Omp target data use_device_ptr vs use_device_addr

Accelerated Computing HPC Compilers nvc, nvc++ and nvfortran
1

Hello,

I have a question when translating OpenACC’s acc host_data use_device(arr), where arr is a Fortran allocatable array, to OpenMP. What’s the difference between omp target data use_device_ptr and use_device_addr? I’ve read the specification but don’t get it. Any advice would be appreciated!

Thanks,
Victor

2

Hi Victor,

I had to ask this too a few weeks ago since the description in the standard isn’t obvious.

use_device_ptr has been in the OpenMP standard longer. It is designed to support systems without a Unified Virtual Address Space. In principle, the pointer returned from use_device_ptr could be a host handle. The use_device_addr is a newer feature that assumes that the system has a Unified Virtual Address Space. For portability there is a requires unified_address to ensure that use_device_addr can safely be used.

However given the compiler can implicitly handle unified memory, the NVHPC compilers treat them the same.

-Mat

3

Hi Mat,

Thanks for your explanation! Good to know that they are treated equally by NVHPC.

Victor

4

Hi,

Just to chime in - it seems they are not treated equally after all, at least when using “nounified,nomanaged” or “mem:separate”.

In that case, when I use “addr” I get incorrect results, but using “ptr” works correctly.

– Ron