libstdc++: Optimise std::future::wait_for

To poll a std::future to see if it's ready you have to call one of the
timed waiting functions. The most obvious way is wait_for(0s) but this
was previously very inefficient because it would turn the relative
timeout to an absolute one by calling system_clock::now(). When the
relative timeout is zero (or less) we're obviously going to get a time
that has already passed, but the overhead of obtaining the current time
can be dozens of microseconds.

This patch makes future::wait_for avoid waiting at all when the relative
timeout is zero or less, to avoid the unnecessary overhead of getting
the current time. This makes polling with wait_for(0s) take only a few
cycles instead of dozens of milliseconds.

libstdc++-v3/ChangeLog:

	* include/std/future (future::wait_for): Do not wait for
	durations less than or equal to zero.
	* testsuite/30_threads/future/members/poll.cc: New test.

(cherry picked from commit 93fc47746815ea9dac413322fcade2931f757e7f)

diff --git a/libstdc++-v3/include/std/future b/libstdc++-v3/include/std/future
index 97506a27e37c821134c90de9dbd95d8360aff396..09f4af6fe7cde8d23ef4b6a6ebe80c3911d72ef4 100644 (file)
--- a/libstdc++-v3/include/std/future
+++ b/libstdc++-v3/include/std/future
@@ -346,10 +346,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          // to synchronize with the thread that made it ready.
          if (_M_status._M_load(memory_order_acquire) == _Status::__ready)
            return future_status::ready;
+
          if (_M_is_deferred_future())
            return future_status::deferred;
-         if (_M_status._M_load_when_equal_for(_Status::__ready,
-             memory_order_acquire, __rel))
+
+         // Don't wait unless the relative time is greater than zero.
+         if (__rel > __rel.zero()
+             && _M_status._M_load_when_equal_for(_Status::__ready,
+                                                 memory_order_acquire,
+                                                 __rel))
            {
              // _GLIBCXX_RESOLVE_LIB_DEFECTS
              // 2100.  timed waiting functions must also join
@@ -378,10 +383,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          // to synchronize with the thread that made it ready.
          if (_M_status._M_load(memory_order_acquire) == _Status::__ready)
            return future_status::ready;
+
          if (_M_is_deferred_future())
            return future_status::deferred;
+
          if (_M_status._M_load_when_equal_until(_Status::__ready,
-             memory_order_acquire, __abs))
+                                                memory_order_acquire,
+                                                __abs))
            {
              // _GLIBCXX_RESOLVE_LIB_DEFECTS
              // 2100.  timed waiting functions must also join

diff --git a/libstdc++-v3/testsuite/30_threads/future/members/poll.cc b/libstdc++-v3/testsuite/30_threads/future/members/poll.cc
new file mode 100644 (file)
index 0000000..1f54ce7
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/future/members/poll.cc
@@ -0,0 +1,106 @@
+// Copyright (C) 2020 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.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-O3" }
+// { dg-do run { target c++11 } }
+// { dg-additional-options "-pthread" { target pthread } }
+// { dg-require-gthreads "" }
+
+#include <future>
+#include <chrono>
+#include <iostream>
+#include <testsuite_hooks.h>
+
+const int iterations = 200;
+
+using namespace std;
+
+template<typename Duration>
+double
+print(const char* desc, Duration dur)
+{
+  auto ns = chrono::duration_cast<chrono::nanoseconds>(dur).count();
+  double d = double(ns) / iterations;
+  cout << desc << ": " << ns << "ns for " << iterations
+    << " calls, avg " << d << "ns per call\n";
+  return d;
+}
+
+int main()
+{
+  promise<int> p;
+  future<int> f = p.get_future();
+
+  auto start = chrono::high_resolution_clock::now();
+  for(int i = 0; i < iterations; i++)
+    f.wait_for(chrono::seconds(0));
+  auto stop = chrono::high_resolution_clock::now();
+  double wait_for_0 = print("wait_for(0s)", stop - start);
+
+  start = chrono::high_resolution_clock::now();
+  for(int i = 0; i < iterations; i++)
+    f.wait_until(chrono::system_clock::time_point::min());
+  stop = chrono::high_resolution_clock::now();
+  double wait_until_sys_min __attribute__((unused))
+    = print("wait_until(system_clock minimum)", stop - start);
+
+  start = chrono::high_resolution_clock::now();
+  for(int i = 0; i < iterations; i++)
+    f.wait_until(chrono::steady_clock::time_point::min());
+  stop = chrono::high_resolution_clock::now();
+  double wait_until_steady_min __attribute__((unused))
+    = print("wait_until(steady_clock minimum)", stop - start);
+
+  start = chrono::high_resolution_clock::now();
+  for(int i = 0; i < iterations; i++)
+    f.wait_until(chrono::system_clock::time_point());
+  stop = chrono::high_resolution_clock::now();
+  double wait_until_sys_epoch __attribute__((unused))
+    = print("wait_until(system_clock epoch)", stop - start);
+
+  start = chrono::high_resolution_clock::now();
+  for(int i = 0; i < iterations; i++)
+    f.wait_until(chrono::steady_clock::time_point());
+  stop = chrono::high_resolution_clock::now();
+  double wait_until_steady_epoch __attribute__((unused))
+    = print("wait_until(steady_clock epoch", stop - start);
+
+  p.set_value(1);
+
+  start = chrono::high_resolution_clock::now();
+  for(int i = 0; i < iterations; i++)
+    f.wait_for(chrono::seconds(0));
+  stop = chrono::high_resolution_clock::now();
+  double ready = print("wait_for when ready", stop - start);
+
+  // Polling before ready with wait_for(0s) should be almost as fast as
+  // after the result is ready.
+  VERIFY( wait_for_0 < (ready * 10) );
+
+  // polling before ready using wait_until(min) should not be terribly slow.
+  VERIFY( wait_until_sys_min < (ready * 100) );
+  // Converting from steady clock to system clock adds overhead before GCC 11.
+  VERIFY( wait_until_steady_min < (ready * 500) );
+
+  // The following two tests fail with GCC 11, see
+  // https://gcc.gnu.org/pipermail/libstdc++/2020-November/051422.html
+
+  // polling before ready using wait_until(epoch) should not be terribly slow.
+  VERIFY( wait_until_sys_epoch < (ready * 100) );
+  // Converting from steady clock to system clock adds overhead before GCC 11.
+  VERIFY( wait_until_steady_epoch < (ready * 500) );
+}