[modules] Don't emit initializers for VarDecls within a module eagerly whenever

we first touch any part of that module. Instead, defer them until the first
time that module is (transitively) imported. The initializer step for a module
then recursively initializes modules that its own headers imported.

For example, this avoids running the <iostream> global initializer in programs
that don't actually use iostreams, but do use other parts of the standard
library.

llvm-svn: 276159

1 files changed, 11 insertions, 5 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 0ef5b74..f73e85d 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -3910,13 +3910,19 @@ void CodeGenModule::EmitTopLevelDecl(Decl *D) {
   case Decl::Import: {
     auto *Import = cast<ImportDecl>(D);
 
-    // Ignore import declarations that come from imported modules.
-    if (Import->getImportedOwningModule())
+    // If we've already imported this module, we're done.
+    if (!ImportedModules.insert(Import->getImportedModule()))
       break;
-    if (CGDebugInfo *DI = getModuleDebugInfo())
-      DI->EmitImportDecl(*Import);
 
-    ImportedModules.insert(Import->getImportedModule());
+    // Emit debug information for direct imports.
+    if (!Import->getImportedOwningModule()) {
+      if (CGDebugInfo *DI = getModuleDebugInfo())
+        DI->EmitImportDecl(*Import);
+    }
+
+    // Emit the module initializers.
+    for (auto *D : Context.getModuleInitializers(Import->getImportedModule()))
+      EmitTopLevelDecl(D);
     break;
   }