Eliminate Builtin.UnknownObject as an AST type (#27378)

This removes it from the AST and largely replaces it with AnyObject
at the SIL and IRGen layers. Some notes:

- Reflection still uses the notion of "unknown object" to mean an
  object with unknown refcounting. There's no real reason to make
  this different from AnyObject (an existential containing a
  single object with unknown refcounting), but this way nothing
  changes for clients of Reflection, and it's consistent with how
  native objects are represented.

- The value witness table and reflection descriptor for AnyObject
  use the mangling "BO" instead of "yXl".

- The demangler and remangler continue to support "BO" because it's
  still in use as a type encoding, even if it's not an AST-level
  Type anymore.

- Type-based alias analysis for Builtin.UnknownObject was incorrect,
  so it's a good thing we weren't using it.

- Same with enum layout. (This one assumed UnknownObject never
  referred to an Objective-C tagged pointer. That certainly wasn't how
  we were using it!)

Author: jrose-apple

docs/ABI/Mangling.rst

@@ -481,7 +481,7 @@ Types
   type ::= 'Bf' NATURAL '_'                  // Builtin.Float<n>
   type ::= 'Bi' NATURAL '_'                  // Builtin.Int<n>
   type ::= 'BI'                              // Builtin.IntLiteral
-  type ::= 'BO'                              // Builtin.UnknownObject
+  type ::= 'BO'                              // Builtin.UnknownObject (no longer a distinct type, but still used for AnyObject)
   type ::= 'Bo'                              // Builtin.NativeObject
   type ::= 'Bp'                              // Builtin.RawPointer
   type ::= 'Bt'                              // Builtin.SILToken

docs/ARCOptimization.rst

@@ -335,7 +335,7 @@ is_unique performs depends on the argument type:
 
     - Objective-C object types require an additional check that the
       dynamic object type uses native Swift reference counting:
-      (Builtin.UnknownObject, unknown class reference, class existential)
+      (unknown class reference, class existential)
 
     - Bridged object types allow the dynamic object type check to be
       bypassed based on the pointer encoding:

include/swift/AST/ASTContext.h

@@ -605,7 +605,6 @@ class ASTContext final {
   const CanType TheAnyType;               /// This is 'Any', the empty protocol composition
   const CanType TheNativeObjectType;      /// Builtin.NativeObject
   const CanType TheBridgeObjectType;      /// Builtin.BridgeObject
-  const CanType TheUnknownObjectType;     /// Builtin.UnknownObject
   const CanType TheRawPointerType;        /// Builtin.RawPointer
   const CanType TheUnsafeValueBufferType; /// Builtin.UnsafeValueBuffer
   const CanType TheSILTokenType;          /// Builtin.SILToken

include/swift/AST/TypeMatcher.h

@@ -107,7 +107,6 @@ class TypeMatcher {
     TRIVIAL_CASE(BuiltinRawPointerType)
     TRIVIAL_CASE(BuiltinNativeObjectType)
     TRIVIAL_CASE(BuiltinBridgeObjectType)
-    TRIVIAL_CASE(BuiltinUnknownObjectType)
     TRIVIAL_CASE(BuiltinUnsafeValueBufferType)
     TRIVIAL_CASE(BuiltinVectorType)
     TRIVIAL_CASE(SILTokenType)

include/swift/AST/TypeNodes.def

@@ -98,7 +98,6 @@ ABSTRACT_TYPE(Builtin, Type)
   BUILTIN_TYPE(BuiltinRawPointer, BuiltinType)
   BUILTIN_TYPE(BuiltinNativeObject, BuiltinType)
   BUILTIN_TYPE(BuiltinBridgeObject, BuiltinType)
-  BUILTIN_TYPE(BuiltinUnknownObject, BuiltinType)
   BUILTIN_TYPE(BuiltinUnsafeValueBuffer, BuiltinType)
   BUILTIN_TYPE(BuiltinVector, BuiltinType)
   TYPE_RANGE(Builtin, BuiltinInteger, BuiltinVector)

include/swift/AST/Types.h

@@ -1311,19 +1311,6 @@ class BuiltinBridgeObjectType : public BuiltinType {
 };
 DEFINE_EMPTY_CAN_TYPE_WRAPPER(BuiltinBridgeObjectType, BuiltinType);
 
-/// BuiltinUnknownObjectType - The builtin opaque Objective-C pointer type.
-/// Useful for pushing an Objective-C type through swift.
-class BuiltinUnknownObjectType : public BuiltinType {
-  friend class ASTContext;
-  BuiltinUnknownObjectType(const ASTContext &C)
-    : BuiltinType(TypeKind::BuiltinUnknownObject, C) {}
-public:
-  static bool classof(const TypeBase *T) {
-    return T->getKind() == TypeKind::BuiltinUnknownObject;
-  }
-};
-DEFINE_EMPTY_CAN_TYPE_WRAPPER(BuiltinUnknownObjectType, BuiltinType);
-
 /// BuiltinUnsafeValueBufferType - The builtin opaque fixed-size value
 /// buffer type, into which storage for an arbitrary value can be
 /// allocated using Builtin.allocateValueBuffer.

include/swift/Reflection/ReflectionContext.h

@@ -671,7 +671,7 @@ class ReflectionContext
     // Class existentials have trivial layout.
     // It is itself the pointer to the instance followed by the witness tables.
     case RecordKind::ClassExistential:
-      // This is just Builtin.UnknownObject
+      // This is just AnyObject.
       *OutInstanceTR = ExistentialRecordTI->getFields()[0].TR;
       *OutInstanceAddress = ExistentialAddress;
       return true;

include/swift/Runtime/BuiltinTypes.def

@@ -56,6 +56,8 @@ BUILTIN_POINTER_TYPE(Bb, "Builtin.BridgeObject")
 BUILTIN_POINTER_TYPE(Bp, "Builtin.RawPointer")
 BUILTIN_TYPE(BB, "Builtin.UnsafeValueBuffer")
 
+// No longer used in the compiler as an AST type, but still used for fields 
+// shaped like AnyObject (normal mangling yXl).
 BUILTIN_POINTER_TYPE(BO, "Builtin.UnknownObject")
 
 // Int8 vector types

include/swift/SIL/SILType.h

@@ -512,8 +512,6 @@ class SILType {
 
   /// Get the NativeObject type as a SILType.
   static SILType getNativeObjectType(const ASTContext &C);
-  /// Get the UnknownObject type as a SILType.
-  static SILType getUnknownObjectType(const ASTContext &C);
   /// Get the BridgeObject type as a SILType.
   static SILType getBridgeObjectType(const ASTContext &C);
   /// Get the RawPointer type as a SILType.

include/swift/Strings.h

@@ -106,6 +106,9 @@ constexpr static BuiltinNameStringLiteral BUILTIN_TYPE_NAME_RAWPOINTER = {
 constexpr static BuiltinNameStringLiteral BUILTIN_TYPE_NAME_UNSAFEVALUEBUFFER =
     {"Builtin.UnsafeValueBuffer"};
 /// The name of the Builtin type for UnknownObject
+///
+/// This no longer exists as an AST-accessible type, but it's still used for 
+/// fields shaped like AnyObject when ObjC interop is enabled.
 constexpr static BuiltinNameStringLiteral BUILTIN_TYPE_NAME_UNKNOWNOBJECT = {
     "Builtin.UnknownObject"};
 /// The name of the Builtin type for Vector

lib/AST/ASTContext.cpp

@@ -551,8 +551,6 @@ ASTContext::ASTContext(LangOptions &langOpts, SearchPathOptions &SearchPathOpts,
                            BuiltinNativeObjectType(*this)),
     TheBridgeObjectType(new (*this, AllocationArena::Permanent)
                            BuiltinBridgeObjectType(*this)),
-    TheUnknownObjectType(new (*this, AllocationArena::Permanent)
-                         BuiltinUnknownObjectType(*this)),
     TheRawPointerType(new (*this, AllocationArena::Permanent)
                         BuiltinRawPointerType(*this)),
     TheUnsafeValueBufferType(new (*this, AllocationArena::Permanent)

lib/AST/ASTDumper.cpp

@@ -3373,7 +3373,6 @@ namespace {
     TRIVIAL_TYPE_PRINTER(BuiltinRawPointer, builtin_raw_pointer)
     TRIVIAL_TYPE_PRINTER(BuiltinNativeObject, builtin_native_object)
     TRIVIAL_TYPE_PRINTER(BuiltinBridgeObject, builtin_bridge_object)
-    TRIVIAL_TYPE_PRINTER(BuiltinUnknownObject, builtin_unknown_object)
     TRIVIAL_TYPE_PRINTER(BuiltinUnsafeValueBuffer, builtin_unsafe_value_buffer)
     TRIVIAL_TYPE_PRINTER(SILToken, sil_token)
 

lib/AST/ASTMangler.cpp

@@ -802,8 +802,6 @@ void ASTMangler::appendType(Type type, const ValueDecl *forDecl) {
       return appendOperator("Bo");
     case TypeKind::BuiltinBridgeObject:
       return appendOperator("Bb");
-    case TypeKind::BuiltinUnknownObject:
-      return appendOperator("BO");
     case TypeKind::BuiltinUnsafeValueBuffer:
       return appendOperator("BB");
     case TypeKind::SILToken:

lib/AST/ASTPrinter.cpp

@@ -3601,7 +3601,6 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
   }
   ASTPRINTER_PRINT_BUILTINTYPE(BuiltinRawPointerType)
   ASTPRINTER_PRINT_BUILTINTYPE(BuiltinNativeObjectType)
-  ASTPRINTER_PRINT_BUILTINTYPE(BuiltinUnknownObjectType)
   ASTPRINTER_PRINT_BUILTINTYPE(BuiltinBridgeObjectType)
   ASTPRINTER_PRINT_BUILTINTYPE(BuiltinUnsafeValueBufferType)
   ASTPRINTER_PRINT_BUILTINTYPE(BuiltinIntegerLiteralType)

lib/AST/Builtins.cpp

@@ -80,8 +80,6 @@ Type swift::getBuiltinType(ASTContext &Context, StringRef Name) {
     return Context.TheRawPointerType;
   if (Name == "NativeObject")
     return Context.TheNativeObjectType;
-  if (Name == "UnknownObject")
-    return Context.TheUnknownObjectType;
   if (Name == "BridgeObject")
     return Context.TheBridgeObjectType;
   if (Name == "SILToken")
@@ -2100,9 +2098,6 @@ StringRef BuiltinType::getTypeName(SmallVectorImpl<char> &result,
   case BuiltinTypeKind::BuiltinNativeObject:
     printer << MAYBE_GET_NAMESPACED_BUILTIN(BUILTIN_TYPE_NAME_NATIVEOBJECT);
     break;
-  case BuiltinTypeKind::BuiltinUnknownObject:
-    printer << MAYBE_GET_NAMESPACED_BUILTIN(BUILTIN_TYPE_NAME_UNKNOWNOBJECT);
-    break;
   case BuiltinTypeKind::BuiltinBridgeObject:
     printer << MAYBE_GET_NAMESPACED_BUILTIN(BUILTIN_TYPE_NAME_BRIDGEOBJECT);
     break;

lib/AST/Type.cpp

@@ -170,7 +170,6 @@ bool CanType::isReferenceTypeImpl(CanType type, GenericSignature *sig,
     llvm_unreachable("sugared canonical type?");
 
   // These types are always class references.
-  case TypeKind::BuiltinUnknownObject:
   case TypeKind::BuiltinNativeObject:
   case TypeKind::BuiltinBridgeObject:
   case TypeKind::Class:
@@ -4348,9 +4347,6 @@ ReferenceCounting TypeBase::getReferenceCounting() {
   case TypeKind::BuiltinBridgeObject:
     return ReferenceCounting::Bridge;
 
-  case TypeKind::BuiltinUnknownObject:
-    return ReferenceCounting::Unknown;
-
   case TypeKind::Class:
     return getClassReferenceCounting(cast<ClassType>(type)->getDecl());
   case TypeKind::BoundGenericClass:

lib/IRGen/GenClangType.cpp

@@ -156,8 +156,6 @@ class GenClangType : public CanTypeVisitor<GenClangType, clang::CanQualType> {
   clang::CanQualType visitBuiltinRawPointerType(CanBuiltinRawPointerType type);
   clang::CanQualType visitBuiltinIntegerType(CanBuiltinIntegerType type);
   clang::CanQualType visitBuiltinFloatType(CanBuiltinFloatType type);
-  clang::CanQualType visitBuiltinUnknownObjectType(
-                                                CanBuiltinUnknownObjectType type);
   clang::CanQualType visitArchetypeType(CanArchetypeType type);
   clang::CanQualType visitSILFunctionType(CanSILFunctionType type);
   clang::CanQualType visitGenericTypeParamType(CanGenericTypeParamType type);
@@ -703,12 +701,6 @@ clang::CanQualType GenClangType::visitBuiltinFloatType(
   llvm_unreachable("cannot translate floating-point format to C");
 }
 
-clang::CanQualType GenClangType::visitBuiltinUnknownObjectType(
-  CanBuiltinUnknownObjectType type) {
-  // Builtin.UnknownObject == AnyObject, so it is also translated to 'id'.
-  return getClangIdType(getClangASTContext());
-}
-
 clang::CanQualType GenClangType::visitArchetypeType(CanArchetypeType type) {
   // We see these in the case where we invoke an @objc function
   // through a protocol.

lib/IRGen/GenMeta.cpp

@@ -2614,7 +2614,7 @@ namespace {
       case ClassMetadataStrategy::Fixed: {
         // FIXME: Should this check HasImported instead?
         auto type = (Target->checkAncestry(AncestryFlags::ObjC)
-                    ? IGM.Context.TheUnknownObjectType
+                    ? IGM.Context.getAnyObjectType()
                     : IGM.Context.TheNativeObjectType);
         auto wtable = IGM.getAddrOfValueWitnessTable(type);
         B.add(wtable);
@@ -4011,7 +4011,7 @@ namespace {
       // Without Objective-C interop, foreign classes must still use
       // Swift native reference counting.
       auto type = (IGM.ObjCInterop
-                   ? IGM.Context.TheUnknownObjectType
+                   ? IGM.Context.getAnyObjectType()
                    : IGM.Context.TheNativeObjectType);
       auto wtable = IGM.getAddrOfValueWitnessTable(type);
       B.add(wtable);

lib/IRGen/GenObjC.cpp

@@ -184,15 +184,16 @@ llvm::Value *irgen::emitObjCAutoreleaseReturnValue(IRGenFunction &IGF,
 }
 
 namespace {
-  /// A type-info implementation suitable for Builtin.UnknownObject.
+  /// A type-info implementation suitable for AnyObject on platforms with ObjC
+  /// interop.
   class UnknownTypeInfo : public HeapTypeInfo<UnknownTypeInfo> {
   public:
     UnknownTypeInfo(llvm::PointerType *storageType, Size size,
                  SpareBitVector spareBits, Alignment align)
       : HeapTypeInfo(storageType, size, spareBits, align) {
     }
 
-    /// Builtin.UnknownObject requires ObjC reference-counting.
+    /// AnyObject requires ObjC reference-counting.
     ReferenceCounting getReferenceCounting() const {
       return ReferenceCounting::Unknown;
     }

lib/IRGen/GenReflection.cpp

@@ -847,7 +847,14 @@ class FixedTypeMetadataBuilder : public ReflectionMetadataBuilder {
   }
 
   void layout() override {
-    addTypeRef(type, CanGenericSignature());
+    if (type->isAnyObject()) {
+      // AnyObject isn't actually a builtin type; we're emitting it as the old
+      // Builtin.UnknownObject type for ABI compatibility.
+      B.addRelativeAddress(
+          IGM.getAddrOfStringForTypeRef("BO", MangledTypeRefRole::Reflection));
+    } else {
+      addTypeRef(type, CanGenericSignature());
+    }
 
     B.addInt32(ti->getFixedSize().getValue());
 
@@ -1251,7 +1258,7 @@ emitAssociatedTypeMetadataRecord(const RootProtocolConformance *conformance) {
 void IRGenModule::emitBuiltinReflectionMetadata() {
   if (getSwiftModule()->isStdlibModule()) {
     BuiltinTypes.insert(Context.TheNativeObjectType);
-    BuiltinTypes.insert(Context.TheUnknownObjectType);
+    BuiltinTypes.insert(Context.getAnyObjectType());
     BuiltinTypes.insert(Context.TheBridgeObjectType);
     BuiltinTypes.insert(Context.TheRawPointerType);
     BuiltinTypes.insert(Context.TheUnsafeValueBufferType);

lib/IRGen/GenType.cpp

@@ -1788,8 +1788,6 @@ const TypeInfo *TypeConverter::convertType(CanType ty) {
   }
   case TypeKind::BuiltinNativeObject:
     return &getNativeObjectTypeInfo();
-  case TypeKind::BuiltinUnknownObject:
-    return &getUnknownObjectTypeInfo();
   case TypeKind::BuiltinBridgeObject:
     return &getBridgeObjectTypeInfo();
   case TypeKind::BuiltinUnsafeValueBuffer:

lib/IRGen/GenValueWitness.cpp

@@ -1001,7 +1001,7 @@ getAddrOfKnownValueWitnessTable(IRGenModule &IGM, CanType type,
     case ReferenceCounting::ObjC:
     case ReferenceCounting::Block:
     case ReferenceCounting::Unknown:
-      witnessSurrogate = C.TheUnknownObjectType;
+      witnessSurrogate = C.getAnyObjectType();
       break;
     case ReferenceCounting::Bridge:
       witnessSurrogate = C.TheBridgeObjectType;

lib/IRGen/IRGenDebugInfo.cpp

@@ -1207,18 +1207,6 @@ class IRGenDebugInfoImpl : public IRGenDebugInfo {
       break;
     }
 
-    case TypeKind::BuiltinUnknownObject: {
-      // The builtin opaque Objective-C pointer type. Useful for pushing
-      // an Objective-C type through swift.
-      unsigned PtrSize = CI.getTargetInfo().getPointerWidth(0);
-      auto IdTy = DBuilder.createForwardDecl(llvm::dwarf::DW_TAG_structure_type,
-                                             MangledName, Scope, File, 0,
-                                             llvm::dwarf::DW_LANG_ObjC, 0, 0);
-      return DBuilder.createPointerType(IdTy, PtrSize, 0,
-                                        /* DWARFAddressSpace */ None,
-                                        MangledName);
-    }
-
     case TypeKind::BuiltinNativeObject: {
       unsigned PtrSize = CI.getTargetInfo().getPointerWidth(0);
       auto PTy =

lib/IRGen/IRGenMangler.h

@@ -80,7 +80,16 @@ class IRGenMangler : public Mangle::ASTMangler {
   std::string mangleValueWitness(Type type, ValueWitness witness);
 
   std::string mangleValueWitnessTable(Type type) {
-    return mangleTypeSymbol(type, "WV");
+    const char * const witnessTableOp = "WV";
+    if (type->isAnyObject()) {
+      // Special-case for AnyObject, whose witness table is under the old name
+      // Builtin.UnknownObject, even though we don't use that as a Type anymore.
+      beginMangling();
+      appendOperator("BO");
+      appendOperator(witnessTableOp);
+      return finalize();
+    }
+    return mangleTypeSymbol(type, witnessTableOp);
   }
 
   std::string mangleTypeMetadataAccessFunction(Type type) {
@@ -382,7 +391,17 @@ class IRGenMangler : public Mangle::ASTMangler {
   }
 
   std::string mangleReflectionBuiltinDescriptor(Type type) {
-    return mangleTypeSymbol(type, "MB");
+    const char * const reflectionDescriptorOp = "MB";
+    if (type->isAnyObject()) {
+      // Special-case for AnyObject, whose reflection descriptor is under the
+      // old name Builtin.UnknownObject, even though we don't use that as a Type
+      // anymore.
+      beginMangling();
+      appendOperator("BO");
+      appendOperator(reflectionDescriptorOp);
+      return finalize();
+    }
+    return mangleTypeSymbol(type, reflectionDescriptorOp);
   }
 
   std::string mangleReflectionFieldDescriptor(Type type) {

lib/IRGen/IRGenSIL.cpp

@@ -3913,8 +3913,7 @@ static bool hasReferenceSemantics(IRGenSILFunction &IGF,
   return (objType->mayHaveSuperclass()
           || objType->isClassExistentialType()
           || objType->is<BuiltinNativeObjectType>()
-          || objType->is<BuiltinBridgeObjectType>()
-          || objType->is<BuiltinUnknownObjectType>());
+          || objType->is<BuiltinBridgeObjectType>());
 }
 
 static llvm::Value *emitIsUnique(IRGenSILFunction &IGF, SILValue operand,