/predict.c | 17 ++---------------
gcc/predict.def | 13 -------------
2 files changed, 2 insertions(+), 28 deletions(-)
diff --git a/gcc/predict.c b/gcc/predict.c
index 51fd14205c2..f53724792e9 100644
--- a/gcc/predict.c
+++ b/gcc/predict.c
@@ -2632,14 +2632,6 @@ return_prediction (tree val, enum prediction *prediction)
}
else if (INTEGRAL_TYPE_P (TREE_TYPE (val)))
{
- /* Negative return values are often used to indicate
- errors. */
- if (TREE_CODE (val) == INTEGER_CST
- && tree_int_cst_sgn (val) < 0)
- {
- *prediction = NOT_TAKEN;
- return PRED_NEGATIVE_RETURN;
- }
/* Constant return values seems to be commonly taken.
Zero/one often represent booleans so exclude them from the
heuristics. */
@@ -2820,9 +2812,6 @@ tree_bb_level_predictions (void)
DECL_ATTRIBUTES (decl)))
predict_paths_leading_to (bb, PRED_COLD_FUNCTION,
NOT_TAKEN);
- if (decl && recursive_call_p (current_function_decl, decl))
- predict_paths_leading_to (bb, PRED_RECURSIVE_CALL,
- NOT_TAKEN);
}
else if (gimple_code (stmt) == GIMPLE_PREDICT)
{
@@ -2880,12 +2869,10 @@ tree_estimate_probability_bb (basic_block bb, bool local_only)
something exceptional. */
&& gimple_has_side_effects (stmt))
{
+ /* Consider just normal function calls, skip indirect and
+ polymorphic calls as these tend to be unreliable. */
if (gimple_call_fndecl (stmt))
predict_edge_def (e, PRED_CALL, NOT_TAKEN);
- else if (virtual_method_call_p (gimple_call_fn (stmt)))
- predict_edge_def (e, PRED_POLYMORPHIC_CALL, NOT_TAKEN);
- else
- predict_edge_def (e, PRED_INDIR_CALL, TAKEN);
break;
}
}
diff --git a/gcc/predict.def b/gcc/predict.def
index fe72080d5bd..7291650d237 100644
--- a/gcc/predict.def
+++ b/gcc/predict.def
@@ -118,16 +118,6 @@ DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)
/* Branch guarding call is probably taken. */
DEF_PREDICTOR (PRED_CALL, "call", HITRATE (67), 0)
-/* PRED_CALL is not very reliable predictor and it turns out to be even
- less reliable for indirect calls and polymorphic calls. For spec2k6
- the predictio nis slightly in the direction of taking the call. */
-DEF_PREDICTOR (PRED_INDIR_CALL, "indirect call", HITRATE (86), 0)
-DEF_PREDICTOR (PRED_POLYMORPHIC_CALL, "polymorphic call", HITRATE (59), 0)
-
-/* Recursive calls are usually not taken or the function will recurse
- indefinitely. */
-DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call", HITRATE (75), 0)
-
/* Branch causing function to terminate is probably not taken. */
DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (66),
0)
@@ -138,9 +128,6 @@ DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (66), 0)
/* Branch ending with return constant is probably not taken. */
DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (65), 0)
-/* Branch ending with return negative constant is probably not taken. */
-DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)
-
/* Branch ending with return; is probably not taken */
DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (71), 0)
--
2.14.3
