On 7/4/19 6:02 PM, Jan Hubicka wrote:
> perhaps we want to also document that builtin-expect can be used this way?
> It owuld be also nice to have a testcase.
Good idea! I'm going to install the following patch.
Martin
>From 6b59938eff83600ae237409de027040b7904f66d Mon Sep 17 00:00:00 2001
From: Martin Liska <mli...@suse.cz>
Date: Tue, 9 Jul 2019 11:10:20 +0200
Subject: [PATCH] Document and test __builtin_expect_with_probability.
gcc/ChangeLog:
2019-07-09 Martin Liska <mli...@suse.cz>
* doc/extend.texi: Document influence on loop
optimizers.
gcc/testsuite/ChangeLog:
2019-07-09 Martin Liska <mli...@suse.cz>
* gcc.dg/predict-17.c: Test loop optimizer assumption
about loop iterations.
---
gcc/doc/extend.texi | 5 ++++-
gcc/testsuite/gcc.dg/predict-17.c | 4 +++-
2 files changed, 7 insertions(+), 2 deletions(-)
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index f2619e12f93..061607411eb 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -13045,8 +13045,11 @@ when testing pointer or floating-point values.
For the purposes of branch prediction optimizations, the probability that
a @code{__builtin_expect} expression is @code{true} is controlled by GCC's
@code{builtin-expect-probability} parameter, which defaults to 90%.
+
You can also use @code{__builtin_expect_with_probability} to explicitly
-assign a probability value to individual expressions.
+assign a probability value to individual expressions. If the built-in
+is used in a loop construct, the provided probability will influence
+the expected number of iterations made by loop optimizations.
@end deftypefn
@deftypefn {Built-in Function} long __builtin_expect_with_probability
diff --git a/gcc/testsuite/gcc.dg/predict-17.c b/gcc/testsuite/gcc.dg/predict-17.c
index 5069aa47c8c..45b618a942c 100644
--- a/gcc/testsuite/gcc.dg/predict-17.c
+++ b/gcc/testsuite/gcc.dg/predict-17.c
@@ -1,5 +1,5 @@
/* { dg-do compile } */
-/* { dg-options "-O2 -fdump-tree-profile_estimate" } */
+/* { dg-options "-O2 -fdump-tree-profile_estimate-details" } */
extern int global;
@@ -11,3 +11,5 @@ void foo (int base)
/* { dg-final { scan-tree-dump "first match heuristics: 5.00%" "profile_estimate"} } */
/* { dg-final { scan-tree-dump "__builtin_expect_with_probability heuristics of edge .*->.*: 5.00%" "profile_estimate"} } */
+/* { dg-final { scan-tree-dump "is probably executed at most 19" "profile_estimate"} } */
+
--
2.22.0
