libstdc++: Optimize synchronization in std::future if futexes are available.

	* src/c++11/futex.cc: New file.
	* include/bits/atomic_futex.h: New file.
	* include/std/future (__future_base::_State_baseV2): Use
	atomic_futex_unsigned instead of mutex+condvar.
	* src/c++11/futex.cc: Likewise.
	* include/Makefile.am: Add atomic_futex.h.
	* include/Makefile.in: Likewise.
	* src/c++11/Makefile.am: Add futex.cc.
	* src/c++11/Makefile.in: Likewise.

From-SVN: r219770

4 files changed, 103 insertions, 6 deletions

diff --git a/libstdc++-v3/src/c++11/Makefile.am b/libstdc++-v3/src/c++11/Makefile.am
index b57e552..90697d0 100644
--- a/libstdc++-v3/src/c++11/Makefile.am
+++ b/libstdc++-v3/src/c++11/Makefile.am
@@ -60,6 +60,7 @@ sources = \
 	debug.cc \
 	functexcept.cc \
 	functional.cc \
+	futex.cc \
 	future.cc \
 	hash_c++0x.cc \
 	hashtable_c++0x.cc \
diff --git a/libstdc++-v3/src/c++11/Makefile.in b/libstdc++-v3/src/c++11/Makefile.in
index 00a5c25..70d5bd7 100644
--- a/libstdc++-v3/src/c++11/Makefile.in
+++ b/libstdc++-v3/src/c++11/Makefile.in
@@ -74,7 +74,7 @@ libc__11convenience_la_LIBADD =
 am__objects_2 = ctype_configure_char.lo ctype_members.lo
 am__objects_3 = chrono.lo codecvt.lo condition_variable.lo \
 	cow-stdexcept.lo ctype.lo debug.lo functexcept.lo \
-	functional.lo future.lo hash_c++0x.lo hashtable_c++0x.lo \
+	functional.lo futex.lo future.lo hash_c++0x.lo hashtable_c++0x.lo \
 	ios.lo limits.lo mutex.lo placeholders.lo random.lo regex.lo \
 	shared_ptr.lo snprintf_lite.lo system_error.lo thread.lo \
 	$(am__objects_1) $(am__objects_2)
@@ -351,6 +351,7 @@ sources = \
 	debug.cc \
 	functexcept.cc \
 	functional.cc \
+	futex.cc \
 	future.cc \
 	hash_c++0x.cc \
 	hashtable_c++0x.cc \
diff --git a/libstdc++-v3/src/c++11/futex.cc b/libstdc++-v3/src/c++11/futex.cc
new file mode 100644
index 0000000..ad766c8
--- /dev/null
+++ b/libstdc++-v3/src/c++11/futex.cc
@@ -0,0 +1,97 @@
+// futex -*- C++ -*-
+
+// Copyright (C) 2015 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This 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 General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+#include <bits/c++config.h>
+#if defined(_GLIBCXX_HAVE_LINUX_FUTEX)
+#include <bits/atomic_futex.h>
+#include <chrono>
+#include <climits>
+#include <syscall.h>
+#include <unistd.h>
+#include <sys/time.h>
+#include <errno.h>
+#include <debug/debug.h>
+
+// Constants for the wait/wake futex syscall operations
+const unsigned futex_wait_op = 0;
+const unsigned futex_wake_op = 1;
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  bool
+  __atomic_futex_unsigned_base::_M_futex_wait_until(unsigned *__addr,
+      unsigned __val,
+      bool __has_timeout, chrono::seconds __s, chrono::nanoseconds __ns)
+  {
+    if (!__has_timeout)
+      {
+	// Ignore whether we actually succeeded to block because at worst,
+	// we will fall back to spin-waiting.  The only thing we could do
+	// here on errors is abort.
+	int ret __attribute__((unused));
+	ret = syscall (SYS_futex, __addr, futex_wait_op, __val);
+	_GLIBCXX_DEBUG_ASSERT(ret == 0 || errno == EINTR || errno == EAGAIN);
+	return true;
+      }
+    else
+      {
+	struct timeval tv;
+	gettimeofday (&tv, NULL);
+	// Convert the absolute timeout value to a relative timeout
+	struct timespec rt;
+	rt.tv_sec = __s.count() - tv.tv_sec;
+	rt.tv_nsec = __ns.count() - tv.tv_usec * 1000;
+	if (rt.tv_nsec < 0)
+	  {
+	    rt.tv_nsec += 1000000000;
+	    --rt.tv_sec;
+	  }
+	// Did we already time out?
+	if (rt.tv_sec < 0)
+	  return false;
+
+	if (syscall (SYS_futex, __addr, futex_wait_op, __val, &rt) == -1)
+	  {
+	    _GLIBCXX_DEBUG_ASSERT(errno == EINTR || errno == EAGAIN
+				  || errno == ETIMEDOUT);
+	    if (errno == ETIMEDOUT)
+	      return false;
+	  }
+	return true;
+      }
+  }
+
+  void
+  __atomic_futex_unsigned_base::_M_futex_notify_all(unsigned* __addr)
+  {
+    // This syscall can fail for various reasons, including in situations
+    // in which there is no real error.  Thus, we don't bother checking
+    // the error codes.  See the futex documentation and glibc for background.
+    syscall (SYS_futex, __addr, futex_wake_op, INT_MAX);
+  }
+
+}
+#endif
diff --git a/libstdc++-v3/src/c++11/future.cc b/libstdc++-v3/src/c++11/future.cc
index 1502ff6..c711a5f 100644
--- a/libstdc++-v3/src/c++11/future.cc
+++ b/libstdc++-v3/src/c++11/future.cc
@@ -89,11 +89,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     unique_ptr<_Make_ready> mr{static_cast<_Make_ready*>(p)};
     if (auto state = mr->_M_shared_state.lock())
       {
-	{
-	  lock_guard<mutex> __lock{state->_M_mutex};
-	  state->_M_ready = true;
-	}
-	state->_M_cond.notify_all();
+	// Use release MO to synchronize with observers of the ready state.
+	state->_M_status._M_store_notify_all(_Status::__ready,
+	    memory_order_release);
       }
   }