[PATCH] D79485: [WIP][BPI] Improve static heuristics for "cold" paths.

Fri Jul 31 04:27:04 PDT 2020

ebrevnov added inline comments.

================
Comment at: llvm/lib/Analysis/BranchProbabilityInfo.cpp:126
+/// with attribute 'cold'.
+static const uint32_t COLD_WEIGHT = 0xffff;
+
----------------
davidxl wrote:
> why is it set to 0xffff?
Short answer is to preserve same branch probability as before.
Old implementation assigned CC_NONTAKEN_WEIGHT weight to "normal" path and  CC_TAKEN_WEIGHT to "cold" path. Absolute values are irrelevant and ratio CC_NONTAKEN_WEIGHT/CC_TAKEN_WEIGHT = 16 defines relative probability of "normal" and "cold" branches. New implementation uses  pre-selected weight for all "nornal" paths DEFAULT_WEIGHT. Thus relative ratio is calculated as DEFAULT_WEIGHT/COLD_WEIGHT = 0xfffff/0xffff = 16.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D79485/new/

https://reviews.llvm.org/D79485