benchtests: Add pthread_once common-case test.

We have a single thread that runs a no-op initialization once and then
repeatedly runs checks of the initialization (i.e., an acquire load and
conditional jump) in a tight loop.  This gives us, on average, the
best-case latency of pthread_once (the initialization is the
exactly-once slow path, and we're not looking at initialization-related
synchronization overheads in this case).

ChangeLog

@@ -1,3 +1,9 @@
+2014-04-10 Torvald Riegel  <triegel@redhat.com>
+
+	* benchtests/pthread_once-inputs: New file.
+	* benchtests/pthread_once-source.c: New file.
+	* benchtests/README: Update documentation.
+
 2014-04-09  Igor Zamyatin  <igor.zamyatin@intel.com>
 	    H.J. Lu  <hongjiu.lu@intel.com>
 

benchtests/Makefile

@@ -23,7 +23,7 @@ subdir := benchtests
 
 include ../Makeconfig
 bench := acos acosh asin asinh atan atanh cos cosh exp exp2 ffs ffsll \
-	log log2 modf pow rint sin sincos sinh sqrt tan tanh
+	log log2 modf pow pthread_once rint sin sincos sinh sqrt tan tanh
 
 # String function benchmarks.
 string-bench := bcopy bzero memccpy memchr memcmp memcpy memmem memmove \
@@ -53,6 +53,7 @@ LDLIBS-bench-exp2 = -lm
 LDLIBS-bench-log = -lm
 LDLIBS-bench-log2 = -lm
 LDLIBS-bench-pow = -lm
+LDLIBS-bench-pthread_once = -lpthread
 LDLIBS-bench-rint = -lm
 LDLIBS-bench-sin = -lm
 LDLIBS-bench-sinh = -lm

benchtests/README

@@ -60,6 +60,8 @@ one to add `foo' to the bench tests:
   - include-sources: This should be assigned a comma-separated list of source
     files that need to be included to provide definitions of global variables
     and functions (specifically, this includes using "#include "source").
+    See pthread_once-inputs and pthreads_once-source.c for an example of how
+    to use this to benchmark a function that needs state across several calls.
   - name: See following section for instructions on how to use this directive.
 
   Lines beginning with a single hash '#' are treated as comments.  See

benchtests/pthread_once-inputs

@@ -0,0 +1,9 @@
+# This tests the common-case scenario for pthread_once.
+# We have a single thread that runs a no-op initialization once and then
+# repeatedly runs checks of the initialization (i.e., an acquire load and
+# conditional jump) in a tight loop.
+# scripts/bench.py doesn't handle function pointers, so we just use void *:
+## args: pthread_once_t *:void *
+## includes: pthread.h
+## include-sources: pthread_once-source.c
+&once, once_handler

benchtests/pthread_once-source.c

@@ -0,0 +1,25 @@
+/* Measure pthread_once best-case latency (i.e., when already initialized.
+
+   Copyright (C) 2014 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+static pthread_once_t once = PTHREAD_ONCE_INIT;
+
+static void
+once_handler (void)
+{
+}