95 lines
3.7 KiB
Fortran
95 lines
3.7 KiB
Fortran
! Copyright (C) 2018-2020 Free Software Foundation, Inc.
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! Contributed by Steven G. Kargl <kargl@gcc.gnu.org>
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!
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! This file is part of the GNU Fortran runtime library (libgfortran).
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!
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! Libgfortran is free software; you can redistribute it and/or
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! modify it under the terms of the GNU General Public
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! License as published by the Free Software Foundation; either
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! version 3 of the License, or (at your option) any later version.
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!
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! Libgfortran is distributed in the hope that it will be useful,
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! but WITHOUT ANY WARRANTY; without even the implied warranty of
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! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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! GNU General Public License for more details.
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!
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! Under Section 7 of GPL version 3, you are granted additional
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! permissions described in the GCC Runtime Library Exception, version
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! 3.1, as published by the Free Software Foundation.
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!
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! You should have received a copy of the GNU General Public License and
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! a copy of the GCC Runtime Library Exception along with this program;
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! see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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! <http://www.gnu.org/licenses/>.
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!
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!
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! WARNING: This file should never be compiled with an option that changes
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! default logical kind from 4 to some other value or changes default integer
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! kind from from 4 to some other value.
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!
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!
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! There are four combinations of repeatable and image_distinct. If a program
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! is compiled without the -fcoarray= option or with -fcoarray=single, then
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! execution of the compiled executable does not use image_distinct as it is
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! irrelevant (although required). The behavior is as follows:
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!
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! call random_init(.true., .true.)
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!
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! The sequence of random numbers is repeatable within an instance of program
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! execution. That is, calls to random_init(.true., .true.) during the
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! execution will reset the sequence of RN to the same sequence. If the
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! program is compiled with -fcoarray=lib and multiple images are instantiated,
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! then each image accesses a repeatable distinct sequence of random numbers.
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! There are no guarantees that multiple execution of the program will access
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! the same sequence.
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!
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! call random_init(.false., .false.)
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! call random_init(.false., .true.)
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!
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! The sequence of random numbers is determined from process-dependent seeds.
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! On each execution of the executable, different seeds will be used. For
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! -fcoarray=lib and multiple instantiated images, each image will use
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! process-dependent seeds. In other words, the two calls have identical
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! behavior.
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!
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! call random_init(.true., .false.)
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!
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! For a program compiled without the -fcoarray= option or with
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! -fcoarray=single, a single image is instantiated when the executable is
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! run. If the executable causes multiple images to be instantiated, then
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! image_distinct=.false. in one image cannot affect the sequence of random
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! numbers in another image. As gfortran gives each image its own independent
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! PRNG, this condition is automatically satisfied.
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!
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impure subroutine _gfortran_random_init(repeatable, image_distinct, hidden)
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implicit none
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logical, value, intent(in) :: repeatable
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logical, value, intent(in) :: image_distinct
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integer, value, intent(in) :: hidden
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logical, save :: once = .true.
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integer :: nseed
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integer, save, allocatable :: seed(:)
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if (once) then
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once = .false.
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call random_seed(size=nseed)
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allocate(seed(nseed))
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call random_seed(get=seed)
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!
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! To guarantee that seed is distinct on multiple images, add the hidden
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! argument (which is the image index).
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!
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if (image_distinct) seed = seed + hidden
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end if
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if (repeatable) then
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call random_seed(put=seed);
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else
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call random_seed();
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end if
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end subroutine _gfortran_random_init
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