[Bug c/120753] is_device_ptr does not compile if given a pointer which is a member of a struct, i.e. is_device_ptr(u.ptr), where mystruct u; and struct mystruct{double *ptr;int something;}; will fail to compile (note that map works in this case)

[schulz.benjamin at googlemail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120753

Benjamin Schulz <schulz.benjamin at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |---
             Status|RESOLVED                    |UNCONFIRMED

--- Comment #8 from Benjamin Schulz <schulz.benjamin at googlemail dot com> ---
Hi tobias,


The standard says:

Unless otherwise specified, a variable that is part of an aggregate variable
must not be a variable list item or an extended list item except 



C++ has no lists. That entire comment does not apply here..


But a member of a struct like 
struct myarray
{
double *data;
double *extents;
}

myarray t;

t.data

here is just a variable that is part of an aggregate variable and thus allowed.

The clauses in the mappings are exactly the same.

but the map macros will accept something like


map (to: u.t[0:20])

because u.t is just an aggregate type...


The workaround:

 double *tmp = u.t;
  #pragma omp target is_device_ptr(tmp)
    tmp[i] = 20;


makes not much sense,

since that will mean that one can not use the struct within the loop but just
needs to unpack it before into several variables...


whereas something like this would work:
#pragma omp target enter data map(alloc:u.t[0:20])
#pragma omp target teams distribute
    for(int i=1;i<20;i++)
    {
        u.t[i]=20;
    }



But the problem is that this needs that the space is allocated on the host.

When you want to create a temporary matrix that is just on gpu, the map macros
are not an option, because assume i want to create a struct on gpu, that
contains 16 gigabyte. The mapping macros would need that i allocate then this
memory on the host before I map them.