[IRGen] Don't emit relative references to Objective-C class references.

Objective-C class references (which show up in the __objc_classrefs
section) are always coalesced by the linker. When we relatively
address them (which occurs in protocol conformance records), the
linker may compute the relative offset *before* coalescing, leading to
an incorrect result. The net effect is a protocol conformance record
that applies to the wrong Objective-C class, causing all sorts of
runtime mayhem.

Switch relatively-addressed Objective-C classes over to using the
Objective-C runtime name of the class. It's a less efficient encoding
(since we need to go through objc_lookUpClass), but it avoids the
linker bug.

Fixes rdar://problem/46428085 by working around the linker bug.

Author: DougGregor

lib/IRGen/GenDecl.cpp

@@ -2565,28 +2565,12 @@ getProtocolDescriptorEntityReference(IRGenModule &IGM, ProtocolDecl *protocol) {
 }
 
 static TypeEntityReference
-getObjCClassRefEntityReference(IRGenModule &IGM, ClassDecl *cls) {
-  // A reference to an Objective-C class object.
-  assert(cls->isObjC() && "Must have an Objective-C class here");
-
-  auto kind = TypeReferenceKind::IndirectObjCClass;
-  auto entity = LinkEntity::forObjCClassRef(cls);
-
-  auto ref = IGM.getAddrOfLLVMVariableOrGOTEquivalent(entity);
-  assert(!ref.isIndirect());
-
-  return TypeEntityReference(kind, ref.getValue());
-}
-
-static TypeEntityReference
-getRuntimeOnlyClassEntityReference(IRGenModule &IGM, ClassDecl *cls) {
-  assert(cls->getForeignClassKind() == ClassDecl::ForeignKind::RuntimeOnly);
-
-  auto namedClangDecl = Mangle::ASTMangler::getClangDeclForMangling(cls);
-  assert(namedClangDecl);
-
+getObjCClassByNameReference(IRGenModule &IGM, ClassDecl *cls) {
   auto kind = TypeReferenceKind::DirectObjCClassName;
-  auto ref = IGM.getAddrOfGlobalString(namedClangDecl->getName());
+  SmallString<64> objcRuntimeNameBuffer;
+  auto ref = IGM.getAddrOfGlobalString(
+                                 cls->getObjCRuntimeName(objcRuntimeNameBuffer),
+                                 /*willByRelativelyAddressed=*/true);
 
   return TypeEntityReference(kind, ref);
 }
@@ -2605,17 +2589,22 @@ IRGenModule::getTypeEntityReference(NominalTypeDecl *decl) {
 
   switch (clas->getForeignClassKind()) {
   case ClassDecl::ForeignKind::RuntimeOnly:
-    return getRuntimeOnlyClassEntityReference(*this, clas);
+    return getObjCClassByNameReference(*this, clas);
 
   case ClassDecl::ForeignKind::CFType:
     return getTypeContextDescriptorEntityReference(*this, clas);
 
   case ClassDecl::ForeignKind::Normal:
     if (hasKnownSwiftMetadata(*this, clas)) {
       return getTypeContextDescriptorEntityReference(*this, clas);
-    } else {
-      return getObjCClassRefEntityReference(*this, clas);
     }
+
+    // Note: we would like to use an Objective-C class reference, but the
+    // Darwin linker currently has a bug where it will coalesce these symbols
+    // *after* computing a relative offset, causing incorrect relative
+    // offsets in the metadata. Therefore, reference Objective-C classes by
+    // their runtime names.
+    return getObjCClassByNameReference(*this, clas);
   }
   llvm_unreachable("bad foreign type kind");
 }

test/IRGen/objc_bridged_generic_conformance.swift

@@ -2,7 +2,9 @@
 
 // CHECK-NOT: _TMnCSo
 
-// CHECK: @"$sSo6ThingyCyxG32objc_bridged_generic_conformance1PADMc" = hidden constant %swift.protocol_conformance_descriptor {{.*}} @"\01l_OBJC_CLASS_REF_$_Thingy"
+// CHECK: @"$sSo6ThingyCyxG32objc_bridged_generic_conformance1PADMc" = hidden constant %swift.protocol_conformance_descriptor {{.*}} @[[THINGY_NAME:[0-9]]]
+
+// CHECK: @[[THINGY_NAME]] = private constant [7 x i8] c"Thingy\00"
 
 // CHECK-NOT: _TMnCSo
 

test/IRGen/objc_runtime_visible_conformance.swift

@@ -10,8 +10,8 @@ extension A : YourProtocol {}
 
 // CHECK-LABEL: @"$sSo1AC32objc_runtime_visible_conformance10MyProtocolACMc"
 // CHECK-SAME:    @"$s32objc_runtime_visible_conformance10MyProtocolMp"
-// CHECK-SAME:    [2 x i8]* [[STRING_A:@[0-9]+]]
+// CHECK-SAME:    [[STRING_A:@[0-9]+]]
 // CHECK-SAME:    @"$sSo1AC32objc_runtime_visible_conformance10MyProtocolACWP"
 //   DirectObjCClassName
 // CHECK-SAME:    i32 16
-// CHECK:       [[STRING_A]] = private unnamed_addr constant [2 x i8] c"A\00"
+// CHECK:       [[STRING_A]] = private constant [22 x i8] c"MyRuntimeVisibleClass\00"

test/IRGen/protocol_conformance_records_objc.swift

@@ -29,17 +29,19 @@ extension NSRect: Runcible {
 // CHECK-LABEL:         @"$sSo5GizmoC33protocol_conformance_records_objc8RuncibleACMc" = constant %swift.protocol_conformance_descriptor {
 // -- protocol descriptor
 // CHECK-SAME:           [[RUNCIBLE]]
-// -- class object reference
-// CHECK-SAME:           @"\01l_OBJC_CLASS_REF_$_Gizmo"
+// -- class name reference
+// CHECK-SAME:           @[[GIZMO_NAME:[0-9]+]]
 // -- witness table
 // CHECK-SAME:           @"$sSo5GizmoC33protocol_conformance_records_objc8RuncibleACWP"
 // -- flags
-// CHECK-SAME:           i32 24
+// CHECK-SAME:           i32 16
 // CHECK-SAME:         }
 extension Gizmo: Runcible {
   public func runce() {}
 }
 
+// CHECK: @[[GIZMO_NAME]] = private constant [6 x i8] c"Gizmo\00"
+
 // CHECK-LABEL: @"\01l_protocol_conformances" = private constant [
 // CHECK-SAME: @"$sSo6NSRectV33protocol_conformance_records_objc8RuncibleACMc"
 // CHECK-SAME: @"$sSo5GizmoC33protocol_conformance_records_objc8RuncibleACMc"