[C++20] [Modules] Skip calls to module initializer to modules if we k…

…now the module doesn't init anything

Close #97244

This is an optimization allowed by the modules's ABI to skip calls to
imported modules for which we know nothing will be initialized.

clang/lib/CodeGen/CGDeclCXX.cpp

@@ -840,6 +840,10 @@ CodeGenModule::EmitCXXGlobalInitFunc() {
       // No Itanium initializer in header like modules.
       if (M->isHeaderLikeModule())
         continue;
+      // We're allowed to skip the initialization if we are sure it doesn't
+      // do any thing.
+      if (!M->isNamedModuleInterfaceHasInit())
+        continue;
       llvm::FunctionType *FTy = llvm::FunctionType::get(VoidTy, false);
       SmallString<256> FnName;
       {

clang/test/Modules/pr97244.cppm

@@ -0,0 +1,30 @@
+// REQUIRES: !system-windows
+//
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/Empty.cppm \
+// RUN:     -emit-module-interface -o %t/Empty.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/Empty2.cppm \
+// RUN:     -fprebuilt-module-path=%t -emit-module-interface -o %t/Empty2.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/main.cpp \
+// RUN:     -fprebuilt-module-path=%t -emit-llvm -o - | FileCheck %t/main.cpp
+// RUN: %clang_cc1 -std=c++20  -triple %itanium_abi_triple %t/Empty2.pcm \
+// RUN:     -fprebuilt-module-path=%t -emit-llvm -o - | FileCheck %t/Empty2.cppm
+
+//--- Empty.cppm
+export module Empty;
+
+//--- Empty2.cppm
+export module Empty2;
+import Empty;
+
+// CHECK-NOT: _ZGIW5Empty
+
+//--- main.cpp
+import Empty;
+import Empty2;
+
+// CHECK-NOT: _ZGIW5Empty
+// CHECK-NOT: _ZGIW6Empty2