PR libstdc++/41861 Add full steady_clock support to condition_variable

The pthread_cond_clockwait function is available in glibc since the 2.30
release. If this function is available in the C library it can be used
to fix PR libstdc++/41861 by supporting std::chrono::steady_clock
properly with std::condition_variable.

This means that code using std::condition_variable::wait_for or
std::condition_variable::wait_until with std::chrono::steady_clock is no
longer subject to timing out early or potentially waiting for much
longer if the system clock is warped at an inopportune moment.

If pthread_cond_clockwait is available then std::chrono::steady_clock is
deemed to be the "best" clock available which means that it is used for
the relative wait_for calls and absolute wait_until calls using
user-defined clocks. Calls explicitly using std::chrono::system_clock
continue to use CLOCK_REALTIME via __gthread_cond_timedwait.

If pthread_cond_clockwait is not available then
std::chrono::system_clock is deemed to be the "best" clock available
which means that the previous suboptimal behaviour remains.

2019-09-04  Mike Crowe  <mac@mcrowe.com>

	PR libstdc++/41861
	* acinclude.m4 (GLIBCXX_CHECK_PTHREAD_COND_CLOCKWAIT): Check for new
	pthread_cond_clockwait function.
	* configure.ac: Use GLIBCXX_CHECK_PTHREAD_COND_CLOCKWAIT.
	* configure: Regenerate.
	* config.h.in: Regenerate.
	* include/std/condition_variable: (condition_variable): Rename
	__steady_clock_t typedef and add system_clock. Change __clock_t to be
	a typedef for the preferred clock to convert arbitrary other clocks to.
	[_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT] (wait_until): Add a steady_clock
	overload.
	(wait_until): Change __clock_t overload to use system_clock.
	[_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT] (__wait_until_impl): Add
	steady_clock overload that calls pthread_cond_clockwait.
	(__wait_until_impl): Change __clock_t overload to use system_clock.
	(condition_variable_any) [_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT]: Use
	steady_clock for __clock_t if pthread_cond_clockwait is available.

From-SVN: r275390

libstdc++-v3/ChangeLog

@@ -1,3 +1,23 @@
+2019-09-04  Mike Crowe  <mac@mcrowe.com>
+
+	PR libstdc++/41861
+	* acinclude.m4 (GLIBCXX_CHECK_PTHREAD_COND_CLOCKWAIT): Check for new
+	pthread_cond_clockwait function.
+	* configure.ac: Use GLIBCXX_CHECK_PTHREAD_COND_CLOCKWAIT.
+	* configure: Regenerate.
+	* config.h.in: Regenerate.
+	* include/std/condition_variable: (condition_variable): Rename
+	__steady_clock_t typedef and add system_clock. Change __clock_t to be
+	a typedef for the preferred clock to convert arbitrary other clocks to.
+	[_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT] (wait_until): Add a steady_clock
+	overload.
+	(wait_until): Change __clock_t overload to use system_clock.
+	[_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT] (__wait_until_impl): Add
+	steady_clock overload that calls pthread_cond_clockwait.
+	(__wait_until_impl): Change __clock_t overload to use system_clock.
+	(condition_variable_any) [_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT]: Use
+	steady_clock for __clock_t if pthread_cond_clockwait is available.
+
 2019-09-04  Mike Crowe  <mac@mcrowe.com>
 
 	* testsuite/30_threads/condition_variable/members/2.cc (test01):

libstdc++-v3/acinclude.m4

@@ -4193,6 +4193,37 @@ AC_DEFUN([GLIBCXX_CHECK_PTHREADS_NUM_PROCESSORS_NP], [
   AC_LANG_RESTORE
 ])
 
+dnl
+dnl Check whether pthread_cond_clockwait is available in <pthread.h> for std::condition_variable to use,
+dnl and define _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT.
+dnl
+AC_DEFUN([GLIBCXX_CHECK_PTHREAD_COND_CLOCKWAIT], [
+
+  AC_LANG_SAVE
+  AC_LANG_CPLUSPLUS
+  ac_save_CXXFLAGS="$CXXFLAGS"
+  CXXFLAGS="$CXXFLAGS -fno-exceptions"
+  ac_save_LIBS="$LIBS"
+  LIBS="$LIBS -lpthread"
+
+  AC_MSG_CHECKING([for pthread_cond_clockwait])
+  AC_CACHE_VAL(glibcxx_cv_PTHREAD_COND_CLOCKWAIT, [
+    GCC_TRY_COMPILE_OR_LINK(
+      [#include <pthread.h>],
+      [pthread_mutex_t mutex; pthread_cond_t cond; struct timespec ts; int n = pthread_cond_clockwait(&cond, &mutex, 0, &ts);],
+      [glibcxx_cv_PTHREAD_COND_CLOCKWAIT=yes],
+      [glibcxx_cv_PTHREAD_COND_CLOCKWAIT=no])
+  ])
+  if test $glibcxx_cv_PTHREAD_COND_CLOCKWAIT = yes; then
+    AC_DEFINE(_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT, 1, [Define if pthread_cond_clockwait is available in <pthread.h>.])
+  fi
+  AC_MSG_RESULT($glibcxx_cv_PTHREAD_COND_CLOCKWAIT)
+
+  CXXFLAGS="$ac_save_CXXFLAGS"
+  LIBS="$ac_save_LIBS"
+  AC_LANG_RESTORE
+])
+
 dnl
 dnl Check whether sysctl is available in <pthread.h>, and define _GLIBCXX_USE_SYSCTL_HW_NCPU.
 dnl

libstdc++-v3/config.h.in

@@ -991,6 +991,9 @@
 /* Define if pthreads_num_processors_np is available in <pthread.h>. */
 #undef _GLIBCXX_USE_PTHREADS_NUM_PROCESSORS_NP
 
+/* Define if pthread_cond_clockwait is available in <pthread.h>. */
+#undef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
+
 /* Define if POSIX read/write locks are available in <gthr.h>. */
 #undef _GLIBCXX_USE_PTHREAD_RWLOCK_T
 

libstdc++-v3/configure

@@ -21581,6 +21581,89 @@ ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
 
+# For pthread_cond_clockwait
+
+
+
+  ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+  ac_save_CXXFLAGS="$CXXFLAGS"
+  CXXFLAGS="$CXXFLAGS -fno-exceptions"
+  ac_save_LIBS="$LIBS"
+  LIBS="$LIBS -lpthread"
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for pthread_cond_clockwait" >&5
+$as_echo_n "checking for pthread_cond_clockwait... " >&6; }
+  if ${glibcxx_cv_PTHREAD_COND_CLOCKWAIT+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+
+    if test x$gcc_no_link = xyes; then
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <pthread.h>
+int
+main ()
+{
+pthread_mutex_t mutex; pthread_cond_t cond; struct timespec ts; int n = pthread_cond_clockwait(&cond, &mutex, 0, &ts);
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  glibcxx_cv_PTHREAD_COND_CLOCKWAIT=yes
+else
+  glibcxx_cv_PTHREAD_COND_CLOCKWAIT=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+else
+  if test x$gcc_no_link = xyes; then
+  as_fn_error $? "Link tests are not allowed after GCC_NO_EXECUTABLES." "$LINENO" 5
+fi
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <pthread.h>
+int
+main ()
+{
+pthread_mutex_t mutex; pthread_cond_t cond; struct timespec ts; int n = pthread_cond_clockwait(&cond, &mutex, 0, &ts);
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_link "$LINENO"; then :
+  glibcxx_cv_PTHREAD_COND_CLOCKWAIT=yes
+else
+  glibcxx_cv_PTHREAD_COND_CLOCKWAIT=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+fi
+
+fi
+
+  if test $glibcxx_cv_PTHREAD_COND_CLOCKWAIT = yes; then
+
+$as_echo "#define _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT 1" >>confdefs.h
+
+  fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $glibcxx_cv_PTHREAD_COND_CLOCKWAIT" >&5
+$as_echo "$glibcxx_cv_PTHREAD_COND_CLOCKWAIT" >&6; }
+
+  CXXFLAGS="$ac_save_CXXFLAGS"
+  LIBS="$ac_save_LIBS"
+  ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
+
+
 
   ac_fn_c_check_header_mongrel "$LINENO" "locale.h" "ac_cv_header_locale_h" "$ac_includes_default"
 if test "x$ac_cv_header_locale_h" = xyes; then :

libstdc++-v3/configure.ac

@@ -220,6 +220,9 @@ GLIBCXX_ENABLE_LIBSTDCXX_TIME
 # Check for tmpnam which is obsolescent in POSIX.1-2008
 GLIBCXX_CHECK_TMPNAM
 
+# For pthread_cond_clockwait
+GLIBCXX_CHECK_PTHREAD_COND_CLOCKWAIT
+
 AC_LC_MESSAGES
 
 # For hardware_concurrency

libstdc++-v3/include/std/condition_variable

@@ -65,8 +65,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   /// condition_variable
   class condition_variable
   {
-    typedef chrono::system_clock	__clock_t;
-    typedef chrono::steady_clock	__steady_clock_t;
+    using steady_clock = chrono::steady_clock;
+    using system_clock = chrono::system_clock;
+#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
+    using __clock_t = steady_clock;
+#else
+    using __clock_t = system_clock;
+#endif
     typedef __gthread_cond_t		__native_type;
 
 #ifdef __GTHREAD_COND_INIT
@@ -101,18 +106,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  wait(__lock);
       }
 
+#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
+    template<typename _Duration>
+      cv_status
+      wait_until(unique_lock<mutex>& __lock,
+		 const chrono::time_point<steady_clock, _Duration>& __atime)
+      { return __wait_until_impl(__lock, __atime); }
+#endif
+
     template<typename _Duration>
       cv_status
       wait_until(unique_lock<mutex>& __lock,
-		 const chrono::time_point<__clock_t, _Duration>& __atime)
+		 const chrono::time_point<system_clock, _Duration>& __atime)
       { return __wait_until_impl(__lock, __atime); }
 
     template<typename _Clock, typename _Duration>
       cv_status
       wait_until(unique_lock<mutex>& __lock,
 		 const chrono::time_point<_Clock, _Duration>& __atime)
       {
-	// DR 887 - Sync unknown clock to known clock.
 	const typename _Clock::time_point __c_entry = _Clock::now();
 	const __clock_t::time_point __s_entry = __clock_t::now();
 	const auto __delta = __atime - __c_entry;
@@ -145,11 +157,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       wait_for(unique_lock<mutex>& __lock,
 	       const chrono::duration<_Rep, _Period>& __rtime)
       {
-	using __dur = typename __steady_clock_t::duration;
+	using __dur = typename steady_clock::duration;
 	auto __reltime = chrono::duration_cast<__dur>(__rtime);
 	if (__reltime < __rtime)
 	  ++__reltime;
-	return wait_until(__lock, __steady_clock_t::now() + __reltime);
+	return wait_until(__lock, steady_clock::now() + __reltime);
       }
 
     template<typename _Rep, typename _Period, typename _Predicate>
@@ -158,11 +170,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	       const chrono::duration<_Rep, _Period>& __rtime,
 	       _Predicate __p)
       {
-	using __dur = typename __steady_clock_t::duration;
+	using __dur = typename steady_clock::duration;
 	auto __reltime = chrono::duration_cast<__dur>(__rtime);
 	if (__reltime < __rtime)
 	  ++__reltime;
-	return wait_until(__lock, __steady_clock_t::now() + __reltime,
+	return wait_until(__lock, steady_clock::now() + __reltime,
 			  std::move(__p));
       }
 
@@ -171,10 +183,34 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return &_M_cond; }
 
   private:
+#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
+    template<typename _Dur>
+      cv_status
+      __wait_until_impl(unique_lock<mutex>& __lock,
+			const chrono::time_point<steady_clock, _Dur>& __atime)
+      {
+	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
+	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
+
+	__gthread_time_t __ts =
+	  {
+	    static_cast<std::time_t>(__s.time_since_epoch().count()),
+	    static_cast<long>(__ns.count())
+	  };
+
+	pthread_cond_clockwait(&_M_cond, __lock.mutex()->native_handle(),
+					 CLOCK_MONOTONIC,
+					 &__ts);
+
+	return (steady_clock::now() < __atime
+		? cv_status::no_timeout : cv_status::timeout);
+      }
+#endif
+
     template<typename _Dur>
       cv_status
       __wait_until_impl(unique_lock<mutex>& __lock,
-			const chrono::time_point<__clock_t, _Dur>& __atime)
+			const chrono::time_point<system_clock, _Dur>& __atime)
       {
 	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
 	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
@@ -188,7 +224,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	__gthread_cond_timedwait(&_M_cond, __lock.mutex()->native_handle(),
 				 &__ts);
 
-	return (__clock_t::now() < __atime
+	return (system_clock::now() < __atime
 		? cv_status::no_timeout : cv_status::timeout);
       }
   };
@@ -208,7 +244,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Like above, but mutex is not required to have try_lock.
   class condition_variable_any
   {
-    typedef chrono::system_clock	__clock_t;
+#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
+    using __clock_t = chrono::steady_clock;
+#else
+    using __clock_t = chrono::system_clock;
+#endif
     condition_variable			_M_cond;
     shared_ptr<mutex>			_M_mutex;