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Test results for Broadwell and Skylake

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Date: 2016-02-22 17:50

Thank you all for the useful information. FYI, the latest Intel architecture optimization manual discusses the Skylake changes for the mixed AVX / SSE problem in great detail, including diagrams and tables. This is in section 11.3 Mixing AVX code with SEE code in the January 2016 edition. Skylake has not eliminated the problem entirely, with "partial register dependency + blend" as the penalty in one mode, and ~XSAVE in another mode. Use of VZEROUPPER is still recommended, in rule 72. "The Skylake microarchitecture implements a different state machine than prior generations to manage the YMM state transition associated with mixing SSE and AVX instructions. It no longer saves the entire upper YMM state transition ... but saves the upper bits of individual register. As a result ... will experience a penalty associated with partial register dependency...".

Other topics discussed include "Align data to 32 bytes", which was recently discussed in this blog too. Section 11.6.1

There is lots and lots of Skylake material, including the tradeoffs between electrical power reduction vs. performance. Like "The latency of the PAUSE instruction in prior generation microarchitecture is about 10 cycles, whereas on Skylake microarchitecture it has been extended to as many as140 cycles... There's also a small power benefit in 2-core and 4-core systems... As the PAUSE latency has been increased significantly, workloads that are sensitive to PAUSE latency will suffer some performance loss." Section 8.4.7

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