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[PATCH] Use __builtin_expect() within gcc_assert().
- From: David Daney <ddaney at caviumnetworks dot com>
- To: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 01 Jul 2009 09:29:08 -0700
- Subject: [PATCH] Use __builtin_expect() within gcc_assert().
This patch adds a __builtin_expect() to the gcc_assert macro. We
expect that all assertions will be true, so it seems plausible to use
__builtin_expect here.
Just above the patched chunk, we already add a definition of
__builtin_expect for compilers that don't implement it, so this should
not adversely affect the ability to bootstrap with non-GCC (or very
old GCC) compilers.
Before the patch in gcc/ (best of three consecutive runs):
$ rm *.o
$ time make cc1
.
.
.
real 6m7.041s
user 5m43.602s
sys 0m21.602s
After the patch
real 6m6.291s
user 5m42.921s
sys 0m21.763s
A savings of 0.681s of user time or 0.681/342 = 0.2%. Not a lot but
it seems to be consistently be a very small amount faster. However, I
don't discount the possibility that it could all be measurement noise
either.
Bootstrapped and tested on x86_64-pc-linux-gnu.
OK to commit?
2009-07-01 David Daney <ddaney@caviumnetworks.com>
* system.h (gcc_assert): Use __builtin_expect.
Index: system.h
===================================================================
--- system.h (revision 149061)
+++ system.h (working copy)
@@ -575,7 +575,8 @@ extern void fancy_abort (const char *, i
/* Use gcc_assert(EXPR) to test invariants. */
#if ENABLE_ASSERT_CHECKING
#define gcc_assert(EXPR) \
- ((void)(!(EXPR) ? fancy_abort (__FILE__, __LINE__, __FUNCTION__), 0 : 0))
+ ((void)(__builtin_expect (!(EXPR), 0) ? \
+ fancy_abort (__FILE__, __LINE__, __FUNCTION__), 0 : 0))
#else
/* Include EXPR, so that unused variable warnings do not occur. */
#define gcc_assert(EXPR) ((void)(0 && (EXPR)))