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From: Neil <nddtwentyone@gmail.com>
Newsgroups: comp.lang.fortran
Subject: system_clock
Date: Mon, 4 Nov 2024 22:43:11 -0000 (UTC)
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Hello,

I have a question about the behaviour of system_clock.

Consider the following little program:

wayland-2022:~> cat t.f90
PROGRAM t
  IMPLICIT NONE
  INTEGER icount,icount_rate,icount_max
  CALL system_CLOCK(icount,icount_rate,icount_max)
  WRITE(*,*)'count, count_rate, count_max = ',icount,icount_rate,icount_max
END PROGRAM t

When I compile and (repeatedly) run this with nvfortran, it always
gives count=0:

wayland-2022:~> nvfortran --version

nvfortran 23.9-0 64-bit target on x86-64 Linux -tp cascadelake 
NVIDIA Compilers and Tools
Copyright (c) 2023, NVIDIA CORPORATION & AFFILIATES.  All rights reserved.
wayland-2022:~> nvfortran t.f90
wayland-2022:~> ./a.out 
 count, count_rate, count_max =             0      1000000   2147483647
wayland-2022:~> ./a.out 
 count, count_rate, count_max =             0      1000000   2147483647
wayland-2022:~> ./a.out 
 count, count_rate, count_max =             0      1000000   2147483647

This is surely incorrect behaviour?  It happens even if I insert some
delaying code before the call to system_clock.


By contrast the behaviour is as expected with gfortran, with a
plausible time interval of a second or so between my re-runs:

wayland-2022:~> gfortran --version
GNU Fortran (GCC) 8.5.0 20210514 (Red Hat 8.5.0-22)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

wayland-2022:~> gfortran t.f90 
wayland-2022:~> ./a.out 
 count, count_rate, count_max =    627874925        1000  2147483647
wayland-2022:~> ./a.out 
 count, count_rate, count_max =    627876335        1000  2147483647
wayland-2022:~> ./a.out 
 count, count_rate, count_max =    627878101        1000  2147483647

(The zero of time seems to be when the machine was powered on in this
case.)


Likewise ifort gives the sort of behaviour I would expect, with the
interval between re-runs being a second or so:

wayland-2022:~> ifort --version
ifort (IFORT) 2021.4.0 20210910
Copyright (C) 1985-2021 Intel Corporation.  All rights reserved.

wayland-2022:~> ifort t.f90
wayland-2022:~> ./a.out 
 count, count_rate, count_max =   1045988618       10000  2147483647
wayland-2022:~> ./a.out 
 count, count_rate, count_max =   1046003030       10000  2147483647
wayland-2022:~> ./a.out 
 count, count_rate, count_max =   1046017558       10000  2147483647


Why is nvfortran so different?  Is this a compiler bug or is it
something permitted by the standard?


Thanks very much,

Neil.