LLVM Enables New SLP Vectorize Option By Default
Written by Michael Larabel in LLVM on 14 November 2015 at 07:38 AM EST. 1 Comment
LLVM developers have decided to enable a new vectorizer option by default that has the potential to boost performance, but the performance benefits aren't immediately clear.

Earlier this week Charlie Turner proposed turn -slp-vectorize-hor on by default. He commented, "I've done compile-time experiments for AArch64 over SPEC{2000,2006} and of course the test-suite. I measure no significant compile-time impact of enabling this feature by default...I also timed running the regression tests with -slp-vectorize-hor enabled and disabled, no significant difference here either. There are no significant performance regressions (or much improvements) on AArch64 in night-test suite. I do see wins in third party benchmarks when using this flag, which is why I'm asking if there would be any objection from the community to making -slp-vectorize-hor default on."

Enabling -slp-vectorize-hor by default did get added to LLVM SVN/Git.

This option attempts to vectorize horizontal reductions. For those unfamiliar with LLVM's SLP Vectorizer, I explained and tested it back in 2013 when this Superword-Level Parallelism Vectorizer was initially added.

With my daily LLVM/Clang compiler development benchmarks done daily via LinuxBenchmarking.com, I haven't noticed any real performance impact of this default enabling of slp-vectorize-hor on my Linux x86_64 systems. This change will be found in LLVM 3.8.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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