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12.1 AMD Radeon (GCN) ¶

On the hardware side, there is the hierarchy (fine to coarse):

• work item (thread)
• wavefront
• work group
• compute unit (CU)

All OpenMP and OpenACC levels are used, i.e.

• OpenMP’s simd and OpenACC’s vector map to work items (thread)
• OpenMP’s threads (“parallel”) and OpenACC’s workers map to wavefronts
• OpenMP’s teams and OpenACC’s gang use a threadpool with the size of the number of teams or gangs, respectively.

The used sizes are

• Number of teams is the specified num_teams (OpenMP) or num_gangs (OpenACC) or otherwise the number of CU. It is limited by two times the number of CU.
• Number of wavefronts is 4 for gfx900 and 16 otherwise; num_threads (OpenMP) and num_workers (OpenACC) overrides this if smaller.
• The wavefront has 102 scalars and 64 vectors
• Number of workitems is always 64
• The hardware permits maximally 40 workgroups/CU and 16 wavefronts/workgroup up to a limit of 40 wavefronts in total per CU.
• 80 scalars registers and 24 vector registers in non-kernel functions (the chosen procedure-calling API).
• For the kernel itself: as many as register pressure demands (number of teams and number of threads, scaled down if registers are exhausted)

The implementation remark:

• I/O within OpenMP target regions and OpenACC parallel/kernels is supported using the C library printf functions and the Fortran print/write statements.
• Reverse offload (i.e. target regions with device(ancestor:1)) are processed serially per target region such that the next reverse offload region is only executed after the previous one returned.
• OpenMP code that has a requires directive with unified_address or unified_shared_memory will remove any GCN device from the list of available devices (“host fallback”).
• The available stack size can be changed using the GCN_STACK_SIZE environment variable; the default is 32 kiB per thread.