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ARM Chooses Variable-Length Vectors

January 31, 2017

Author: David Kanter

Pushing further into the server market, ARM has announced the Scalable Vector Extension (SVE), a novel 128- to 2,048-bit vector extension with a vector-length-agnostic programming model. SVE offers compatibility across a range of implementations. Its ambitious goal is to create a “universal” vector extension for the ARMv8-A ecosystem that focuses on high-performance computing (HPC), is orthogonal to the existing 128-bit Neon SIMD, and can compete with Intel’s AVX-512 extensions.

SVE makes complicated tradeoffs and sacrifices to fit in ARM’s fixed-length instruction encoding while allowing variable-length vectors, predication, and vector partitioning. Variable-length vectors with length-agnostic programming date back to the supercomputers of the 1970s. They present a unique challenge for software to exploit efficiently, but they promise value for the ecosystem. ARM wants to enable licensees to differentiate by customizing their own vector implementations yet ensure cross-vendor and forward compatibility. For example, server processors may implement wide vectors whereas mobile processors take a narrower approach.

Longer vector extensions are necessary but insufficient to effectively compete for HPC systems. Established vendors such as AMD, IBM, Intel, and Nvidia all rely on fixed-length vector extensions such as AltiVec, AVX, and the Cuda environment to provide massive computational throughput. Using its Neon extensions, ARM lags most of these vendors, particularly Intel, in the width of its vectors and therefore the throughput of its CPUs on vector code. SVE provides ARM licensees with a tool to fix this shortcoming.

ARM is finishing the SVE instruction-set architecture (ISA) and application binary interface (ABI) with major partners while it’s developing the software environment. Its lead partner for SVE is Fujitsu, a leading HPC vendor that has switched from SPARC. We expect other ARM server-processor vendors, such as Cavium and Qualcomm, to adopt SVE to expand their offerings into the HPC market.

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