Decoder Override event on AMD

By: Chester (, November 3, 2020 5:27 pm
Room: Moderated Discussions
Does anyone know what event 0x91, 'Decoder Overrides Existing Branch Prediction (speculative)' (BpDeReDirect), on AMD Zen1/Zen 2 counts?

I thought it might be similar to Intel's baclear event, which I understand to mean a BTB miss. But on AMD Zen 1 which has a 4096 entry L2 BTB (same size as Haswell's 4096 entry BTB), I get far lower counts.

In Cinebench R20 ST mode, it's around 0.44 decoder overrides per 1K instructions on Zen 1, and 1.11 baclears on Haswell. On Zen 2, it's even lower - 0.27 per 1K instr.

Is Intel's BAClear event counting something other than BTB misses? Or is 'decoder overrides' on AMD something totally different?
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Decoder Override event on AMDChester2020/11/03 05:27 PM
  Decoder Override event on AMDanon2020/11/04 07:19 AM
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      AMD AMD64 Architecture new revisionAdrian2020/11/05 03:23 AM
    Decoder Override event on AMDChester2020/11/05 11:37 AM
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