[PATCH] D43945: [dsymutil] Perform analyzing and cloning DIE in parallel

[Jonas Devlieghere via Phabricator via llvm-commits]

JDevlieghere created this revision.
JDevlieghere added reviewers: friss, aprantl, davide.

This patch makes dsymutil perform `analyzeContextInfo` and `CloneDIEs` in
parallel. For the same object file, there is a dependency between the two.
However, we can do `analyzeContextInfo` for the next object file while cloning
DIEs for the current. This is exactly the approach taken in this patch.

For WebCore, this leads to a performance improvement of **29%** and for clang we
see similar results with at **32%** improvement.

A large part of the change is related to passing around objects during linking,
instead of having one "current" counterpart accessible by all the linker's
methods. We group these objects into a new struct called `LinkContext` which
exists for every object file.

A big thanks to Pete Cooper (@pete) who came up with the idea and a PoC.

  ---------------------------------------------
  WebCore (before)
  ---------------------------------------------
  78.13 real        72.85 user         4.80 sys
  77.92 real        72.75 user         4.87 sys
  77.91 real        72.69 user         4.91 sys
  78.38 real        73.01 user         5.00 sys
  79.48 real        73.99 user         5.03 sys
  ---------------------------------------------
  78.36 real
  ---------------------------------------------
  
  ---------------------------------------------
  WebCore (after)
  ---------------------------------------------
  55.22 real        74.76 user         6.16 sys
  54.22 real        73.72 user         5.89 sys
  55.13 real        74.62 user         6.02 sys
  55.83 real        74.89 user         6.49 sys
  54.87 real        74.23 user         6.10 sys
  ---------------------------------------------
  55.05 real
  ---------------------------------------------
  
  ---------------------------------------------
  Clang (before)
  ---------------------------------------------
  82.16 real        78.16 user         3.61 sys
  82.92 real        78.79 user         3.69 sys
  84.59 real        79.85 user         3.95 sys
  84.45 real        79.80 user         3.93 sys
  85.11 real        80.23 user         4.07 sys
  ---------------------------------------------
  83.85 real
  ---------------------------------------------
  
  ---------------------------------------------
  Clang (after)
  ---------------------------------------------
  56.28 real        81.32 user         6.53 sys
  56.04 real        81.24 user         6.39 sys
  56.65 real        81.42 user         6.36 sys
  57.06 real        81.75 user         6.89 sys
  55.65 real        80.40 user         6.59 sys
  ---------------------------------------------
  56.34 real
  ---------------------------------------------


Repository:
  rL LLVM

https://reviews.llvm.org/D43945

Files:
  test/tools/dsymutil/basic-linking.test
  tools/dsymutil/DebugMap.h
  tools/dsymutil/DwarfLinker.cpp

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