AMD VLIW4 GPUs and double precision floating point (fp64)

UPDATE: I think the 6900 series support for double-precision seems fine, but Trinity/Richland remains unexplained.

AMD has two VLIW4 based parts: Radeon 6900 series and Trinity/Richland APU’s GPU products (Radeon 7660D, 8650G etc.). Some of the launch media coverage stated that these GPUs have fp64 capability. I recently got a Richland based system to work on and realized the following:

a) AMD does not support cl_khr_fp64 (i.e. the standard OpenCL extension for fp64) on the 8650G GPU and only supports cl_amd_fp64. But AMD’s documentation is not very clear about the difference.

b) Earlier driver versions for Trinity (which afaik has the same silicon as Richland) definitely had cl_khr_fp64 support, but it was later removed and demoted to only cl_amd_fp64.

c) Richland’s GPU (8650G) does not seem to support double precision under Direct3D either.

d) Forum postings indicate latest drivers for 6900 series GPUs also do not support cl_khr_fp64, and only support cl_amd_fp64. I am not sure about the fp64 support status under DirectCompute.

My speculation is that AMD discovered some issue with IEEE compliance on fp64 units in the VLIW4 GPUs and hence AMD is unable to support APIs where full IEEE compliance is required. If anyone has any insight into the issue, then let me know.

One thought on “AMD VLIW4 GPUs and double precision floating point (fp64)”

  1. Hi Rahul – AMD has a number of products that have VLIW4 architecture graphics in them. Cayman GPU based designs can be found in consumer and professional graphics, and Trinity/Richland A-series APU’s feature VLIW4 architecture also – as do the FirePro equipped APU’s. Likely what you have found is a driver feature support matrix, not evidence of a lack of compliance in hardware implementation.

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