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

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Mar 14, 2022 at 04:42:14PM +0100, Marcel Vollweiler wrote:
> --- a/libgomp/libgomp.map
> +++ b/libgomp/libgomp.map
> @@ -226,6 +226,11 @@ OMP_5.1 {
>  	omp_get_teams_thread_limit_;
>  } OMP_5.0.2;
>  
> +OMP_5.1.1 {
> +  global:
> +	omp_target_is_accessible;
> +} OMP_5.1;
> +

You've already added another OMP_5.1.1 symbol, so this hunk will need to be
adjusted.  Keep the names in there alphabetically sorted.

> --- a/libgomp/omp_lib.f90.in
> +++ b/libgomp/omp_lib.f90.in
> @@ -835,6 +835,16 @@
>            end function omp_target_disassociate_ptr
>          end interface
>  
> +        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 :(.

> --- a/libgomp/target.c
> +++ b/libgomp/target.c
> @@ -3666,6 +3666,24 @@ omp_target_disassociate_ptr (const void *ptr, int device_num)
>  }
>  
>  int
> +omp_target_is_accessible (const void *ptr, size_t size, int device_num)
> +{
> +  if (device_num < 0 || device_num > gomp_get_num_devices ())
> +    return false;
> +
> +  if (device_num == gomp_get_num_devices ())
> +    return true;
> +
> +  struct gomp_device_descr *devicep = resolve_device (device_num);
> +  if (devicep == NULL)
> +    return false;
> +
> +  /* TODO: Unified shared memory must be handled when available.  */
> +
> +  return devicep->capabilities & GOMP_OFFLOAD_CAP_SHARED_MEM;

I guess for now it is reasonable, but I wonder if even without
GOMP_OFFLOAD_CAP_SHARED_MEM one can't for CUDA or GCN allocate host
memory (not all, but just some subset) that will be accessible on the
device (I bet that means accessible through the same address on the host and
device, aka partial shared mem).

So, ok for trunk.

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.

	Jakub