Intel i7 loop performance anomaly (2013)

Re:Intel i7 loop performance anomaly (2013) by rayiner on Saturday December 16, 2017 @02:24AM (#15938315)

The loop stream detector can’t handle a call/ret in the loop.

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Re:Intel i7 loop performance anomaly (2013) by nkurz on Saturday December 16, 2017 @03:24AM (#15938487)

Yes. https://www.intel.com/content/www/us/en/architecture-and-tec...

Section 2.3.2.4 describes it for Sandy Bridge (and the rules don't seem to have changed in later generations):

  The loops with the following attributes qualify for 
  LSD/micro-op queue replay:
  • Up to eight chunk fetches of 32-instruction-bytes.
  • Up to 28 micro-ops (~28 instructions).
  • All micro-ops are also resident in the Decoded ICache.
  • Can contain no more than eight taken branches and 
    none of them can be a CALL or RET.
  • Cannot have mismatched stack operations. For example,
    more PUSH than POP instructions.

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