Better codegen support for DLL attributes being dropped after the first declaration (PR20792)

For the following code:

  __declspec(dllimport) int f(int x);
  int user(int x) {
    return f(x);
  }
  int f(int x) { return 1; }

Clang will drop the dllimport attribute in the AST, but CodeGen would have
already put it on the LLVM::Function, and that would never get updated.
(The same thing happens for global variables.)

This makes Clang check dropped DLL attribute case each time the LLVM object
is referenced.

This isn't perfect, because we will still get it wrong if the function is
never referenced by codegen after the attribute is dropped, but this handles
the common cases and makes us not fail in the verifier.

llvm-svn: 216699

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1449,6 +1449,10 @@ CodeGenModule::GetOrCreateLLVMFunction(StringRef MangledName,
         Entry->setLinkage(llvm::Function::ExternalLinkage);
     }
 
+    // Handle dropped DLL attributes.
+    if (D && !D->hasAttr<DLLImportAttr>() && !D->hasAttr<DLLExportAttr>())
+      Entry->setDLLStorageClass(llvm::GlobalValue::DefaultStorageClass);
+
     if (Entry->getType()->getElementType() == Ty)
       return Entry;
 
@@ -1614,6 +1618,10 @@ CodeGenModule::GetOrCreateLLVMGlobal(StringRef MangledName,
         Entry->setLinkage(llvm::Function::ExternalLinkage);
     }
 
+    // Handle dropped DLL attributes.
+    if (D && !D->hasAttr<DLLImportAttr>() && !D->hasAttr<DLLExportAttr>())
+      Entry->setDLLStorageClass(llvm::GlobalValue::DefaultStorageClass);
+
     if (Entry->getType() == Ty)
       return Entry;
 

clang/test/CodeGen/dllimport.c

@@ -44,12 +44,19 @@ __declspec(dllimport) extern int GlobalRedecl3;
                       extern int GlobalRedecl3; // dllimport ignored
 USEVAR(GlobalRedecl3)
 
+// Make sure this works even if the decl has been used before it's defined (PR20792).
+// CHECK: @GlobalRedecl4 = common global i32
+__declspec(dllimport) extern int GlobalRedecl4;
+USEVAR(GlobalRedecl4)
+                      int GlobalRedecl4; // dllimport ignored
+
+
 // Redeclaration in local context.
-// CHECK: @GlobalRedecl4 = external dllimport global i32
-__declspec(dllimport) int GlobalRedecl4;
+// CHECK: @GlobalRedecl5 = external dllimport global i32
+__declspec(dllimport) int GlobalRedecl5;
 int functionScope() {
-  extern int GlobalRedecl4; // still dllimport
-  return GlobalRedecl4;
+  extern int GlobalRedecl5; // still dllimport
+  return GlobalRedecl5;
 }
 
 
@@ -99,3 +106,9 @@ USE(redecl2)
 __declspec(dllimport) void redecl3(void);
                       void redecl3(void) {} // dllimport ignored
 USE(redecl3)
+
+// Make sure this works even if the decl is used before it's defined (PR20792).
+// CHECK-DAG: define void @redecl4()
+__declspec(dllimport) void redecl4(void);
+USE(redecl4)
+                      void redecl4(void) {} // dllimport ignored