Be more careful in applying pragma weak. Fixes pr14974.

GCC applies a pragma weak to a decl if it matches the mangled name. We used
to apply if it matched the plain name.

This patch is a compromise: we apply the pragma only if it matches the name
and the decl has C language linkage.

llvm-svn: 176110

Author: espindola

clang/lib/Sema/SemaDeclAttr.cpp

@@ -19,6 +19,7 @@
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/Mangle.h"
 #include "clang/Basic/CharInfo.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TargetInfo.h"
@@ -5096,11 +5097,18 @@ void Sema::ProcessDeclAttributes(Scope *S, Decl *D, const Declarator &PD,
   if (Inheritable) {
     LoadExternalWeakUndeclaredIdentifiers();
     if (!WeakUndeclaredIdentifiers.empty()) {
-      if (NamedDecl *ND = dyn_cast<NamedDecl>(D)) {
+      NamedDecl *ND = NULL;
+      if (VarDecl *VD = dyn_cast<VarDecl>(D))
+        if (VD->isExternC())
+          ND = VD;
+      if (FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
+        if (FD->isExternC())
+          ND = FD;
+      if (ND) {
         if (IdentifierInfo *Id = ND->getIdentifier()) {
           llvm::DenseMap<IdentifierInfo*,WeakInfo>::iterator I
             = WeakUndeclaredIdentifiers.find(Id);
-          if (I != WeakUndeclaredIdentifiers.end() && ND->hasLinkage()) {
+          if (I != WeakUndeclaredIdentifiers.end()) {
             WeakInfo W = I->second;
             DeclApplyPragmaWeak(S, ND, W);
             WeakUndeclaredIdentifiers[Id] = W;

clang/test/CodeGenCXX/pragma-weak.cpp

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -emit-llvm %s -o - | FileCheck %s
+
+#pragma weak zex
+int zex;
+// GCC produces a weak symbol for this because it matches mangled names.
+// Different c++ ABIs may or may not mangle this, so we produce a strong
+// symbol.
+// CHECK: @zex = global i32
+
+#pragma weak foo
+struct S {  void foo(); };
+void S::foo() {}
+// CHECK: define void @_ZN1S3fooEv(
+
+#pragma weak zed
+namespace bar {  void zed() {} }
+// CHECK: define void @_ZN3bar3zedEv(
+
+#pragma weak bah
+void bah() {}
+// CHECK: define void @_Z3bahv(
+
+#pragma weak baz
+extern "C" void baz() {}
+// CHECK: define weak void @baz(
+
+#pragma weak _Z3baxv
+void bax() {}
+// GCC produces a weak symbol for this one, but it doesn't look like a good
+// idea to expose the mangling to the pragma unless we really have to.
+// CHECK: define void @_Z3baxv(