[clang] [C++20] [Modules] Don't update MarkAsUsed information for decls from named modules (PR #174687)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-modules

Author: Chuanqi Xu (ChuanqiXu9)

<details>
<summary>Changes</summary>


Declarations from named modules are used naturally. Thet are declarations in other TU. We don't need to record the information for updating them.

---
Full diff: https://github.com/llvm/llvm-project/pull/174687.diff


2 Files Affected:

- (modified) clang/lib/Serialization/ASTWriter.cpp (+4) 
- (added) clang/test/Modules/class-instantiate-no-change.cppm (+50) 


``````````diff
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index f9176b7e68f73..a211e98667829 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -7769,6 +7769,10 @@ void ASTWriter::DeclarationMarkedUsed(const Decl *D) {
   if (Chain && Chain->isProcessingUpdateRecords()) return;
   assert(!WritingAST && "Already writing the AST!");
 
+  // If the declaration comes from module unit, it is meaningless to mark it as used.
+  if (auto *M = D->getOwningModule(); M && M->getTopLevelModule()->isNamedModule())
+    return;
+
   // If there is *any* declaration of the entity that's not from an AST file,
   // we can skip writing the update record. We make sure that isUsed() triggers
   // completion of the redeclaration chain of the entity.
diff --git a/clang/test/Modules/class-instantiate-no-change.cppm b/clang/test/Modules/class-instantiate-no-change.cppm
new file mode 100644
index 0000000000000..6ea7d24252ab5
--- /dev/null
+++ b/clang/test/Modules/class-instantiate-no-change.cppm
@@ -0,0 +1,50 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 %t/impl.cppm -emit-reduced-module-interface -o %t/impl.pcm
+// RUN: %clang_cc1 -std=c++20 %t/impl.v2.cppm -emit-reduced-module-interface -o %t/impl.v2.pcm
+// RUN: %clang_cc1 -std=c++20 %t/m.cppm -emit-reduced-module-interface -o %t/m.pcm \
+// RUN:     -fmodule-file=impl=%t/impl.pcm
+// RUN: %clang_cc1 -std=c++20 %t/m.cppm -emit-reduced-module-interface -o %t/m.v2.pcm \
+// RUN:     -fmodule-file=impl=%t/impl.v2.pcm
+
+// Since m only uses impl in the definition, the change in impl shouldn't affect m.
+// RUN: diff %t/m.pcm %t/m.v2.pcm &> /dev/null
+
+//--- impl.cppm
+export module impl;
+export struct Impl {
+    Impl() {}
+    Impl(const Impl &) {}
+    ~Impl() {}
+    int get() {
+        return 43;
+    }
+};
+
+//--- impl.v2.cppm
+export module impl;
+export struct Impl {
+    Impl() {}
+    Impl(const Impl &) {}
+    ~Impl() {}
+    int get() {
+        return 43;
+    }
+};
+
+export struct ImplV2 {
+    int get() {
+        return 43;
+    }
+};
+
+//--- m.cppm
+export module m;
+import impl;
+
+export int interface() {
+    Impl impl;
+    return impl.get();
+};

``````````

</details>


https://github.com/llvm/llvm-project/pull/174687