[clang codegen] Add dso_local/hidden/etc. markings to VTT declarations

[efriedma]

Summary

We were marking definitions, but not declarations. Marking declarations makes computing the address more efficient.

Fixes issue reported at https://discourse.llvm.org/t/63090

[MaskRay]

Looks great!

[rjmccall]

LGTM

[justincady]

LGTM!

Just to double check I tested this exact patch locally with the setup that found the problem. I confirmed it resolved that issue.

clang/lib/CodeGen/CGVTT.cpp

CodeGenVTables::EmitVTTDefinition(llvm::GlobalVariable *VTT,
 
   VTT->setInitializer(Init);
 
   // Set the correct linkage.
   VTT->setLinkage(Linkage);
 
   if (CGM.supportsCOMDAT() && VTT->isWeakForLinker())
     VTT->setComdat(CGM.getModule().getOrInsertComdat(VTT->getName()));
-
-  // Set the right visibility.
-  CGM.setGVProperties(VTT, RD);
 }
 
 llvm::GlobalVariable *CodeGenVTables::GetAddrOfVTT(const CXXRecordDecl *RD) {
   assert(RD->getNumVBases() && "Only classes with virtual bases need a VTT");
 
   SmallString<256> OutName;
   llvm::raw_svector_ostream Out(OutName);
   cast<ItaniumMangleContext>(CGM.getCXXABI().getMangleContext())
       .mangleCXXVTT(RD, Out);
   StringRef Name = OutName.str();
 
   // This will also defer the definition of the VTT.
   (void) CGM.getCXXABI().getAddrOfVTable(RD, CharUnits());
 
   VTTBuilder Builder(CGM.getContext(), RD, /*GenerateDefinition=*/false);
 
   llvm::ArrayType *ArrayType =
     llvm::ArrayType::get(CGM.Int8PtrTy, Builder.getVTTComponents().size());
   unsigned Align = CGM.getDataLayout().getABITypeAlignment(CGM.Int8PtrTy);
 
   llvm::GlobalVariable *GV = CGM.CreateOrReplaceCXXRuntimeVariable(
       Name, ArrayType, llvm::GlobalValue::ExternalLinkage, Align);
   GV->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
+  CGM.setGVProperties(GV, RD);
   return GV;
 }
 
 uint64_t CodeGenVTables::getSubVTTIndex(const CXXRecordDecl *RD,
                                         BaseSubobject Base) {
   BaseSubobjectPairTy ClassSubobjectPair(RD, Base);
 
   SubVTTIndiciesMapTy::iterator I = SubVTTIndicies.find(ClassSubobjectPair);

clang/test/CodeGenCXX/visibility.cpp

 // CHECK: @_ZN6Test131B1aE = hidden global
 // CHECK: @_ZN6Test131C1aE = global
 // CHECK-HIDDEN: @_ZN6Test131B1aE = hidden global
 // CHECK-HIDDEN: @_ZN6Test131C1aE = global
 // CHECK: @_ZN6Test143varE = external global
 // CHECK-HIDDEN: @_ZN6Test143varE = external global
 // CHECK: @_ZN6Test154TempINS_1AEE5Inner6bufferE = external global [0 x i8]
 // CHECK-HIDDEN: @_ZN6Test154TempINS_1AEE5Inner6bufferE = external global [0 x i8]
+// CHECK: @_ZTVN6test701BE = external hidden unnamed_addr constant { [5 x ptr] }, align 8
+// CHECK: @_ZTTN6test701BE = external hidden unnamed_addr constant [2 x ptr], align 8
 
 namespace test27 {
   template<typename T>
   class C {
     class DEFAULT D {
       void f();
     };
   };
namespace test69 {
   namespace foo {
     void f() {};
   }
   namespace foo __attribute__((visibility("hidden"))) {
   }
   // CHECK-LABEL: define void @_ZN6test693foo1fEv
   // CHECK-HIDDEN-LABEL: define hidden void @_ZN6test693foo1fEv
 }
+
+namespace test70 {
+  // Make sure both the vtable and VTT declarations are marked "hidden"
+  class HIDDEN A {
+    virtual void a();
+  };
+  class HIDDEN B : virtual A {
+    void a() override;
+    ~B();
+  };
+  B::~B() {}
+  // Check lines at top of file.
+}