[Lldb-commits] [PATCH] D122974: prevent ConstString from calling djbHash() more than once

[Luboš Luňák via Phabricator via lldb-commits]

llunak created this revision.
llunak added reviewers: clayborg, chandlerc.
Herald added subscribers: dexonsmith, hiraditya.
Herald added a project: All.
llunak requested review of this revision.
Herald added projects: LLDB, LLVM.
Herald added subscribers: llvm-commits, lldb-commits.

Pool::GetConstCStringWithStringRef() may call djbHash() up to 3 times for the same string: First from Pool::hash() when deciding which StringMap in the pool to use, then from read-locked StringMap::find() and then possibly from StringMap::insert() if insertion is needed.

If LLDB's index cache is enabled and everything is cached, calls to djbHash() are ~30% of the total CPU time here when I profile lldb start. Modifying StringMap to allow explicitly passing in a precomputed hash and calling djbHash() just once reduces startup CPU time by ~19%.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D122974

Files:
  lldb/source/Utility/ConstString.cpp
  llvm/include/llvm/ADT/StringMap.h
  llvm/lib/Support/StringMap.cpp

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