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Benchmarks Of JCC Erratum: A New Intel CPU Bug With Performance Implications On Skylake Through Cascade Lake

Submitted by Rianne Schestowitz on Tuesday 12th of November 2019 06:18:03 PM Filed under
Graphics/Benchmarks

Intel is today making public the Jump Conditional Code (JCC) erratum. This is a bug involving the CPU's Decoded ICache where on Skylake and derived CPUs where unpredictable behavior could happen when jump instructions cross cache lines. Unfortunately addressing this error in software comes with a performance penalty but ultimately Intel engineers are working to offset that through a toolchain update. Here are the exclusive benchmarks out today of the JCC erratum performance impact as well as when trying to recover that performance through the updated GNU Assembler.

The microcode update prevents jump instructions from being cached in the Decoded Icache when those instructions cross a 32-byte boundary or where they end on a 32-bit boundary. Due to that change there will be more misses from the Decoded ICache and switches back to the legacy decode pipeline -- resulting in a new performance penalty. The Decoded ICache / Decoded Streaming Buffer has been around since Sandy Bridge but only Skylake and newer is affected by this erratum. Cascade Lake is affected by this erratum but Ice Lake and future iterations appears unaffected. The erratum notice officially lists Amber Lake, Cascade Lake, Coffee Lake, Comet Lake, Kaby Lake, Skylake, and Whiskey Lake as affected generations for the JCC bug.

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The Gaming Performance Impact From The Intel JCC Erratum

Submitted by Roy Schestowitz on Wednesday 13th of November 2019 10:15:35 AM.
  • The Gaming Performance Impact From The Intel JCC Erratum Microcode Update

    This morning I provided a lengthy look at the performance impact of Intel's JCC Erratum around the CPU microcode update issued for Skylake through Cascade Lake for mitigating potentially unpredictable behavior when jump instructions cross cache lines. Of the many benchmarks shared this morning in that overview, there wasn't time for any gaming tests prior to publishing. Now with more time passed, here is an initial look at how the Linux gaming performance is impacted by the newly-released Intel CPU microcode for this Jump Conditional Code issue.

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