Re: Willing to contribute to the project Idea "Fortran – DO CONCURRENT"

[Tobias Burnus <tobias@codesourcery.com>]

Hi,

and welcome to the GCC / gfortran community.

On 23.02.23 14:15, varma datla via Fortran wrote:
> I am willing to contribute to the project idea "Fortran – DO CONCURRENT".

I hope the following helps a bit – it is admittedly a bit chaotic, but I
try to write something cleaner later.

But to have something to think of and to startwith, it should be sufficient:

I think there are two parts to it: First, to add the changes of newer
Fortran to gfortran and then to actually use them to generate
concurrently running code. (Internally, gfortran currently handles 'do
concurrent' to run mostly like a normal loop – except that it annotates
the loops are independent. – Real parallelization would be useful, however.)

If you want to see examples, see do_concurrent_1.f90 to
do_concurrent_6.f90 in gfortran's testsuite, i.e.
gcc/testsuite/gfortran.dg/ in the GCC sources. That's at
https://gcc.gnu.org/git/?p=gcc.git;a=tree;f=gcc/testsuite/gfortran.dg;hb=refs/heads/master
/ But it is best to download GCC yourself via Git as described at
https://gcc.gnu.org/git.html

I want to note that the DO CONCURRENT syntax also permits a mask
argument, like in 'do concurrent (i=1:5, j=1:5, (i/=j))' where the last
argument selects a subset.

For the Fortran standard, see https://gcc.gnu.org/wiki/GFortranStandards

Fortran 2018 (= 18-007r1) adds for locality specifiers: LOCAL,
LOCAL_INIT, SHARED and DEFAULT(NONE).

Fortran 202x alias 2023 adds 'reduce' as in 'do concurrent (i = 1, n)
reduce(+:a, b, c) reduce(max:d, e, f)'

I think the first step is to add parsing support for those new features,
i.e. store them, check for issues (diagnostic) and then fail with a
'sorry not yet implemented'.

The next step would be to implement LOCAL/LOCAL_INIT for running on the
host.

And then, finally, would be to translate into code which can then be run
concurrently. I was thinking of mapping it internally to OpenMP or
OpenACC, to be toggled via a commandline option like
-fdo-concurrent=<openmp,openmp-target,openacc,...>.

* * *

I think the first step would be to download GCC and build it. Something
like "git clone" as described above, then "mkdir build" (some
directory); "cd build" and then "../configure --prefix=where-to-install"
followed by "make -j12" and "make install". The "-j12" runs 12 build
jobs in parallel. How much to use depends on your system.

You probably need to install some development versions of libraries such
as ISL, gmp, mpfr and mpc. If you don't have them readily, an option is
to run ./contrib/download_prerequisites to download those and build them
automatically alongside GCC.

So far for now. If you have questions, please ask. — And I will try to
write something more structured later.

Tobias

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