GeneratedEventTimestamp_plugin.cc broken in clang.
GeneratedEventTimestamp_plugin.cc gives a different and incorrect time in clang as compared to gcc.
I made a stand alone test program test.cxx (attached) containing the essential code copied from GeneratedEventTimestamp_plugin.cc (nutools v2_21_01)
$ clang++ test.cxx
$ g++ test.cxx
Timestamp in clang is too small by factor or 10^4. This is consistent with behavior observed in the wild using the art framework EmptyEvent source module.
#2 Updated by Lynn Garren about 2 years ago
One must, of course, use --std=c++17 when compiling with clang v5_0_1 and --std=c++14 when compiling with gcc v6_4_0. Using these gets rid of the warnings. Also, this code has not been changed in some time.
$ clang++ --std=c++17 -o clangtest test.cxx $ ./clangtest 8072336450453934
$ g++ --std=c++14 -o gcctest test.cxx $ ./gcctest 1524010889101558187
#3 Updated by Lynn Garren about 2 years ago
Here is a sample of a failure message:
%MSG-s ArtException: PostEndJob 18-Apr-2018 11:26:17 CDT ModuleEndJob cet::exception caught in art ---- OtherArt BEGIN ^[[93mTimeStampDecoder: I do not know how to convert this timestamp: 8130267357464517^[[00m ---- OtherArt END %MSG
#4 Updated by Kyle Knoepfel about 2 years ago
time_since_epoch() function returns an implementation-defined duration--i.e. the definition of the epoch does not necessarily correspond to January 1, 1970. See this StackOverflow response by Howard Hinnant, who along with Walter Brown and Marc Paterno designed the C++
#5 Updated by Gianluca Petrillo about 2 years ago
- Status changed from Assigned to Feedback
art::TimeStamp does not prescribe a specific interpretation of its value, nor LArSoft endorses any.
The intention of this plugin was to provide some source of randomness, which both GCC and Clang in their own ways do. From this point of view, the feature is not broken.
This said, it may be that you are using it beyond the way it was designed for, and maybe that extended use can be accommodated.
If you want to, please detail the use case and requirement this plugin fails to fulfill, and we can work out a solution.
If you don't have any follow up, instead, you can close this ticket.
#7 Updated by Gianluca Petrillo about 2 years ago
- Status changed from Feedback to Resolved
The timestamp plugin
GeneratedEventTimeStamp has been modified to add an offset to the high resolution clock to bring the timestamp in the vicinity of the number of nanoseconds past from the UNIX epoch (January 1, 1970). This is not the official definition of the timestamp.
The timestamp preserves the high resolution. The timestamp may be off the number of nanoseconds from the epoch, depending on the implementation of the C++ standard library. This error is currently none with GCC 6.4.0 (where the high resolution clock is already based on that definition) and it may be of about 1 µs in Clang 5.0 (which is the resolution of the system clock; the high resolution clock is not relative to the epoch).
Note that formally the code will be guaranteed to conform the definition only with C++20, which is going to require that the system clock be relative to the epoch, which is only de facto true now.
#8 Updated by Gianluca Petrillo about 2 years ago
The fix is uploaded in branch
It has been checked that on the same machine, Clang and GCC return timestamps with a similar value.
A "unit test" is provided, which does not really test anything and just runs
EmptyEvent source module with the plugin.
An analyzer module could be written to check that the timestamp is not too far from the expected value — I did not do that though.