[PATCH] PR libstdc++/68519 use native duration to avoid rounding errors

[Jonathan Wakely]

The result of system_clock::now() + duration<float>(1) is a
duration<float> with the same value as now(), due to the limited
precision of float.

This patch converts the duration to system_clock::duration before
doing the sum, so that we don't lose precision.

	PR libstdc++/68519
	* include/std/condition_variable (condition_variable::wait_for):
	Convert duration to native clock's duration before addition.
	* testsuite/30_threads/condition_variable/members/68519.cc: New test.

Tested powerpc64le-linux, committed to trunk.

-------------- next part --------------
commit bf3d2e0ce692a43f9370a3c98fd7dec62334ffe7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Dec 14 20:18:30 2017 +0000

    PR libstdc++/68519 use native duration to avoid rounding errors
    
            PR libstdc++/68519
            * include/std/condition_variable (condition_variable::wait_for):
            Convert duration to native clock's duration before addition.
            * testsuite/30_threads/condition_variable/members/68519.cc: New test.

diff --git a/libstdc++-v3/include/std/condition_variable b/libstdc++-v3/include/std/condition_variable
index 1d8f057ceb6..6d20d365531 100644
--- a/libstdc++-v3/include/std/condition_variable
+++ b/libstdc++-v3/include/std/condition_variable
@@ -135,14 +135,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       cv_status
       wait_for(unique_lock<mutex>& __lock,
 	       const chrono::duration<_Rep, _Period>& __rtime)
-      { return wait_until(__lock, __clock_t::now() + __rtime); }
+      {
+	using __dur = typename __clock_t::duration;
+	auto __reltime = chrono::duration_cast<__dur>(__rtime);
+	if (__reltime < __rtime)
+	  ++__reltime;
+	return wait_until(__lock, __clock_t::now() + __reltime);
+      }
 
     template<typename _Rep, typename _Period, typename _Predicate>
       bool
       wait_for(unique_lock<mutex>& __lock,
 	       const chrono::duration<_Rep, _Period>& __rtime,
 	       _Predicate __p)
-      { return wait_until(__lock, __clock_t::now() + __rtime, std::move(__p)); }
+      {
+	using __dur = typename __clock_t::duration;
+	auto __reltime = chrono::duration_cast<__dur>(__rtime);
+	if (__reltime < __rtime)
+	  ++__reltime;
+	return wait_until(__lock, __clock_t::now() + __reltime, std::move(__p));
+      }
 
     native_handle_type
     native_handle()
diff --git a/libstdc++-v3/testsuite/30_threads/condition_variable/members/68519.cc b/libstdc++-v3/testsuite/30_threads/condition_variable/members/68519.cc
new file mode 100644
index 00000000000..71c1d29e231
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/condition_variable/members/68519.cc
@@ -0,0 +1,51 @@
+// Copyright (C) 2017 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-do run }
+// { dg-options "-pthread"  }
+// { dg-require-effective-target c++11 }
+// { dg-require-effective-target pthread }
+// { dg-require-cstdint "" }
+// { dg-require-gthreads "" }
+
+#include <condition_variable>
+#include <testsuite_hooks.h>
+
+// PR libstdc++/68519
+
+bool val = false;
+std::mutex mx;
+std::condition_variable cv;
+
+void
+test01()
+{
+  for (int i = 0; i < 3; ++i)
+  {
+    std::unique_lock<std::mutex> l(mx);
+    auto start = std::chrono::system_clock::now();
+    cv.wait_for(l, std::chrono::duration<float>(1), [] { return val; });
+    auto t = std::chrono::system_clock::now();
+    VERIFY( (t - start) >= std::chrono::seconds(1) );
+  }
+}
+
+int
+main()
+{
+  test01();
+}