Hi, >> since the first version of my SYSTEM_CLOCK patch was not so well >> received, I propose here an alternative patch: It does not use a fixed >> COUNT_RATE, but adjusts it to match the resolution of the underlying >> library function which is used. For the int4 version it additionally >> limits COUNT_RATE to 1000, in order to provide a reasonable maximum >> timespan. The patch also has the effect that the reported COUNT_RATE >> can now depend e.g. on the linker flags used (as does the precision of >> the clock: 1 nanosec with -lrt and 1 microsec without). >> >> Does anyone agree with me that this represents an improvement of the >> SYSTEM_CLOCK behavior? > > this looks better than the first path, IMHO. I worry slightly that a > changing count rate could confuse users, but OTOH I agree you have a > point in that the current behavior can give users a false impression > of better accuracy than what is actually provided. > > An issue with the idea of changing count rate is that using an OS API > providing some particular resolution doesn't in itself guarantee that > the underlying hardware clock is actually capable of that resolution. > But I suppose there's not much we can do about that anyway (if one has > clock_gettime, one probably also has clock_getres, but at least on > Linux that seems to always return 1 ns as the resolution). > > Unless somebody else has objections, together with a documentation > patch and a ChangeLog entry, this is Ok for trunk.
thanks, Janne. Here is an updated patch with docu and ChangeLog. I
will wait another day before committing, in order to allow for further
comments.
Cheers,
Janus
2012-12-02 Janus Weil <ja...@gcc.gnu.org>
PR fortran/55548
* intrinsics/system_clock.c (gf_gettime_mono): Add argument 'tck',
which returns the clock resolution.
(system_clock_4): Get resolution from gf_gettime_mono, but limit to
1000/s.
(system_clock_8): Get resolution from gf_gettime_mono.
2012-12-02 Janus Weil <ja...@gcc.gnu.org>
PR fortran/55548
* intrinsic.texi (SYSTEM_CLOCK): Update documentation of SYSTEM_CLOCK.
>> 2012/12/1 Janus Weil <ja...@gcc.gnu.org>:
>>> Hi all,
>>>
>>> here is a straightforward patch for the intrinsic procedure
>>> SYSTEM_CLOCK. It does two things:
>>> 1) It reduces the resolution of the int8 version from 1 nanosecond to
>>> 1 microsecond (COUNT_RATE = 1000000).
>>> 2) It adds an int16 version with nanosecond precision.
>>>
>>> The motivation for item #1 was mainly that the actual precision is
>>> usually not better than 1 microsec anyway (unless linking with -lrt).
>>> This results in SYSTEM_CLOCK giving values whose last three digits are
>>> zero. One can argue that this is not a dramatic misbehavior, but it
>>> has disadvantages for certain applications, like e.g. using
>>> SYSTEM_CLOCK to initialize the random seed in a Monte-Carlo
>>> simulation. In general, I would say that the value of COUNT_RATE
>>> should not be larger than the actual precision of the clock used.
>>>
>>> Moreover, the microsecond resolution for int8 arguments has the
>>> advantage that it is compatible with ifort's behavior. Also I think a
>>> resolution of 1 microsecond is sufficient for most applications. If
>>> someone really needs more, he can now use the int16 version (and link
>>> with -lrt).
>>>
>>> Regtested on x86_64-unknown-linux-gnu (although we don't actually seem
>>> to have any test cases for SYSTEM_CLOCK yet). Ok for trunk?
>>>
>>> Btw, does it make sense to also add an int2 version? If yes, which
>>> resolution? Note that most other compilers seem to have an int2
>>> version of SYSTEM_CLOCK ...
>>>
>>> Cheers,
>>> Janus
>>>
>>>
>>> 2012-12-01 Janus Weil <ja...@gcc.gnu.org>
>>>
>>> PR fortran/55548
>>> * gfortran.map (GFORTRAN_1.5): Add _gfortran_system_clock_16.
>>> * intrinsics/system_clock.c (system_clock_8): Change resolution to
>>> one microsec.
>>> (system_clock_16): New function (with nanosecond resolution).
>>>
>>> 2012-12-01 Janus Weil <ja...@gcc.gnu.org>
>>>
>>> PR fortran/55548
>>> * intrinsic.texi (SYSTEM_CLOCK): Update documentation of SYSTEM_CLOCK.
>>>
>>> 2012-12-01 Janus Weil <ja...@gcc.gnu.org>
>>>
>>> PR fortran/55548
>>> * gfortran.dg/system_clock_1.f90: New test case.
>
>
>
> --
> Janne Blomqvist
pr55548_v3.diff
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