Re: [Patch] OpenMP, libgomp: Add new runtime routine omp_target_is_accessible.

[Tobias Burnus <Tobias_Burnus@mentor.com>]

Hi,

On 05.05.22 11:33, Jakub Jelinek via Gcc-patches wrote:
> On Mon, Mar 14, 2022 at 04:42:14PM +0100, Marcel Vollweiler wrote:
>> +        interface
>> +          function omp_target_is_accessible (ptr, size, device_num) bind(c)
>> +            use, intrinsic :: iso_c_binding, only : c_ptr, c_size_t, c_int
>> +            integer(c_int) :: omp_target_is_accessible
> The function returning integer(c_int) rather than logical seems like
> a screw up in the standard, but too late to fix that :(.

I think the idea is that it can directly call the C function without
needing a wrapper. And as default-kind 'logical' != 'integer(c_int)' in
general, it cannot return logical. (In case of GCC, just claiming that
it is logical would work. But some Fortran compilers use -1 for .true.
and only flip a single bit for .not. For those,
"if(.not.omp_target_is_accessible(..)) will not work properly, if the C
function returns 1.

But I concur that requiring "/= 0" is ugly!

> OT, tried to look how libomptarget implements it and they don't at least
> on llvm-project trunk, but while looking at that, noticed that for
> omp_target_is_present they do return false from omp_target_is_present
> while we return true.  It is unclear if NULL has corresponding storage
> on the device (NULL always corresponds to NULL on the device) or not.

Regarding NULL: no idea what's the best semantic – we could ask
for clarification.

Regarding target:
I think "false" from on device makes more sense in general, especially
if the device number points to a different device. It might work in some
cases – but false simply plays save. Note that the spec states:

"When called from within a target region the effect is unspecified."

Thus, either behavior is fine.

Tobias

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