16.10.2019

AMD “Zen” Does Support FMA4, Just Not Exposed

With its “Zen” CPU microarchitecture, AMD removed support for the FMA4 instruction-set, on paper. This, while retaining FMA3. Level1Techs discovered that “Zen” CPUs do support FMA4 instructions, even through the instruction-set is not exposed to the operating system. FMA, or fused multiply add, is an efficient way to compute linear algebra. FMA3 and FMA4 are not generations of the instruction-set (unlike SSE3 and SSE4), but rather the digit denotes the number of operands per instruction. Support for both were introduced by AMD in 2012 with its FX-series processors, while Intel added FMA3 support in 2013 with “Ivy Bridge.”

The exact reasons why AMD deprecated FMA4 with “Zen” are unknown, but some developers speculate it’s because AMD’s implementation of FMA4 is buggy, even though it’s more efficient (33% more throughput). Intel’s adoption of FMA3 made it more popular, and hence more stable over the years. Level1Techs used an OpenBLAS FMA4 test-program to confirm that feeding “Zen” processors with FMA4 instructions won’t just return a “illegal instruction” error, but also the processor will go ahead and complete the operation. This is interesting because FMA4 isn’t exposed as a CPUID bit, and the operating system has no idea the processor even supports the instruction. For linear algebra, FMA4 has proven more efficient than AVX in both single- and double-precision.

Best of British brands: Jonathan Mildenhall on Cadbury Dairy Milk "nothing tastes better"

When I think about Cadbury Dairy Milk, the saying “absence makes the heart grow fonder” has never been more relevant. Twelve years ago, I emigrated to the US and, with that move, my favourite British brand and I became estranged. Thankfully, our home in San Francisco is always full of British friends and family and they all know what to bring with them when they visit. “If in doubt, get the CDM out!” So what is it that makes this brand so iconic in my heart? First, it’s one of the earliest brand preferences I can recall. Why would anyone eat any other chocolate? Second, the line “a glass and a half of full cream milk” just sounded so moreish and wholesome to me as a child. Third, the personality of the brand was nailed in my youth when Cilla Black fronted up the advertising. Finally, but most brilliantly, Cadbury “Gorilla” is one of the best ads of all time. More than a decade later, I still marvel on how it was able to capture that mind-altering moment when you pop that first chunk of CDM in your mouth. Nothing, I repeat nothing, tastes better than a bar of CDM.

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