[Patch, Fortran] PR 55548: SYSTEM_CLOCK with integer(8) provides nanosecond resolution, but only microsecond precision (without -lrt)

[Janus Weil <janus at gcc dot gnu dot 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  <janus@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  <janus@gcc.gnu.org>

    PR fortran/55548
    * intrinsic.texi (SYSTEM_CLOCK): Update documentation of SYSTEM_CLOCK.

2012-12-01  Janus Weil  <janus@gcc.gnu.org>

    PR fortran/55548
    * gfortran.dg/system_clock_1.f90: New test case.

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