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

[Jonathan Wakely]

On 20/07/18 17:49 +0100, Mike Crowe wrote:
>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

Also committed to trunk. Thanks again.

>---
> 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 4657af7..432cb84 100644
>--- a/libstdc++-v3/ChangeLog
>+++ b/libstdc++-v3/ChangeLog
>@@ -1,4 +1,7 @@
> 2018-07-20  Mike Crowe <mac@mcrowe.com>
>+       * include/std/condition_variable (wait_for): Use steady_clock.
>+
>+2018-07-20  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 a2d146a..ce58399 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>
>--
>git-series 0.9.1
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