Zen found to be compatible with fma4 instructions
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With the advent of the successful “Zen” microarchitecture, AMD removed support for the FMA4 instruction set, at least on paper. Level1Techs discovered that Zen-based CPUs support FMA4 instructions, only the instruction set is not exposed to the operating system.
The Zen architecture is really compatible with FMA4
FMA is an efficient way of calculating linear algebra. FMA3 and FMA4 are not generations of the instruction set, but the digit denotes the number of operands per instruction. AMD introduced support for FMA3 in 2012 with its FX series processors, while Intel added support for FMA3 in 2013 with Haswell. The exact reasons AMD disabled FMA4 with ”Zen are unknown, but some developers speculate that it is because AMD's implementation of FMA4 is flawed, although it is more efficient with 33% more performance. Intel's adoption of FMA3 made it more popular and therefore more stable over the years.
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Level1Techs used an OpenBLAS FMA4 test program to confirm that powering Zen processors with FMA4 instructions will not only return an "illegal instruction" error, but the processor will also go ahead and complete the operation. This is interesting because FMA4 is not exposed as a CPUID bit, and the operating system has no idea that the processor supports the instruction. For linear algebra, FMA4 has proven to be more efficient than AVX in single and double precision.
We'll be watching for new information about AMD's decision to disable support for FMA4 on Zen-based processors, sure they had their reasons. What do you think of this AMD decision?
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