When an internal-linkage function or variable is declared within an extern "C"

linkage specification, and is marked as __attribute__((used)), try to also give
it the unmangled name (by emitting an internal linkage alias) if nothing else
within the translation unit would use that name. This allows inline asm in that
translation unit to use the entity via its unmangled name, which people
apparently rely on.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178950 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 96 insertions, 0 deletions

diff --git a/lib/CodeGen/CodeGenModule.cpp b/lib/CodeGen/CodeGenModule.cpp
index c518a5554e..670f06620d 100644
--- a/lib/CodeGen/CodeGenModule.cpp
+++ b/lib/CodeGen/CodeGenModule.cpp
@@ -186,6 +186,7 @@ void CodeGenModule::Release() {
   EmitCtorList(GlobalCtors, "llvm.global_ctors");
   EmitCtorList(GlobalDtors, "llvm.global_dtors");
   EmitGlobalAnnotations();
+  EmitStaticExternCAliases();
   EmitLLVMUsed();
 
   if (CodeGenOpts.ModulesAutolink) {
@@ -1707,6 +1708,39 @@ unsigned CodeGenModule::GetGlobalVarAddressSpace(const VarDecl *D,
   return AddrSpace;
 }
 
+template<typename SomeDecl>
+void CodeGenModule::MaybeHandleStaticInExternC(const SomeDecl *D,
+                                               llvm::GlobalValue *GV) {
+  if (!getLangOpts().CPlusPlus)
+    return;
+
+  // Must have 'used' attribute, or else inline assembly can't rely on
+  // the name existing.
+  if (!D->template hasAttr<UsedAttr>())
+    return;
+
+  // Must have internal linkage and an ordinary name.
+  if (!D->getIdentifier() || D->getLinkage() != InternalLinkage)
+    return;
+
+  // Must be in an extern "C" context. Entities declared directly within
+  // a record are not extern "C" even if the record is in such a context.
+  const DeclContext *DC = D->getFirstDeclaration()->getDeclContext();
+  if (DC->isRecord() || !DC->isExternCContext())
+    return;
+
+  // OK, this is an internal linkage entity inside an extern "C" linkage
+  // specification. Make a note of that so we can give it the "expected"
+  // mangled name if nothing else is using that name.
+  StaticExternCMap::iterator I =
+      StaticExternCValues.insert(std::make_pair(D->getIdentifier(), GV)).first;
+
+  // If we have multiple internal linkage entities with the same name
+  // in extern "C" regions, none of them gets that name.
+  if (I->second != GV)
+    I->second = 0;
+}
+
 void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D) {
   llvm::Constant *Init = 0;
   QualType ASTTy = D->getType();
@@ -1805,6 +1839,8 @@ void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D) {
     cast<llvm::GlobalValue>(Entry)->eraseFromParent();
   }
 
+  MaybeHandleStaticInExternC(D, GV);
+
   if (D->hasAttr<AnnotateAttr>())
     AddGlobalAnnotations(D, GV);
 
@@ -2083,6 +2119,8 @@ void CodeGenModule::EmitGlobalFunctionDefinition(GlobalDecl GD) {
   // FIXME: this is redundant with part of SetFunctionDefinitionAttributes
   setGlobalVisibility(Fn, D);
 
+  MaybeHandleStaticInExternC(D, Fn);
+
   CodeGenFunction(*this).GenerateCode(D, Fn, FI);
 
   SetFunctionDefinitionAttributes(D, Fn);
@@ -2903,6 +2941,21 @@ static void EmitGlobalDeclMetadata(CodeGenModule &CGM,
   GlobalMetadata->addOperand(llvm::MDNode::get(CGM.getLLVMContext(), Ops));
 }
 
+/// For each function which is declared within an extern "C" region and marked
+/// as 'used', but has internal linkage, create an alias from the unmangled
+/// name to the mangled name if possible. People expect to be able to refer
+/// to such functions with an unmangled name from inline assembly within the
+/// same translation unit.
+void CodeGenModule::EmitStaticExternCAliases() {
+  for (StaticExternCMap::iterator I = StaticExternCValues.begin(),
+                                  E = StaticExternCValues.end();
+       I != E; ++I)
+    if (I->second && !getModule().getNamedValue(I->first->getName()))
+      AddUsedGlobal(
+        new llvm::GlobalAlias(I->second->getType(), I->second->getLinkage(),
+                              I->first->getName(), I->second, &getModule()));
+}
+
 /// Emits metadata nodes associating all the global values in the
 /// current module with the Decls they came from.  This is useful for
 /// projects using IR gen as a subroutine.
diff --git a/lib/CodeGen/CodeGenModule.h b/lib/CodeGen/CodeGenModule.h
index 5b2153e5ff..ccac4371c4 100644
--- a/lib/CodeGen/CodeGenModule.h
+++ b/lib/CodeGen/CodeGenModule.h
@@ -305,6 +305,12 @@ class CodeGenModule : public CodeGenTypeCache {
   llvm::DenseMap<QualType, llvm::Constant *> AtomicSetterHelperFnMap;
   llvm::DenseMap<QualType, llvm::Constant *> AtomicGetterHelperFnMap;
 
+  /// Map used to track internal linkage functions declared within
+  /// extern "C" regions.
+  typedef llvm::DenseMap<IdentifierInfo *,
+                         llvm::GlobalValue *> StaticExternCMap;
+  StaticExternCMap StaticExternCValues;
+
   /// CXXGlobalInits - Global variables with initializers that need to run
   /// before main.
   std::vector<llvm::Constant*> CXXGlobalInits;
@@ -736,6 +742,12 @@ public:
   // variable has been instantiated.
   void HandleCXXStaticMemberVarInstantiation(VarDecl *VD);
 
+  /// \brief If the declaration has internal linkage but is inside an
+  /// extern "C" linkage specification, prepare to emit an alias for it
+  /// to the expected name.
+  template<typename SomeDecl>
+  void MaybeHandleStaticInExternC(const SomeDecl *D, llvm::GlobalValue *GV);
+
   /// AddUsedGlobal - Add a global which should be forced to be
   /// present in the object file; these are emitted to the llvm.used
   /// metadata global.
@@ -1048,6 +1060,10 @@ private:
   /// \brief Emit the link options introduced by imported modules.
   void EmitModuleLinkOptions();
 
+  /// \brief Emit aliases for internal-linkage declarations inside "C" language
+  /// linkage specifications, giving them the "expected" name where possible.
+  void EmitStaticExternCAliases();
+
   void EmitDeclMetadata();
 
   /// EmitCoverageFile - Emit the llvm.gcov metadata used to tell LLVM where
diff --git a/test/CodeGenCXX/extern-c.cpp b/test/CodeGenCXX/extern-c.cpp
index a8c4f0cdbd..5899b9348c 100644
--- a/test/CodeGenCXX/extern-c.cpp
+++ b/test/CodeGenCXX/extern-c.cpp
@@ -36,3 +36,30 @@ namespace test2 {
   extern "C" X test2_b;
   X test2_b;
 }
+
+extern "C" {
+  static int unused_var;
+  static int unused_fn() { return 0; }
+
+  __attribute__((used)) static int internal_var;
+  __attribute__((used)) static int internal_fn() { return 0; }
+
+  __attribute__((used)) static int duplicate_internal_var;
+  __attribute__((used)) static int duplicate_internal_fn() { return 0; }
+
+  namespace N {
+    __attribute__((used)) static int duplicate_internal_var;
+    __attribute__((used)) static int duplicate_internal_fn() { return 0; }
+  }
+
+  // CHECK: @llvm.used = appending global {{.*}} @internal_var {{.*}} @internal_fn 
+
+  // CHECK-NOT: @unused
+  // CHECK-NOT: @duplicate_internal
+  // CHECK: @internal_var = alias internal i32* @_Z12internal_var
+  // CHECK-NOT: @unused
+  // CHECK-NOT: @duplicate_internal
+  // CHECK: @internal_fn = alias internal i32 ()* @_Z11internal_fnv
+  // CHECK-NOT: @unused
+  // CHECK-NOT: @duplicate_internal
+}