[all-commits] [llvm/llvm-project] 3e5173: [BPI] Compile time improvement when erasing blocks...

[Teresa Johnson via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 3e5173dbc352317712ca35788333c6e118cbf79c
      https://github.com/llvm/llvm-project/commit/3e5173dbc352317712ca35788333c6e118cbf79c
  Author: Teresa Johnson <tejohnson at google.com>
  Date:   2020-07-10 (Fri, 10 Jul 2020)

  Changed paths:
    M llvm/lib/Analysis/BranchProbabilityInfo.cpp

  Log Message:
  -----------
  [BPI] Compile time improvement when erasing blocks (NFC)

Summary:
eraseBlock is trying to erase all probability info for the given BB.
This info is stored in a DenseMap organized like so:
   using Edge = std::pair<const BasicBlock *, unsigned>;
   DenseMap<Edge, BranchProbability> Probs;
where the unsigned in the Edge key is the successor id.

It was walking through every single map entry, checking if the BB in the
key's pair matched the given BB. Much more efficient is to do what
another method (getEdgeProbability) was already doing, which is to walk
the successors of the BB, and simply do a map lookup on the key formed
from each <BB, successor id> pair.

Doing this dropped the overall compile time for a file containing a
very large function by around 32%.

Reviewers: davidxl, xur

Subscribers: llvm-commits, hiraditya

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83596