[all-commits] [llvm/llvm-project] 83f4c3: [modules] Do not cache invalid state for modules t...

[Volodymyr Sapsai via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 83f4c3af021cd5322ea10fd1c4e839874c1dae49
      https://github.com/llvm/llvm-project/commit/83f4c3af021cd5322ea10fd1c4e839874c1dae49
  Author: Volodymyr Sapsai <vsapsai at apple.com>
  Date:   2020-01-16 (Thu, 16 Jan 2020)

  Changed paths:
    M clang/include/clang/Serialization/InMemoryModuleCache.h
    M clang/lib/Serialization/ASTReader.cpp
    M clang/lib/Serialization/ASTWriter.cpp
    M clang/lib/Serialization/InMemoryModuleCache.cpp
    M clang/lib/Serialization/ModuleManager.cpp
    R clang/test/Modules/Inputs/implicit-invalidate-chain/A.h
    R clang/test/Modules/Inputs/implicit-invalidate-chain/B.h
    R clang/test/Modules/Inputs/implicit-invalidate-chain/C.h
    R clang/test/Modules/Inputs/implicit-invalidate-chain/module.modulemap
    R clang/test/Modules/implicit-invalidate-chain.c
    M clang/unittests/Frontend/FrontendActionTest.cpp
    M clang/unittests/Serialization/InMemoryModuleCacheTest.cpp

  Log Message:
  -----------
  [modules] Do not cache invalid state for modules that we attempted to load.

Partially reverts 0a2be46cfdb698fefcc860a56b47dde0884d5335 as it turned
out to cause redundant module rebuilds in multi-process incremental builds.
When a module was getting out of date, all compilation processes started at the
same time were marking it as `ToBuild`. So each process was building the same
module instead of checking if it was built by someone else and using that
result. In addition to the work duplication, contention on the same .pcm file
wasn't making builds faster.

Note that for a single-process build this change would cause redundant module
reads and validations. But reading a module is faster than building it and
multi-process builds are more common than single-process. So I'm willing to
make such a trade-off.

rdar://problem/54395127

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D72860