[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.

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index 2ddf9d7..67658c9 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/ASTConsumer.h"
+#include "clang/AST/ASTMutationListener.h"
 #include "clang/Lex/HeaderSearch.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/SemaInternal.h"
@@ -475,6 +476,9 @@ Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
 
   getASTContext().setCurrentNamedModule(Mod);
 
+  if (auto *Listener = getASTMutationListener())
+    Listener->EnteringModulePurview();
+
   // We already potentially made an implicit import (in the case of a module
   // implementation unit importing its interface).  Make this module visible
   // and return the import decl to be added to the current TU.