[2/2] PR libstdc++/41861 Add full steady_clock support to condition_variable
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Message ID 813a2918734ae172846d52f7d33f3dac34aabf9a.1563209229.git-series.mac@mcrowe.com
State New
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  • PR libstdc++/41861 Add full steady_clock support to condition_variable
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Commit Message

Mike Crowe July 15, 2019, 4:47 p.m. UTC
The pthread_cond_clockwait function was recently added[1] to glibc, and is
due to be released in glibc 2.30. If this function is available in the C
library it can be used it to fix
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=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.

[1] https://sourceware.org/git/?p=glibc.git;a=commit;h=afe4de7d283ebd88157126c5494ce1796194c16e

libstdc++-v3/

	* include/std/condition_variable: Add include of <bits/c++config.h>
	to make _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT available.
	(condition_variable): Split __clock_t into __system_clock_t, which
	is always system_clock and __best_clock_t, which is the clock that
	we wish to convert arbitrary other clocks to.  If we know that
	pthread_cond_clockwait is available then it is best to convert
	clocks to steady_clock, otherwise it's best to use
	system_clock. (wait_until): If pthread_cond_clockwait is available,
	provide a steady_clock overload.  Convert previous __clock_t
	overload to use __system_clock_t.  Convert generic overload to
	convert passed clock to __best_clock_t.  (wait_until_impl): Add
	steady_clock overload that calls pthread_cond_clockwait.  Convert
	previous __clock_t overload to use
	__system_clock_t. (condition_variable_any): Use steady_clock for
	__clock_t if pthread_cond_clockwait is available.

	* acinclude.m4:	Detect the availability of POSIX-proposed
	pthread_cond_clockwait function.
	* configure: Likewise.
	* configure.ac: Likewise.
	* config.h.in: Add _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT define to
	indicate presence of pthread_cond_clockwait function.
---
 libstdc++-v3/ChangeLog                      | 25 +++++++-
 libstdc++-v3/acinclude.m4                   | 31 ++++++++-
 libstdc++-v3/config.h.in                    |  3 +-
 libstdc++-v3/configure                      | 83 ++++++++++++++++++++++-
 libstdc++-v3/configure.ac                   |  3 +-
 libstdc++-v3/include/std/condition_variable | 52 ++++++++++++--
 6 files changed, 191 insertions(+), 6 deletions(-)

Patch
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diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index a52a704..597000b 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,5 +1,30 @@ 
 2019-07-15  Mike Crowe  <mac@mcrowe.com>
 
+	* include/std/condition_variable: Add include of <bits/c++config.h>
+	to make _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT available.
+	(condition_variable): Split __clock_t into __system_clock_t, which
+	is always system_clock and __best_clock_t, which is the clock that
+	we wish to convert arbitrary other clocks to.  If we know that
+	pthread_cond_clockwait is available then it is best to convert
+	clocks to steady_clock, otherwise it's best to use
+	system_clock. (wait_until): If pthread_cond_clockwait is available,
+	provide a steady_clock overload.  Convert previous __clock_t
+	overload to use __system_clock_t.  Convert generic overload to
+	convert passed clock to __best_clock_t.  (wait_until_impl): Add
+	steady_clock overload that calls pthread_cond_clockwait.  Convert
+	previous __clock_t overload to use
+	__system_clock_t. (condition_variable_any): Use steady_clock for
+	__clock_t if pthread_cond_clockwait is available.
+
+	* acinclude.m4:	Detect the availability of POSIX-proposed
+	pthread_cond_clockwait function.
+	* configure: Likewise.
+	* configure.ac: Likewise.
+	* config.h.in: Add _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT define to
+	indicate presence of pthread_cond_clockwait function.
+
+2019-07-15  Mike Crowe  <mac@mcrowe.com>
+
 	* testsuite/30_threads/condition_variable/members/2.cc (test01):
 	Parameterise so that test can be run against an arbitrary clock.
 	(main): Test using std::chrono::steady_clock and a user-defined
diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 24145fd..35979e7 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -4194,6 +4194,37 @@  AC_DEFUN([GLIBCXX_CHECK_PTHREADS_NUM_PROCESSORS_NP], [
 ])
 
 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
 AC_DEFUN([GLIBCXX_CHECK_SYSCTL_HW_NCPU], [
diff --git a/libstdc++-v3/config.h.in b/libstdc++-v3/config.h.in
index 99286e6..3d13402 100644
--- a/libstdc++-v3/config.h.in
+++ b/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
 
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index ab46399..c0b98d8 100755
--- a/libstdc++-v3/configure
+++ b/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
+$as_echo_n "checking for pthread_cond_clockwait... " >&6; }
+  if test "${glibcxx_cv_PTHREAD_COND_CLOCKWAIT+set}" = set; 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 :
diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
index 80d8202..ad4ae0c 100644
--- a/libstdc++-v3/configure.ac
+++ b/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
diff --git a/libstdc++-v3/include/std/condition_variable b/libstdc++-v3/include/std/condition_variable
index a83996a..0e1bdf7 100644
--- a/libstdc++-v3/include/std/condition_variable
+++ b/libstdc++-v3/include/std/condition_variable
@@ -35,6 +35,7 @@ 
 # include <bits/c++0x_warning.h>
 #else
 
+#include <bits/c++config.h>
 #include <chrono>
 #include <bits/std_mutex.h>
 #include <bits/unique_lock.h>
@@ -65,8 +66,13 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
   /// condition_variable
   class condition_variable
   {
-    typedef chrono::system_clock	__clock_t;
     typedef chrono::steady_clock	__steady_clock_t;
+#if defined(_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT)
+    typedef chrono::steady_clock	__best_clock_t;
+#else
+    typedef chrono::system_clock	__best_clock_t;
+#endif
+    typedef chrono::system_clock	__system_clock_t;
     typedef __gthread_cond_t		__native_type;
 
 #ifdef __GTHREAD_COND_INIT
@@ -101,10 +107,18 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  wait(__lock);
       }
 
+#if defined(_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT)
+    template<typename _Duration>
+      cv_status
+      wait_until(unique_lock<mutex>& __lock,
+		 const chrono::time_point<__steady_clock_t, _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_t, _Duration>& __atime)
       { return __wait_until_impl(__lock, __atime); }
 
     template<typename _Clock, typename _Duration>
@@ -112,9 +126,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       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 __best_clock_t::time_point __s_entry = __best_clock_t::now();
 	const auto __delta = __atime - __c_entry;
 	const auto __s_atime = __s_entry + __delta;
 
@@ -171,10 +184,33 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return &_M_cond; }
 
   private:
+#if defined(_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT)
+    template<typename _Dur>
+      cv_status
+      __wait_until_impl(unique_lock<mutex>& __lock,
+			const chrono::time_point<__steady_clock_t, _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_t::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_t, _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_t::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
   {
+#if defined(_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT)
+    typedef chrono::steady_clock	__clock_t;
+#else
     typedef chrono::system_clock	__clock_t;
+#endif
     condition_variable			_M_cond;
     shared_ptr<mutex>			_M_mutex;