CodeGen: Mark functions used in vtables as unnamed_addr.

[pcc]

Summary

This marks virtual function declarations, as well as runtime library functions
cxa_pure_virtual, cxa_deleted_virtual and _purecall, as unnamed_addr. This
will allow us to correctly form relative references to them from vtables in
the relative vtable ABI.

[rjmccall]

The underscore check seems over-broad; we support a lot of language dialects and runtimes besides the Itanium C++ runtime. This would be both more conservative and more convincing if it were constrained to the functions that we expect to put in v-tables.

[pcc]

Restrict to functions referenced in virtual tables

[pcc]

Done (this also affects _purecall in the MS ABI; I don't think this change should break anything there, but someone working on that might want to comment). Since this patch is now specific to virtual tables anyway, I've incorporated another relevant change, which is to mark virtual function declarations as unnamed_addr.

[rjmccall]

I see, they weren't being set on declarations, just definitions. Yes, this seems reasonable.

[rnk]

In D18071#373687, @pcc wrote:

Done (this also affects _purecall in the MS ABI; I don't think this change should break anything there, but someone working on that might want to comment). Since this patch is now specific to virtual tables anyway, I've incorporated another relevant change, which is to mark virtual function declarations as unnamed_addr.

That shouldn't matter. Virtual member functions are wrapped with a thunk, so comparisons won't actually involve pointers to __purecall.

[rjmccall]

Member function comparisons of virtual functions aren't required to work anyway. Itanium just happens to implement them in a way that kindof gets it right (for simple cases).

lib/CodeGen/CodeGenModule.cpp

 /// type and name.
 llvm::Constant *
 CodeGenModule::CreateRuntimeFunction(llvm::FunctionType *FTy,
                                      StringRef Name,
                                      llvm::AttributeSet ExtraAttrs) {
   llvm::Constant *C =
       GetOrCreateLLVMFunction(Name, FTy, GlobalDecl(), /*ForVTable=*/false,
                               /*DontDefer=*/false, /*IsThunk=*/false, ExtraAttrs);
-  if (auto *F = dyn_cast<llvm::Function>(C))
+  if (auto *F = dyn_cast<llvm::Function>(C)) {
+    // Mark runtime functions with reserved names as unnamed_addr, as user
+    // programs shouldn't be able to observe their addresses without invoking
+    // undefined behavior.
+    if (Name[0] == '_')
+      F->setUnnamedAddr(true);
     if (F->empty())
       F->setCallingConv(getRuntimeCC());
+  }
   return C;
 }
 
 /// CreateBuiltinFunction - Create a new builtin function with the specified
 /// type and name.
 llvm::Constant *
 CodeGenModule::CreateBuiltinFunction(llvm::FunctionType *FTy,
                                      StringRef Name,

test/CodeGenCXX/dynamic-cast.cpp

try {
     dynamic_cast<const B&>(*a);
   } catch (...) {
     // CHECK:      landingpad { i8*, i32 }
     // CHECK-NEXT:   catch i8* null
   }
   return fail;
 }
 
-// CHECK: declare i8* @__dynamic_cast(i8*, i8*, i8*, i64) [[NUW_RO:#[0-9]+]]
+// CHECK: declare i8* @__dynamic_cast(i8*, i8*, i8*, i64) unnamed_addr [[NUW_RO:#[0-9]+]]
 
 // CHECK: attributes [[NUW_RO]] = { nounwind readonly }
 // CHECK: attributes [[NR]] = { noreturn }

test/CodeGenCXX/exceptions.cpp

A *c() {
     // CHECK:      [[ISACTIVE:%.*]] = load i1, i1* [[ACTIVE]]
     // CHECK-NEXT: br i1 [[ISACTIVE]]
     // CHECK:      call void @_ZdlPv(i8* [[NEW]])
     return new A(B().x);
   }
 
   //   rdar://11904428
   //   Terminate landing pads should call __cxa_begin_catch first.
-  // CHECK:      define linkonce_odr hidden void @__clang_call_terminate(i8*) [[NI_NR_NUW:#[0-9]+]] comdat
+  // CHECK:      define linkonce_odr hidden void @__clang_call_terminate(i8*) unnamed_addr [[NI_NR_NUW:#[0-9]+]] comdat
   // CHECK-NEXT:   [[T0:%.*]] = call i8* @__cxa_begin_catch(i8* %0) [[NUW:#[0-9]+]]
   // CHECK-NEXT:   call void @_ZSt9terminatev() [[NR_NUW:#[0-9]+]]
   // CHECK-NEXT:   unreachable
 
   A *d() {
     // CHECK:    define [[A:%.*]]* @_ZN5test11dEv()
     // CHECK:      [[ACTIVE:%.*]] = alloca i1
     // CHECK-NEXT: [[NEW:%.*]] = call noalias i8* @_Znwm(i64 8)

test/CodeGenCXX/runtimecc.cpp

namespace test0 {
 
   A global;
 // CHECK-LABEL:    define internal void @__cxx_global_var_init()
 // CHECK:      call [[A]]* @_ZN5test01AC1Ev([[A]]* @_ZN5test06globalE)
 // CHECK-NEXT: call i32 @__cxa_atexit(void (i8*)* bitcast ([[A]]* ([[A]]*)* @_ZN5test01AD1Ev to void (i8*)*), i8* bitcast ([[A]]* @_ZN5test06globalE to i8*), i8* @__dso_handle) [[NOUNWIND:#[0-9]+]]
 // CHECK-NEXT: ret void
 }
 
-// CHECK: declare i32 @__cxa_atexit(void (i8*)*, i8*, i8*) [[NOUNWIND]]
+// CHECK: declare i32 @__cxa_atexit(void (i8*)*, i8*, i8*) unnamed_addr [[NOUNWIND]]
 
 namespace test1 {
   void test() {
     throw 0;
   }
 
 // CHECK-LABEL:    define void @_ZN5test14testEv()
 // CHECK:      [[T0:%.*]] = call i8* @__cxa_allocate_exception(i32 4) [[NOUNWIND]]