llvm-gcc treats a tentative definition with a previous

(or follow up) extern declaration with weak_import as 
an actual definition. make clang follows this behavior. 
// rdar://9538608
llvm-gcc treats an extern declaration with weak_import


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133450 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CodeGenModule.cpp

@@ -1351,7 +1351,8 @@ CodeGenModule::GetLLVMLinkageVarDefinition(const VarDecl *D,
            ((!CodeGenOpts.NoCommon && !D->getAttr<NoCommonAttr>()) ||
              D->getAttr<CommonAttr>()) &&
            !D->hasExternalStorage() && !D->getInit() &&
-           !D->getAttr<SectionAttr>() && !D->isThreadSpecified()) {
+           !D->getAttr<SectionAttr>() && !D->isThreadSpecified() &&
+           !D->getAttr<WeakImportAttr>()) {
     // Thread local vars aren't considered common linkage.
     return llvm::GlobalVariable::CommonLinkage;
   }

lib/Sema/SemaDecl.cpp

@@ -2039,6 +2039,12 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
   }
   
   mergeDeclAttributes(New, Old, Context);
+  // weak_import on current declaration is applied to previous
+  // tentative definiton.
+  if (New->getAttr<WeakImportAttr>() &&
+      Old->getStorageClass() == SC_None &&
+      !Old->getAttr<WeakImportAttr>())
+    Old->addAttr(::new (Context) WeakImportAttr(SourceLocation(), Context));
 
   // Merge the types.
   MergeVarDeclTypes(New, Old);

test/CodeGen/attr-weak-import.c

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -triple x86_64-darwin-apple -emit-llvm -o - %s | FileCheck %s
+// rdar://9538608
+
+extern int A __attribute__((weak_import));
+int A;
+
+extern int B __attribute__((weak_import));
+extern int B;
+
+int C;
+extern int C __attribute__((weak_import));
+
+extern int D __attribute__((weak_import));
+extern int D __attribute__((weak_import));
+int D;
+
+extern int E __attribute__((weak_import));
+int E;
+extern int E __attribute__((weak_import));
+
+// CHECK: @A = global i32
+// CHECK-NOT: @B =
+// CHECK: @C = global i32
+// CHECK: @D = global i32
+// CHECK: @E = global i32
+