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[PATCH 2/2] condition_variable: Use steady_clock to implement wait_for


I believe[1][2] that the C++ standard says that
std::condition_variable::wait_for should be implemented to be equivalent
to:

 return wait_until(lock, chrono::steady_clock::now() + rel_time);

But the existing implementation uses chrono::system_clock. Now that
wait_until has potentially-different behaviour for chrono::steady_clock,
let's at least try to wait using the correct clock.

[1] https://en.cppreference.com/w/cpp/thread/condition_variable/wait_for
[2] https://github.com/cplusplus/draft/blob/master/source/threads.tex
---
 libstdc++-v3/ChangeLog                      | 3 +++
 libstdc++-v3/include/std/condition_variable | 5 +++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index ea7875ace9f..4500273ace7 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,4 +1,7 @@
 2018-07-09  Mike Crowe <mac@mcrowe.com>
+       * include/std/condition_variable (wait_for): Use steady_clock.
+
+2018-07-09  Mike Crowe <mac@mcrowe.com>
        * include/std/condition_variable (wait_until): Only report timeout
        if we really have timed out when measured against the
        caller-supplied clock.
diff --git a/libstdc++-v3/include/std/condition_variable b/libstdc++-v3/include/std/condition_variable
index a2d146a9b09..ce583990b9d 100644
--- a/libstdc++-v3/include/std/condition_variable
+++ b/libstdc++-v3/include/std/condition_variable
@@ -65,6 +65,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   class condition_variable
   {
     typedef chrono::system_clock       __clock_t;
+    typedef chrono::steady_clock       __steady_clock_t;
     typedef __gthread_cond_t           __native_type;

 #ifdef __GTHREAD_COND_INIT
@@ -142,11 +143,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       wait_for(unique_lock<mutex>& __lock,
               const chrono::duration<_Rep, _Period>& __rtime)
       {
-       using __dur = typename __clock_t::duration;
+       using __dur = typename __steady_clock_t::duration;
        auto __reltime = chrono::duration_cast<__dur>(__rtime);
        if (__reltime < __rtime)
          ++__reltime;
-       return wait_until(__lock, __clock_t::now() + __reltime);
+       return wait_until(__lock, __steady_clock_t::now() + __reltime);
       }

     template<typename _Rep, typename _Period, typename _Predicate>
--
2.11.0

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