[libcxx-commits] [PATCH] D74341: [libc++][Apple] Use CLOCK_MONOTONIC_RAW instead of CLOCK_UPTIME_RAW for steady_clock

[Louis Dionne via Phabricator via libcxx-commits]

ldionne created this revision.
ldionne added reviewers: bruno, howard.hinnant.
Herald added a reviewer: EricWF.
Herald added subscribers: libcxx-commits, dexonsmith, jkorous, christof.
Herald added a project: libc++.
ldionne added a comment.
Herald added a reviewer: mclow.lists.

Specifically, I'm looking to understand why Eric's comment at that time (https://reviews.llvm.org/D27429#618296) about using `CLOCK_MONOTONIC_RAW` wasn't acted upon -- did we simply overlook it or is there a reason why `CLOCK_MONOTONIC_RAW` is not a suitable implementation?


In D27429 <https://reviews.llvm.org/D27429>, we switched the Apple implementation of steady_clock::now()
from clock_gettime(CLOCK_MONOTONIC) to clock_gettime(CLOCK_UPTIME_RAW).
The purpose was to get nanosecond precision, and also to improve the
performance of the implementation.

However, it appears that CLOCK_UPTIME_RAW does not satisfy the requirements
of the Standard, since it is not strictly speaking monotonic. Indeed, the
clock does not increment while the system is asleep, which had been
mentioned in D27429 <https://reviews.llvm.org/D27429> but somehow not addressed.

This patch switches to CLOCK_MONOTONIC_RAW, which is monotonic, increased
during sleep, and also has nanosecond precision.

https://llvm.org/PR44773


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D74341

Files:
  libcxx/src/chrono.cpp


Index: libcxx/src/chrono.cpp
===================================================================
--- libcxx/src/chrono.cpp
+++ libcxx/src/chrono.cpp
@@ -114,14 +114,14 @@
 
 #if defined(__APPLE__)
 
-// Darwin libc versions >= 1133 provide ns precision via CLOCK_UPTIME_RAW
-#if defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_UPTIME_RAW)
+// Darwin libc versions >= 1133 provide ns precision via CLOCK_MONOTONIC_RAW
+#if defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC_RAW)
 steady_clock::time_point
 steady_clock::now() _NOEXCEPT
 {
     struct timespec tp;
-    if (0 != clock_gettime(CLOCK_UPTIME_RAW, &tp))
-        __throw_system_error(errno, "clock_gettime(CLOCK_UPTIME_RAW) failed");
+    if (0 != clock_gettime(CLOCK_MONOTONIC_RAW, &tp))
+        __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC_RAW) failed");
     return time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
 }
 
@@ -177,7 +177,7 @@
     static FP fp = init_steady_clock();
     return time_point(duration(fp()));
 }
-#endif // defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_UPTIME_RAW)
+#endif // defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC_RAW)
 
 #elif defined(_LIBCPP_WIN32API)
 
@@ -206,9 +206,9 @@
 
 #elif defined(CLOCK_MONOTONIC)
 
-// On Apple platforms only CLOCK_UPTIME_RAW or mach_absolute_time are able to
-// time functions in the nanosecond range. Thus, they are the only acceptable
-// implementations of steady_clock.
+// On Apple platforms only CLOCK_UPTIME_RAW, CLOCK_MONOTONIC_RAW or
+// mach_absolute_time are able to time functions in the nanosecond range.
+// Thus, they are the only acceptable implementations of steady_clock.
 #ifdef __APPLE__
 #error "Never use CLOCK_MONOTONIC for steady_clock::now on Apple platforms"
 #endif


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