[all-commits] [llvm/llvm-project] f2695a: [C++20] [Modules] Avoid writing untouched DeclUpda...

[Chuanqi Xu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f2695a1c2f561da42b973187a254b6eeb7da76a9
      https://github.com/llvm/llvm-project/commit/f2695a1c2f561da42b973187a254b6eeb7da76a9
  Author: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
  Date:   2024-04-18 (Thu, 18 Apr 2024)

  Changed paths:
    M clang/include/clang/AST/ASTMutationListener.h
    M clang/include/clang/AST/Decl.h
    M clang/include/clang/Serialization/ASTWriter.h
    M clang/lib/AST/ASTContext.cpp
    M clang/lib/AST/Decl.cpp
    M clang/lib/Frontend/MultiplexConsumer.cpp
    M clang/lib/Sema/SemaModule.cpp
    M clang/lib/Serialization/ASTWriter.cpp
    A clang/test/Modules/reduced-bmi-empty-module-purview.cppm

  Log Message:
  -----------
  [C++20] [Modules] Avoid writing untouched DeclUpdates from GMF in
Reduced BMI

Mitigate https://github.com/llvm/llvm-project/issues/61447

The root cause of the above problem is that when we write a declaration,
we need to lookup all the redeclarations in the imported modules. Then
it will be pretty slow if there are too many redeclarations in different
modules. This patch doesn't solve the porblem.

What the patchs mitigated is, when we writing a named module, we shouldn't
write the declarations from GMF if it is unreferenced **in current
module unit**. The difference here is that, if the declaration is used
in the imported modules, we used to emit it as an update. But we
definitely want to avoid that after this patch.

For that reproducer in
https://github.com/llvm/llvm-project/issues/61447, it used to take 2.5s
to compile and now it only takes 0.49s to compile, which is a big win.



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