[clang] [C++20][Modules] Improve namespace look-up performance for modules. (PR #171769)

[Chuanqi Xu via cfe-commits]

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@@ -4432,14 +4432,35 @@ class ASTDeclContextNameLookupTrait
           TULocalDeclsMap.insert({D->getDeclName(), DeclIDsTy{ID}});
         else
           Iter->second.push_back(ID);
-        continue;
+        return;
       }
       case LookupVisibility::GenerallyVisibile:
         // Generally visible decls go into the general lookup table.
         break;
       }
 
       DeclIDs.push_back(ID);
+    };
+    for (NamedDecl *D : Decls) {
+      if (isa<NamespaceDecl>(D) && D->isFromASTFile()) {
+        // In ASTReader, we stored only the key declaration of a namespace decl
+        // for this TU rather than storing all of the key declarations from each
+        // imported module. If we have an external namespace decl, this is that
+        // key declaration and we need to re-expand it to write out all of the
+        // key declarations from each imported module again.
+        //
+        // See comment 'ASTReader::FindExternalVisibleDeclsByName' for details.
+        ASTReader *Chain = Writer.getChain();
+        assert(Chain && "An external namespace decl without an ASTReader");
+        assert(D == Chain->getKeyDeclaration(D) &&
+               "An external namespace decl that is not "
+               "key declaration of this TU");
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ChuanqiXu9 wrote:

Why is this true? 

https://github.com/llvm/llvm-project/pull/171769