On Sun, 27 Jul 2025 16:57:14 +0200
Mikael Morin <morin-mik...@orange.fr> wrote:
> Le 27/07/2025 à 12:57, Andre Vehreschild a écrit :
> > Hi Mikael,
> >
> >>> In this example, image 1, i.e., for
> >>> Opencoarrays a thread on image one takes the data from the executing
> >>> image and writes it into the memory of image 1.
> >> When you say it takes data, do you mean it takes the assignment right
> >> hand side (named "data"), or do you mean that it takes all required data
> >> (right hand side "data" and index value initialized with the result of
> >> "get_val()") from the executing image?
> >
> > Both! Always keep in mind that an expression like
> >
> > res(this_image())[1] = 42
> >
> > executed on image 2, manipulates memory of process/image 1. When those
> > images are not running on the same machine (like in MPI possible), then the
> > (evaluated) index, here this_image(), and the evaluated rhs need to be
> > send to image 1 like in this example. On image 1 a routine is called,
> > that looks like this (pseudo C, abbreviated):
> >
> > void caf_accessor_res_1(struct array_integer_t &res, void *rhs, struct
> > add_data_t *add_data) {
> >
> > int *int_rhs = (int *)rhs;
> > res.data[add_data->this_image_val) = *int_rhs;
> > }
> >
> > The above routine is generated by the Fortran compiler from a gfc_code
> > structure that models it in Fortran. I went that way to have exactly the
> > assignment behavior of Fortran. This way assigning res(1:N)[...] = rhs(1:N)
> > does no trigger N communication for assigning scalars, but the vector is
> > send as a block and the loop to modify the data in res is done in the
> > accessor (significantly faster).
> >
> > This routine is executed on the remote image, here image 1. Note, that it
> > lacks the coindex now, because that is the implementation of coindexing.
> > For brevity I left out all the boilerplate that is implemented in
> > OpenCoarrays.
>
> If I rephrase the above on my example:
> https://gcc.gnu.org/pipermail/fortran/2025-July/062591.html
> with the assignment:
> res(get_val())[1] = data
>
> every image <n> does:
> rhs_n = data;
> idx_n = get_val();
>
> and the image <1> does:
> for each n:
> res(idx_n) = rhs_n;
>
> Do you confirm?
Absolutely confirmed!
>
> And now if I come back to your patch:
> https://gcc.gnu.org/pipermail/fortran/2025-July/062530.html
> the behaviour before the patch was different;
>
> every image <n> was doing:
> rhs_n = data;
>
> and the image <1> was doing:
> for each n:
> res(get_val()) = rhs_n;
>
> because get_val() is pure and takes zero argument.
>
> Still confirming?
And again confirmed. You got it!
- Andre
--
Andre Vehreschild * Email: vehre ad gmx dot de
