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SGPR usage trippled on GCN-1.2 (v8) GPUs

Question asked by Linuxhippy on May 4, 2015
Latest reply on Aug 26, 2015 by youwei



I've analyzed an OpenCL-kernel using CodeXL and I am quite happy with the register-usage - on GCN 1.0/1.1 devices per SIMD the maximum of 10 wavefronts can be queued, so hopefully memory latencies can be hidden efficiently.

However on GCN-1.2 devices (Tonga), SGPRs usage exploded - while on Capverde the same kernel consumes 32 SPGRs, on Tonga 94 SGPRs are required which limits the kernel to 5 parallel waves per SIMD (screenshots attached).


Any idea why the same Code running on Tonga requires almost 3 times the SGPRs?

Have there been architectural changes to Tonga or are there pitfalls when it comes to SGPR usage?


Thank you in advance, Clemens