stdlib/public/runtime/MetadataLookup.cpp

//===--- MetadataLookup.cpp - Swift Language Type Name Lookup -------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// Implementations of runtime functions for looking up a type by name.
//
//===----------------------------------------------------------------------===//

#include "swift/Basic/Lazy.h"
#include "swift/Demangling/Demangler.h"
#include "swift/Demangling/TypeDecoder.h"
#include "swift/Reflection/Records.h"
#include "swift/ABI/TypeIdentity.h"
#include "swift/Runtime/Casting.h"
#include "swift/Runtime/Concurrent.h"
#include "swift/Runtime/Debug.h"
#include "swift/Runtime/HeapObject.h"
#include "swift/Runtime/Metadata.h"
#include "swift/Runtime/Mutex.h"
#include "swift/Strings.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/PointerUnion.h"
#include "llvm/ADT/StringExtras.h"
#include "Private.h"
#include "CompatibilityOverride.h"
#include "ImageInspection.h"
#include <functional>
#include <vector>
#include <list>

using namespace swift;
using namespace Demangle;
using namespace reflection;

#if SWIFT_OBJC_INTEROP
#include <objc/runtime.h>
#include <objc/message.h>
#include <objc/objc.h>
#include <dlfcn.h>
#endif

/// A Demangler suitable for resolving runtime type metadata strings.
template <class Base = Demangler>
class DemanglerForRuntimeTypeResolution : public Base {
public:
  using Base::demangleSymbol;
  using Base::demangleType;

  // Force callers to explicitly pass `nullptr` to demangleSymbol or
  // demangleType if they don't want to demangle symbolic references.
  NodePointer demangleSymbol(StringRef symbolName) = delete;
  NodePointer demangleType(StringRef typeName) = delete;

  NodePointer demangleTypeRef(StringRef symbolName) {
    // Resolve symbolic references to type contexts into the absolute address of
    // the type context descriptor, so that if we see a symbolic reference in
    // the mangled name we can immediately find the associated metadata.
    return Base::demangleType(symbolName,
                              ResolveAsSymbolicReference(*this));
  }
};

/// Resolve the relative reference in a mangled symbolic reference.
static uintptr_t resolveSymbolicReferenceOffset(SymbolicReferenceKind kind,
                                                Directness isIndirect,
                                                int32_t offset,
                                                const void *base) {
  auto ptr = detail::applyRelativeOffset(base, offset);

  // Indirect references may be authenticated in a way appropriate for the
  // referent.
  if (isIndirect == Directness::Indirect) {
    switch (kind) {
    case SymbolicReferenceKind::Context: {
      ContextDescriptor *contextPtr =
        *(const TargetSignedContextPointer<InProcess> *)ptr;
      return (uintptr_t)contextPtr;
    }
    case SymbolicReferenceKind::AccessorFunctionReference: {
      swift_unreachable("should not be indirectly referenced");
    }
    }
    swift_unreachable("unknown symbolic reference kind");
  } else {
    return ptr;
  }
}

NodePointer
ResolveAsSymbolicReference::operator()(SymbolicReferenceKind kind,
                                       Directness isIndirect,
                                       int32_t offset,
                                       const void *base) {
  // Resolve the absolute pointer to the entity being referenced.
  auto ptr = resolveSymbolicReferenceOffset(kind, isIndirect, offset, base);

  // Figure out this symbolic reference's grammatical role.
  Node::Kind nodeKind;
  bool isType;
  switch (kind) {
  case Demangle::SymbolicReferenceKind::Context: {
    auto descriptor = (const ContextDescriptor *)ptr;
    switch (descriptor->getKind()) {
    case ContextDescriptorKind::Protocol:
      nodeKind = Node::Kind::ProtocolSymbolicReference;
      isType = false;
      break;
    
    case ContextDescriptorKind::OpaqueType:
      nodeKind = Node::Kind::OpaqueTypeDescriptorSymbolicReference;
      isType = false;
      break;
        
    default:
      if (auto typeContext = dyn_cast<TypeContextDescriptor>(descriptor)) {
        nodeKind = Node::Kind::TypeSymbolicReference;
        isType = true;
        break;
      }
      
      // References to other kinds of context aren't yet implemented.
      return nullptr;
    }
    break;
  }
  case Demangle::SymbolicReferenceKind::AccessorFunctionReference: {
    // Save the pointer to the accessor function. We can't demangle it any
    // further as AST, but the consumer of the demangle tree may be able to
    // invoke the function to resolve the thing they're trying to access.
    nodeKind = Node::Kind::AccessorFunctionReference;
    isType = false;
#if SWIFT_PTRAUTH
    // The pointer refers to an accessor function, which we need to sign.
    ptr = (uintptr_t)ptrauth_sign_unauthenticated((void*)ptr,
      ptrauth_key_function_pointer, 0);
#endif
    break;
  }
  }
  
  auto node = Dem.createNode(nodeKind, ptr);
  if (isType) {
    auto typeNode = Dem.createNode(Node::Kind::Type);
    typeNode->addChild(node, Dem);
    node = typeNode;
  }
  return node;
}

static NodePointer
_buildDemanglingForSymbolicReference(SymbolicReferenceKind kind,
                                     const void *resolvedReference,
                                     Demangler &Dem) {
  switch (kind) {
  case SymbolicReferenceKind::Context:
    return _buildDemanglingForContext(
      (const ContextDescriptor *)resolvedReference, {}, Dem);
  case SymbolicReferenceKind::AccessorFunctionReference:
#if SWIFT_PTRAUTH
    // The pointer refers to an accessor function, which we need to sign.
    resolvedReference = ptrauth_sign_unauthenticated(resolvedReference,
      ptrauth_key_function_pointer, 0);
#endif
    return Dem.createNode(Node::Kind::AccessorFunctionReference,
                          (uintptr_t)resolvedReference);
  }
  
  swift_unreachable("invalid symbolic reference kind");
}
  
NodePointer
ResolveToDemanglingForContext::operator()(SymbolicReferenceKind kind,
                                          Directness isIndirect,
                                          int32_t offset,
                                          const void *base) {
  auto ptr = resolveSymbolicReferenceOffset(kind, isIndirect, offset, base);

  return _buildDemanglingForSymbolicReference(kind, (const void *)ptr, Dem);
}

NodePointer
ExpandResolvedSymbolicReferences::operator()(SymbolicReferenceKind kind,
                                             const void *ptr) {
  return _buildDemanglingForSymbolicReference(kind, (const void *)ptr, Dem);
}

#pragma mark Nominal type descriptor cache
// Type Metadata Cache.

namespace {
  struct TypeMetadataSection {
    const TypeMetadataRecord *Begin, *End;
    const TypeMetadataRecord *begin() const {
      return Begin;
    }
    const TypeMetadataRecord *end() const {
      return End;
    }
  };

  struct NominalTypeDescriptorCacheEntry {
  private:
    const char *Name;
    size_t NameLength;
    const ContextDescriptor *Description;

  public:
    NominalTypeDescriptorCacheEntry(const llvm::StringRef name,
                                    const ContextDescriptor *description)
        : Description(description) {
      char *nameCopy = reinterpret_cast<char *>(malloc(name.size()));
      memcpy(nameCopy, name.data(), name.size());
      Name = nameCopy;
      NameLength = name.size();
    }

    const ContextDescriptor *getDescription() const { return Description; }

    bool matchesKey(llvm::StringRef aName) {
      return aName == llvm::StringRef{Name, NameLength};
    }

    friend llvm::hash_code
    hash_value(const NominalTypeDescriptorCacheEntry &value) {
      return hash_value(llvm::StringRef{value.Name, value.NameLength});
    }

    template <class... T>
    static size_t getExtraAllocationSize(T &&... ignored) {
      return 0;
    }
  };
} // end anonymous namespace

struct TypeMetadataPrivateState {
  ConcurrentReadableHashMap<NominalTypeDescriptorCacheEntry> NominalCache;
  ConcurrentReadableArray<TypeMetadataSection> SectionsToScan;
  
  TypeMetadataPrivateState() {
    initializeTypeMetadataRecordLookup();
  }

};

static Lazy<TypeMetadataPrivateState> TypeMetadataRecords;

static void
_registerTypeMetadataRecords(TypeMetadataPrivateState &T,
                             const TypeMetadataRecord *begin,
                             const TypeMetadataRecord *end) {
  T.SectionsToScan.push_back(TypeMetadataSection{begin, end});
}

void swift::addImageTypeMetadataRecordBlockCallbackUnsafe(
    const void *records, uintptr_t recordsSize) {
  assert(recordsSize % sizeof(TypeMetadataRecord) == 0
         && "weird-sized type metadata section?!");

  // If we have a section, enqueue the type metadata for lookup.
  auto recordBytes = reinterpret_cast<const char *>(records);
  auto recordsBegin
    = reinterpret_cast<const TypeMetadataRecord*>(records);
  auto recordsEnd
    = reinterpret_cast<const TypeMetadataRecord*>(recordBytes + recordsSize);

  // Type metadata cache should always be sufficiently initialized by this
  // point. Attempting to go through get() may also lead to an infinite loop,
  // since we register records during the initialization of
  // TypeMetadataRecords.
  _registerTypeMetadataRecords(TypeMetadataRecords.unsafeGetAlreadyInitialized(),
                               recordsBegin, recordsEnd);
}

void swift::addImageTypeMetadataRecordBlockCallback(const void *records,
                                                    uintptr_t recordsSize) {
  TypeMetadataRecords.get();
  addImageTypeMetadataRecordBlockCallbackUnsafe(records, recordsSize);
}

void
swift::swift_registerTypeMetadataRecords(const TypeMetadataRecord *begin,
                                         const TypeMetadataRecord *end) {
  auto &T = TypeMetadataRecords.get();
  _registerTypeMetadataRecords(T, begin, end);
}

static const ContextDescriptor *
_findContextDescriptor(Demangle::NodePointer node,
                           Demangle::Demangler &Dem);

/// Find the context descriptor for the type extended by the given extension.
///
/// If \p maybeExtension isn't actually an extension context, returns nullptr.
static const ContextDescriptor *
_findExtendedTypeContextDescriptor(const ContextDescriptor *maybeExtension,
                                   Demangler &demangler,
                                   Demangle::NodePointer *demangledNode
                                     = nullptr) {
  auto extension = dyn_cast<ExtensionContextDescriptor>(maybeExtension);
  if (!extension)
    return nullptr;

  Demangle::NodePointer localNode;
  Demangle::NodePointer &node = demangledNode ? *demangledNode : localNode;

  auto mangledName = extension->getMangledExtendedContext();
  node = demangler.demangleType(mangledName,
                                ResolveAsSymbolicReference(demangler));
  if (!node)
    return nullptr;
  if (node->getKind() == Node::Kind::Type) {
    if (node->getNumChildren() < 1)
      return nullptr;
    node = node->getChild(0);
  }
  if (Demangle::isSpecialized(node)) {
    node = Demangle::getUnspecialized(node, demangler);
  }

  return _findContextDescriptor(node, demangler);
}

/// Recognize imported tag types, which have a special mangling rule.
///
/// This should be kept in sync with the AST mangler and with
/// buildContextDescriptorMangling in MetadataReader.
bool swift::_isCImportedTagType(const TypeContextDescriptor *type,
                                const ParsedTypeIdentity &identity) {
  // Tag types are always imported as structs or enums.
  if (type->getKind() != ContextDescriptorKind::Enum &&
      type->getKind() != ContextDescriptorKind::Struct)
    return false;

  // Not a typedef imported as a nominal type.
  if (identity.isCTypedef())
    return false;

  // Not a related entity.
  if (identity.isAnyRelatedEntity())
    return false;

  // Imported from C.
  return type->Parent->isCImportedContext();
}

ParsedTypeIdentity
ParsedTypeIdentity::parse(const TypeContextDescriptor *type) {
  ParsedTypeIdentity result;

  // The first component is the user-facing name and (unless overridden)
  // the ABI name.
  StringRef component = type->Name.get();
  result.UserFacingName = component;

  // If we don't have import info, we're done.
  if (!type->getTypeContextDescriptorFlags().hasImportInfo()) {
    result.FullIdentity = result.UserFacingName;
    return result;
  }

  // Otherwise, start parsing the import information.
  result.ImportInfo.emplace();

  // The identity starts with the user-facing name.
  const char *startOfIdentity = component.begin();
  const char *endOfIdentity = component.end();

#ifndef NDEBUG
  enum {
    AfterName,
    AfterABIName,
    AfterSymbolNamespace,
    AfterRelatedEntityName,
    AfterIdentity,
  } stage = AfterName;
#endif

  while (true) {
    // Parse the next component.  If it's empty, we're done.
    component = StringRef(component.end() + 1);
    if (component.empty()) break;

    // Update the identity bounds and assert that the identity
    // components are in the right order.
    auto kind = TypeImportComponent(component[0]);
    if (kind == TypeImportComponent::ABIName) {
#ifndef NDEBUG
      assert(stage < AfterABIName);
      stage = AfterABIName;
      assert(result.UserFacingName != component.drop_front(1) &&
             "user-facing name was same as the ABI name");
#endif
      startOfIdentity = component.begin() + 1;
      endOfIdentity = component.end();
    } else if (kind == TypeImportComponent::SymbolNamespace) {
#ifndef NDEBUG
      assert(stage < AfterSymbolNamespace);
      stage = AfterSymbolNamespace;
#endif
      endOfIdentity = component.end();
    } else if (kind == TypeImportComponent::RelatedEntityName) {
#ifndef NDEBUG
      assert(stage < AfterRelatedEntityName);
      stage = AfterRelatedEntityName;
#endif
      endOfIdentity = component.end();
    } else {
#ifndef NDEBUG
      // Anything else is assumed to not be part of the identity.
      stage = AfterIdentity;
#endif
    }

    // Collect the component, whatever it is.
    result.ImportInfo->collect</*asserting*/true>(component);
  }

  assert(stage != AfterName && "no components?");

  // Record the full identity.
  result.FullIdentity =
    StringRef(startOfIdentity, endOfIdentity - startOfIdentity);

  return result;
}

#if SWIFT_OBJC_INTEROP
/// Determine whether the two demangle trees both refer to the same
/// Objective-C class or protocol referenced by name.
static bool sameObjCTypeManglings(Demangle::NodePointer node1,
                                  Demangle::NodePointer node2) {
  // Entities need to be of the same kind.
  if (node1->getKind() != node2->getKind())
    return false;

  auto name1 = Demangle::getObjCClassOrProtocolName(node1);
  if (!name1) return false;

  auto name2 = Demangle::getObjCClassOrProtocolName(node2);
  if (!name2) return false;

  return *name1 == *name2;
}
#endif

bool
swift::_contextDescriptorMatchesMangling(const ContextDescriptor *context,
                                         Demangle::NodePointer node) {
  while (context) {
    if (node->getKind() == Demangle::Node::Kind::Type)
      node = node->getChild(0);
    
    // We can directly match symbolic references to the current context.
    if (node) {
      if (node->getKind() == Demangle::Node::Kind::TypeSymbolicReference
         || node->getKind() == Demangle::Node::Kind::ProtocolSymbolicReference){
        if (equalContexts(context,
               reinterpret_cast<const ContextDescriptor *>(node->getIndex()))) {
          return true;
        }
      }
    }

    switch (context->getKind()) {
    case ContextDescriptorKind::Module: {
      auto module = cast<ModuleContextDescriptor>(context);
      // Match to a mangled module name.
      if (node->getKind() != Demangle::Node::Kind::Module)
        return false;
      if (!node->getText().equals(module->Name.get()))
        return false;
      
      node = nullptr;
      break;
    }
    
    case ContextDescriptorKind::Extension: {
      auto extension = cast<ExtensionContextDescriptor>(context);
      
      // Check whether the extension context matches the mangled context.
      if (node->getKind() != Demangle::Node::Kind::Extension)
        return false;
      if (node->getNumChildren() < 2)
        return false;
      
      // Check that the context being extended matches as well.
      auto extendedContextNode = node->getChild(1);
      DemanglerForRuntimeTypeResolution<> demangler;

      auto extendedDescriptorFromNode =
        _findContextDescriptor(extendedContextNode, demangler);

      Demangle::NodePointer extendedContextDemangled;
      auto extendedDescriptorFromDemangled =
        _findExtendedTypeContextDescriptor(extension, demangler,
                                           &extendedContextDemangled);

      // Determine whether the contexts match.
      bool contextsMatch =
        extendedDescriptorFromNode && extendedDescriptorFromDemangled &&
        equalContexts(extendedDescriptorFromNode,
                      extendedDescriptorFromDemangled);
      
#if SWIFT_OBJC_INTEROP
      // If we have manglings of the same Objective-C type, the contexts match.
      if (!contextsMatch &&
          (!extendedDescriptorFromNode || !extendedDescriptorFromDemangled) &&
          sameObjCTypeManglings(extendedContextNode,
                                extendedContextDemangled)) {
        contextsMatch = true;
      }
#endif

      if (!contextsMatch)
        return false;
      
      // Check whether the generic signature of the extension matches the
      // mangled constraints, if any.

      if (node->getNumChildren() >= 3) {
        // NB: If we ever support extensions with independent generic arguments
        // like `extension <T> Array where Element == Optional<T>`, we'd need
        // to look at the mangled context name to match up generic arguments.
        // That would probably need a new extension mangling form, though.
        
        // TODO
      }
      
      // The parent context of the extension should match in the mangling and
      // context descriptor.
      node = node->getChild(0);
      break;
    }

    case ContextDescriptorKind::Protocol:
      // Match a protocol context.
      if (node->getKind() == Demangle::Node::Kind::Protocol) {
        auto proto = llvm::cast<ProtocolDescriptor>(context);
        auto nameNode = node->getChild(1);
        if (nameNode->getKind() != Demangle::Node::Kind::Identifier)
          return false;
        if (nameNode->getText() == proto->Name.get()) {
          node = node->getChild(0);
          break;
        }
      }
      return false;

    default:
      if (auto type = llvm::dyn_cast<TypeContextDescriptor>(context)) {
        llvm::Optional<ParsedTypeIdentity> _identity;
        auto getIdentity = [&]() -> const ParsedTypeIdentity & {
          if (_identity) return *_identity;
          _identity = ParsedTypeIdentity::parse(type);
          return *_identity;
        };

        switch (node->getKind()) {
        // If the mangled name doesn't indicate a type kind, accept anything.
        // Otherwise, try to match them up.
        case Demangle::Node::Kind::OtherNominalType:
          break;
        case Demangle::Node::Kind::Structure:
          // We allow non-structs to match Kind::Structure if they are
          // imported C tag types.  This is necessary because we artificially
          // make imported C tag types Kind::Structure.
          if (type->getKind() != ContextDescriptorKind::Struct &&
              !_isCImportedTagType(type, getIdentity()))
            return false;
          break;
        case Demangle::Node::Kind::Class:
          if (type->getKind() != ContextDescriptorKind::Class)
            return false;
          break;
        case Demangle::Node::Kind::Enum:
          if (type->getKind() != ContextDescriptorKind::Enum)
            return false;
          break;
        case Demangle::Node::Kind::TypeAlias:
          if (!getIdentity().isCTypedef())
            return false;
          break;

        default:
          return false;
        }

        auto nameNode = node->getChild(1);
        
        // Declarations synthesized by the Clang importer get a small tag
        // string in addition to their name.
        if (nameNode->getKind() == Demangle::Node::Kind::RelatedEntityDeclName){          
          if (!getIdentity().isRelatedEntity(
                                        nameNode->getFirstChild()->getText()))
            return false;
          
          nameNode = nameNode->getChild(1);
        } else if (getIdentity().isAnyRelatedEntity()) {
          return false;
        }
        
        // We should only match public or internal declarations with stable
        // names. The runtime metadata for private declarations would be
        // anonymized.
        if (nameNode->getKind() == Demangle::Node::Kind::Identifier) {
          if (nameNode->getText() != getIdentity().getABIName())
            return false;
          
          node = node->getChild(0);
          break;
        }
        
        return false;

      }

      // We don't know about this kind of context, or it doesn't have a stable
      // name we can match to.
      return false;
    }
    
    context = context->Parent;
  }
  
  // We should have reached the top of the node tree at the same time we reached
  // the top of the context tree.
  if (node)
    return false;
  
  return true;
}

// returns the nominal type descriptor for the type named by typeName
static const ContextDescriptor *
_searchTypeMetadataRecords(TypeMetadataPrivateState &T,
                           Demangle::NodePointer node) {
  for (auto &section : T.SectionsToScan.snapshot()) {
    for (const auto &record : section) {
      if (auto context = record.getContextDescriptor()) {
        if (_contextDescriptorMatchesMangling(context, node)) {
          return context;
        }
      }
    }
  }

  return nullptr;
}

#define DESCRIPTOR_MANGLING_SUFFIX_Structure Mn
#define DESCRIPTOR_MANGLING_SUFFIX_Enum Mn
#define DESCRIPTOR_MANGLING_SUFFIX_Protocol Mp

#define DESCRIPTOR_MANGLING_SUFFIX_(X) X
#define DESCRIPTOR_MANGLING_SUFFIX(KIND) \
  DESCRIPTOR_MANGLING_SUFFIX_(DESCRIPTOR_MANGLING_SUFFIX_ ## KIND)

#define DESCRIPTOR_MANGLING_(CHAR, SUFFIX) \
  $sS ## CHAR ## SUFFIX
#define DESCRIPTOR_MANGLING(CHAR, SUFFIX) DESCRIPTOR_MANGLING_(CHAR, SUFFIX)

#define STANDARD_TYPE(KIND, MANGLING, TYPENAME) \
  extern "C" const ContextDescriptor DESCRIPTOR_MANGLING(MANGLING, DESCRIPTOR_MANGLING_SUFFIX(KIND));

#if !SWIFT_OBJC_INTEROP
# define OBJC_INTEROP_STANDARD_TYPE(KIND, MANGLING, TYPENAME)
#endif

#include "swift/Demangling/StandardTypesMangling.def"

static const ContextDescriptor *
_findContextDescriptor(Demangle::NodePointer node,
                       Demangle::Demangler &Dem) {
  NodePointer symbolicNode = node;
  if (symbolicNode->getKind() == Node::Kind::Type)
    symbolicNode = symbolicNode->getChild(0);

  // If we have a symbolic reference to a context, resolve it immediately.
  if (symbolicNode->getKind() == Node::Kind::TypeSymbolicReference) {
    return cast<TypeContextDescriptor>(
      (const ContextDescriptor *)symbolicNode->getIndex());
  }

  // Fast-path lookup for standard library type references with short manglings.
  if (symbolicNode->getNumChildren() >= 2
      && symbolicNode->getChild(0)->getKind() == Node::Kind::Module
      && symbolicNode->getChild(0)->getText().equals("Swift")
      && symbolicNode->getChild(1)->getKind() == Node::Kind::Identifier) {
    auto name = symbolicNode->getChild(1)->getText();

#define STANDARD_TYPE(KIND, MANGLING, TYPENAME) \
    if (name.equals(#TYPENAME)) { \
      return &DESCRIPTOR_MANGLING(MANGLING, DESCRIPTOR_MANGLING_SUFFIX(KIND)); \
    }
#if !SWIFT_OBJC_INTEROP
# define OBJC_INTEROP_STANDARD_TYPE(KIND, MANGLING, TYPENAME)
#endif

#include "swift/Demangling/StandardTypesMangling.def"
  }
  
  const ContextDescriptor *foundContext = nullptr;
  auto &T = TypeMetadataRecords.get();

  // Nothing to resolve if have a generic parameter.
  if (symbolicNode->getKind() == Node::Kind::DependentGenericParamType)
    return nullptr;

  StringRef mangledName =
    Demangle::mangleNode(node, ExpandResolvedSymbolicReferences(Dem), Dem);

  // Look for an existing entry.
  // Find the bucket for the metadata entry.
  {
    auto snapshot = T.NominalCache.snapshot();
    if (auto Value = snapshot.find(mangledName))
      return Value->getDescription();
  }

  // Check type metadata records		   
  // Scan any newly loaded images for context descriptors, then try the context
  foundContext = _searchTypeMetadataRecords(T, node);
  
  // Check protocol conformances table. Note that this has no support for		
  // resolving generic types yet.
  if (!foundContext)
    foundContext = _searchConformancesByMangledTypeName(node);
  
  if (foundContext)
    T.NominalCache.getOrInsert(mangledName, [&](NominalTypeDescriptorCacheEntry
                                                    *entry,
                                                bool created) {
      if (created)
        new (entry) NominalTypeDescriptorCacheEntry{mangledName, foundContext};
      return true;
    });

  return foundContext;
}

#pragma mark Protocol descriptor cache
namespace {
  struct ProtocolSection {
    const ProtocolRecord *Begin, *End;

    const ProtocolRecord *begin() const {
      return Begin;
    }
    const ProtocolRecord *end() const {
      return End;
    }
  };

  struct ProtocolDescriptorCacheEntry {
  private:
    const char *Name;
    size_t NameLength;
    const ProtocolDescriptor *Description;

  public:
    ProtocolDescriptorCacheEntry(const llvm::StringRef name,
                                 const ProtocolDescriptor *description)
        : Description(description) {
      char *nameCopy = reinterpret_cast<char *>(malloc(name.size()));
      memcpy(nameCopy, name.data(), name.size());
      Name = nameCopy;
      NameLength = name.size();
    }

    const ProtocolDescriptor *getDescription() const { return Description; }

    bool matchesKey(llvm::StringRef aName) {
      return aName == llvm::StringRef{Name, NameLength};
    }

    friend llvm::hash_code
    hash_value(const ProtocolDescriptorCacheEntry &value) {
      return hash_value(llvm::StringRef{value.Name, value.NameLength});
    }

    template <class... T>
    static size_t getExtraAllocationSize(T &&... ignored) {
      return 0;
    }
  };

  struct ProtocolMetadataPrivateState {
    ConcurrentReadableHashMap<ProtocolDescriptorCacheEntry> ProtocolCache;
    ConcurrentReadableArray<ProtocolSection> SectionsToScan;

    ProtocolMetadataPrivateState() {
      initializeProtocolLookup();
    }
  };

  static Lazy<ProtocolMetadataPrivateState> Protocols;
}

static void
_registerProtocols(ProtocolMetadataPrivateState &C,
                   const ProtocolRecord *begin,
                   const ProtocolRecord *end) {
  C.SectionsToScan.push_back(ProtocolSection{begin, end});
}

void swift::addImageProtocolsBlockCallbackUnsafe(const void *protocols,
                                                 uintptr_t protocolsSize) {
  assert(protocolsSize % sizeof(ProtocolRecord) == 0 &&
         "protocols section not a multiple of ProtocolRecord");

  // If we have a section, enqueue the protocols for lookup.
  auto protocolsBytes = reinterpret_cast<const char *>(protocols);
  auto recordsBegin
    = reinterpret_cast<const ProtocolRecord *>(protocols);
  auto recordsEnd
    = reinterpret_cast<const ProtocolRecord *>(protocolsBytes + protocolsSize);

  // Conformance cache should always be sufficiently initialized by this point.
  _registerProtocols(Protocols.unsafeGetAlreadyInitialized(),
                     recordsBegin, recordsEnd);
}

void swift::addImageProtocolsBlockCallback(const void *protocols,
                                           uintptr_t protocolsSize) {
  Protocols.get();
  addImageProtocolsBlockCallbackUnsafe(protocols, protocolsSize);
}

void swift::swift_registerProtocols(const ProtocolRecord *begin,
                                    const ProtocolRecord *end) {
  auto &C = Protocols.get();
  _registerProtocols(C, begin, end);
}

static const ProtocolDescriptor *
_searchProtocolRecords(ProtocolMetadataPrivateState &C,
                       NodePointer node) {
  for (auto &section : C.SectionsToScan.snapshot()) {
    for (const auto &record : section) {
      if (auto protocol = record.Protocol.getPointer()) {
        if (_contextDescriptorMatchesMangling(protocol, node))
          return protocol;
      }
    }
  }

  return nullptr;
}

static const ProtocolDescriptor *
_findProtocolDescriptor(NodePointer node,
                        Demangle::Demangler &Dem,
                        std::string &mangledName) {
  const ProtocolDescriptor *foundProtocol = nullptr;
  auto &T = Protocols.get();

  // If we have a symbolic reference to a context, resolve it immediately.
  NodePointer symbolicNode = node;
  if (symbolicNode->getKind() == Node::Kind::Type)
    symbolicNode = symbolicNode->getChild(0);
  if (symbolicNode->getKind() == Node::Kind::ProtocolSymbolicReference)
    return cast<ProtocolDescriptor>(
      (const ContextDescriptor *)symbolicNode->getIndex());

  mangledName =
    Demangle::mangleNode(node, ExpandResolvedSymbolicReferences(Dem), Dem).str();

  // Look for an existing entry.
  // Find the bucket for the metadata entry.
  {
    auto snapshot = T.ProtocolCache.snapshot();
    if (auto Value = snapshot.find(mangledName))
      return Value->getDescription();
  }

  // Check type metadata records
  foundProtocol = _searchProtocolRecords(T, node);

  if (foundProtocol) {
    T.ProtocolCache.getOrInsert(mangledName, [&](ProtocolDescriptorCacheEntry
                                                     *entry,
                                                 bool created) {
      if (created)
        new (entry) ProtocolDescriptorCacheEntry{mangledName, foundProtocol};
      return true;
    });
  }

  return foundProtocol;
}

#pragma mark Type field descriptor cache
namespace {
struct FieldDescriptorCacheEntry {
private:
  const Metadata *Type;
  const FieldDescriptor *Description;

public:
  FieldDescriptorCacheEntry(const Metadata *type,
                            const FieldDescriptor *description)
      : Type(type), Description(description) {}

  const FieldDescriptor *getDescription() { return Description; }

  int compareWithKey(const Metadata *other) const {
    auto a = (uintptr_t)Type;
    auto b = (uintptr_t)other;
    return a == b ? 0 : (a < b ? -1 : 1);
  }

  template <class... Args>
  static size_t getExtraAllocationSize(Args &&... ignored) {
    return 0;
  }
};

} // namespace

#pragma mark Metadata lookup via mangled name

llvm::Optional<unsigned>
swift::_depthIndexToFlatIndex(unsigned depth, unsigned index,
                              llvm::ArrayRef<unsigned> paramCounts) {
  // Out-of-bounds depth.
  if (depth >= paramCounts.size()) return None;

  // Compute the flat index.
  unsigned flatIndex = index + (depth == 0 ? 0 : paramCounts[depth - 1]);

  // Out-of-bounds index.
  if (flatIndex >= paramCounts[depth]) return None;

  return flatIndex;
}

/// Gather generic parameter counts from a context descriptor.
///
/// \returns true if the innermost descriptor is generic.
bool swift::_gatherGenericParameterCounts(
    const ContextDescriptor *descriptor,
    llvm::SmallVectorImpl<unsigned> &genericParamCounts,
    Demangler &BorrowFrom) {
  DemanglerForRuntimeTypeResolution<> demangler;
  demangler.providePreallocatedMemory(BorrowFrom);

  if (auto extension = _findExtendedTypeContextDescriptor(descriptor,
                                                          demangler)) {
    // If we have a nominal type extension descriptor, extract the extended type
    // and use that. If the extension is not nominal, then we can use the
    // extension's own signature.
    descriptor = extension;
  }

  // Once we hit a non-generic descriptor, we're done.
  if (!descriptor->isGeneric()) return false;

  // Recurse to record the parent context's generic parameters.
  auto parent = descriptor->Parent.get();
  (void)_gatherGenericParameterCounts(parent, genericParamCounts, demangler);

  // Record a new level of generic parameters if the count exceeds the
  // previous count.
  unsigned parentCount = parent->getNumGenericParams();
  unsigned myCount = descriptor->getNumGenericParams();
  if (myCount > parentCount) {
    genericParamCounts.push_back(myCount);
    return true;
  }

  return false;
}

/// Retrieve the generic parameters introduced in this context.
static llvm::ArrayRef<GenericParamDescriptor>
getLocalGenericParams(const ContextDescriptor *context) {
  if (!context->isGeneric())
    return { };

  // Determine where to start looking at generic parameters.
  unsigned startParamIndex;
  if (auto parent = context->Parent.get())
    startParamIndex = parent->getNumGenericParams();
  else
    startParamIndex = 0;

  auto genericContext = context->getGenericContext();
  return genericContext->getGenericParams().slice(startParamIndex);
}

static llvm::Optional<TypeLookupError>
_gatherGenericParameters(const ContextDescriptor *context,
                         llvm::ArrayRef<const Metadata *> genericArgs,
                         const Metadata *parent,
                         llvm::SmallVectorImpl<unsigned> &genericParamCounts,
                         llvm::SmallVectorImpl<const void *> &allGenericArgsVec,
                         Demangler &demangler) {
  auto makeCommonErrorStringGetter = [&] {
    auto metadataVector = genericArgs.vec();
    return [=] {
      std::string str;

      str += "_gatherGenericParameters: context: ";

#if !defined(SWIFT_RUNTIME_MACHO_NO_DYLD)
      SymbolInfo contextInfo;
      if (lookupSymbol(context, &contextInfo)) {
        str += contextInfo.symbolName.get();
        str += " ";
      }
#endif

      char *contextStr;
      swift_asprintf(&contextStr, "%p", context);
      str += contextStr;
      free(contextStr);

      str += " <";

      bool first = true;
      for (const Metadata *metadata : genericArgs) {
        if (!first)
          str += ", ";
        first = false;
        str += nameForMetadata(metadata);
      }

      str += "> ";

      str += "parent: ";
      if (parent)
        str += nameForMetadata(parent);
      else
        str += "<null>";
      str += " - ";

      return str;
    };
  };

  // Figure out the various levels of generic parameters we have in
  // this type.
  (void)_gatherGenericParameterCounts(context,
                                      genericParamCounts, demangler);
  unsigned numTotalGenericParams =
    genericParamCounts.empty() ? 0 : genericParamCounts.back();
  
  // Check whether we have the right number of generic arguments.
  if (genericArgs.size() == getLocalGenericParams(context).size()) {
    // Okay: genericArgs is the innermost set of generic arguments.
  } else if (genericArgs.size() == numTotalGenericParams && !parent) {
    // Okay: genericArgs is the complete set of generic arguments.
  } else {
    auto commonString = makeCommonErrorStringGetter();
    auto genericArgsSize = genericArgs.size();
    return TypeLookupError([=] {
      return commonString() + "incorrect number of generic args (" +
             std::to_string(genericArgsSize) + "), " +
             std::to_string(getLocalGenericParams(context).size()) +
             " local params, " + std::to_string(numTotalGenericParams) +
             " total params";
    });
  }
  
  // If there are generic parameters at any level, check the generic
  // requirements and fill in the generic arguments vector.
  if (!genericParamCounts.empty()) {
    // Compute the set of generic arguments "as written".
    llvm::SmallVector<const Metadata *, 8> allGenericArgs;

    // If we have a parent, gather it's generic arguments "as written".
    if (parent) {
      gatherWrittenGenericArgs(parent, parent->getTypeContextDescriptor(),
                               allGenericArgs, demangler);
    }
    
    // Add the generic arguments we were given.
    allGenericArgs.insert(allGenericArgs.end(),
                          genericArgs.begin(), genericArgs.end());
    
    // Copy the generic arguments needed for metadata from the generic
    // arguments "as written".
    auto generics = context->getGenericContext();
    assert(generics);
    {
      auto genericParams = generics->getGenericParams();
      unsigned n = genericParams.size();
      if (allGenericArgs.size() != n) {
        auto commonString = makeCommonErrorStringGetter();
        auto argsVecSize = allGenericArgsVec.size();
        return TypeLookupError([=] {
          return commonString() + "have " + std::to_string(argsVecSize) +
                 "generic args, expected " + std::to_string(n);
        });
      }
      for (unsigned i = 0; i != n; ++i) {
        const auto &param = genericParams[i];
        if (param.getKind() != GenericParamKind::Type) {
          auto commonString = makeCommonErrorStringGetter();
          return TypeLookupError([=] {
            return commonString() + "param " + std::to_string(i) +
                   " has unexpected kind " +
                   std::to_string(static_cast<uint8_t>(param.getKind()));
          });
        }
        if (param.hasExtraArgument()) {
          auto commonString = makeCommonErrorStringGetter();
          return TypeLookupError([=] {
            return commonString() + "param " + std::to_string(i) +
                   "has extra argument";
          });
        }
        if (param.hasKeyArgument())
          allGenericArgsVec.push_back(allGenericArgs[i]);
      }
    }

    // If we have the wrong number of generic arguments, fail.

    // Check whether the generic requirements are satisfied, collecting
    // any extra arguments we need for the instantiation function.
    SubstGenericParametersFromWrittenArgs substitutions(allGenericArgs,
                                                        genericParamCounts);
    auto error = _checkGenericRequirements(
        generics->getGenericRequirements(), allGenericArgsVec,
        [&substitutions](unsigned depth, unsigned index) {
          return substitutions.getMetadata(depth, index);
        },
        [&substitutions](const Metadata *type, unsigned index) {
          return substitutions.getWitnessTable(type, index);
        });
    if (error)
      return *error;

    // If we still have the wrong number of generic arguments, this is
    // some kind of metadata mismatch.
    if (generics->getGenericContextHeader().getNumArguments() !=
        allGenericArgsVec.size()) {
      auto commonString = makeCommonErrorStringGetter();
      auto argsVecSize = allGenericArgsVec.size();
      return TypeLookupError([=] {
        return commonString() + "generic argument count mismatch, expected " +
               std::to_string(
                   generics->getGenericContextHeader().getNumArguments()) +
               ", have " + std::to_string(argsVecSize);
      });
    }
  }

  return llvm::None;
}

namespace {

/// Find the offset of the protocol requirement for an associated type with
/// the given name in the given protocol descriptor.
llvm::Optional<const ProtocolRequirement *>
findAssociatedTypeByName(const ProtocolDescriptor *protocol, StringRef name) {
  // If we don't have associated type names, there's nothing to do.
  const char *associatedTypeNamesPtr = protocol->AssociatedTypeNames.get();
  if (!associatedTypeNamesPtr) return None;

  // Look through the list of associated type names.
  StringRef associatedTypeNames(associatedTypeNamesPtr);
  unsigned matchingAssocTypeIdx = 0;
  bool found = false;
  while (!associatedTypeNames.empty()) {
    // Avoid using StringRef::split because its definition is not
    // provided in the header so that it requires linking with libSupport.a.
    auto splitIdx = associatedTypeNames.find(' ');
    if (associatedTypeNames.substr(0, splitIdx) == name) {
      found = true;
      break;
    }

    ++matchingAssocTypeIdx;
    associatedTypeNames = associatedTypeNames.substr(splitIdx).substr(1);
  }

  if (!found) return None;

  // We have a match on the Nth associated type; go find the Nth associated
  // type requirement.
  unsigned currentAssocTypeIdx = 0;
  unsigned numRequirements = protocol->NumRequirements;
  auto requirements = protocol->getRequirements();
  for (unsigned reqIdx = 0; reqIdx != numRequirements; ++reqIdx) {
    if (requirements[reqIdx].Flags.getKind() !=
        ProtocolRequirementFlags::Kind::AssociatedTypeAccessFunction)
      continue;

    if (currentAssocTypeIdx == matchingAssocTypeIdx)
      return requirements.begin() + reqIdx;

    ++currentAssocTypeIdx;
  }

  swift_unreachable("associated type names don't line up");
}

namespace {
  static Lazy<Mutex> DynamicReplacementLock;
}

namespace {
struct OpaqueTypeMappings {
  llvm::DenseMap<const OpaqueTypeDescriptor *, const OpaqueTypeDescriptor *>
      descriptorMapping;
  const OpaqueTypeDescriptor* find(const OpaqueTypeDescriptor *orig) {
    const OpaqueTypeDescriptor *replacement = nullptr;
    DynamicReplacementLock.get().withLock([&] {
      auto entry = descriptorMapping.find(orig);
      if (entry != descriptorMapping.end())
        replacement = entry->second;
    });
    return replacement;
  }

  // We take a mutex argument to make sure someone is holding the lock.
  void insert(const OpaqueTypeDescriptor *orig,
              const OpaqueTypeDescriptor *replacement, const Mutex &) {
    descriptorMapping[orig] = replacement;
  }
};
} // end unnamed namespace

static Lazy<OpaqueTypeMappings> opaqueTypeMappings;


static const OpaqueTypeDescriptor *
_findOpaqueTypeDescriptor(NodePointer demangleNode,
                          Demangler &dem) {
  // Directly resolve a symbolic reference.
  if (demangleNode->getKind()
        == Node::Kind::OpaqueTypeDescriptorSymbolicReference) {
    auto context = (const ContextDescriptor *)demangleNode->getIndex();
    auto *orig = cast<OpaqueTypeDescriptor>(context);
    if (auto *entry = opaqueTypeMappings.get().find(orig)) {
      return entry;
    }
    return orig;
  }
  
  // TODO: Find non-symbolic-referenced opaque decls.
  return nullptr;
}

/// Constructs metadata by decoding a mangled type name, for use with
/// \c TypeDecoder.
class DecodedMetadataBuilder {
private:
  /// The demangler we'll use when building new nodes.
  Demangler &demangler;

  /// Substitute generic parameters.
  SubstGenericParameterFn substGenericParameter;

  /// Substitute dependent witness tables.
  SubstDependentWitnessTableFn substWitnessTable;

  /// Ownership information related to the metadata we are trying to lookup.
  TypeReferenceOwnership ReferenceOwnership;

public:
  DecodedMetadataBuilder(Demangler &demangler,
                         SubstGenericParameterFn substGenericParameter,
                         SubstDependentWitnessTableFn substWitnessTable)
    : demangler(demangler),
      substGenericParameter(substGenericParameter),
      substWitnessTable(substWitnessTable) { }

  using BuiltType = const Metadata *;
  using BuiltTypeDecl = const ContextDescriptor *;
  using BuiltProtocolDecl = ProtocolDescriptorRef;

  Demangle::NodeFactory &getNodeFactory() { return demangler; }

  TypeLookupErrorOr<BuiltType>
  resolveOpaqueType(NodePointer opaqueDecl,
                    llvm::ArrayRef<llvm::ArrayRef<BuiltType>> genericArgs,
                    unsigned ordinal) {
    auto descriptor = _findOpaqueTypeDescriptor(opaqueDecl, demangler);
    if (!descriptor)
      return BuiltType();
    auto outerContext = descriptor->Parent.get();

    llvm::SmallVector<BuiltType, 8> allGenericArgs;
    for (auto argSet : genericArgs) {
      allGenericArgs.append(argSet.begin(), argSet.end());
    }
    
    // Gather the generic parameters we need to parameterize the opaque decl.
    llvm::SmallVector<unsigned, 8> genericParamCounts;
    llvm::SmallVector<const void *, 8> allGenericArgsVec;

    if (auto error = _gatherGenericParameters(
            outerContext, allGenericArgs, BuiltType(), /* no parent */
            genericParamCounts, allGenericArgsVec, demangler))
      return *error;

    auto mangledName = descriptor->getUnderlyingTypeArgument(ordinal);
    SubstGenericParametersFromMetadata substitutions(descriptor,
                                                     allGenericArgsVec.data());
    return swift_getTypeByMangledName(MetadataState::Complete,
                                      mangledName, allGenericArgsVec.data(),
      [&substitutions](unsigned depth, unsigned index) {
        return substitutions.getMetadata(depth, index);
      },
      [&substitutions](const Metadata *type, unsigned index) {
        return substitutions.getWitnessTable(type, index);
      }).getType().getMetadata();
  }

  BuiltTypeDecl createTypeDecl(NodePointer node,
                               bool &typeAlias) const {
    // Look for a nominal type descriptor based on its mangled name.
    return _findContextDescriptor(node, demangler);
  }

  BuiltProtocolDecl createProtocolDecl(NodePointer node) const {
    // Look for a protocol descriptor based on its mangled name.
    std::string mangledName;
    if (auto protocol = _findProtocolDescriptor(node, demangler, mangledName))
      return ProtocolDescriptorRef::forSwift(protocol);;

#if SWIFT_OBJC_INTEROP
    // Look for a Swift-defined @objc protocol with the Swift 3 mangling that
    // is used for Objective-C entities.
    const char *objcMangledName = mangleNodeAsObjcCString(node, demangler);
    if (auto protocol = objc_getProtocol(objcMangledName))
      return ProtocolDescriptorRef::forObjC(protocol);
#endif

    return ProtocolDescriptorRef();
  }

  BuiltProtocolDecl createObjCProtocolDecl(
                                         const std::string &mangledName) const {
#if SWIFT_OBJC_INTEROP
    return ProtocolDescriptorRef::forObjC(
        objc_getProtocol(mangledName.c_str()));
#else
    return ProtocolDescriptorRef();
#endif
  }

  TypeLookupErrorOr<BuiltType>
  createObjCClassType(const std::string &mangledName) const {
#if SWIFT_OBJC_INTEROP
    auto objcClass = objc_getClass(mangledName.c_str());
    return swift_getObjCClassMetadata((const ClassMetadata *)objcClass);
#else
    return BuiltType();
#endif
  }

  TypeLookupErrorOr<BuiltType>
  createBoundGenericObjCClassType(const std::string &mangledName,
                                  llvm::ArrayRef<BuiltType> args) const {
    // Generic arguments of lightweight Objective-C generic classes are not
    // reified in the metadata.
    return createObjCClassType(mangledName);
  }

  TypeLookupErrorOr<BuiltType>
  createNominalType(BuiltTypeDecl metadataOrTypeDecl, BuiltType parent) const {
    // Treat nominal type creation the same way as generic type creation,
    // but with no generic arguments at this level.
    return createBoundGenericType(metadataOrTypeDecl, { }, parent);
  }

  TypeLookupErrorOr<BuiltType> createTypeAliasType(BuiltTypeDecl typeAliasDecl,
                                                   BuiltType parent) const {
    // We can't support sugared types here since we have no way to
    // resolve the underlying type of the type alias. However, some
    // CF types are mangled as type aliases.
    return createNominalType(typeAliasDecl, parent);
  }

  TypeLookupErrorOr<BuiltType>
  createBoundGenericType(BuiltTypeDecl anyTypeDecl,
                         llvm::ArrayRef<BuiltType> genericArgs,
                         BuiltType parent) const {
    auto typeDecl = dyn_cast<TypeContextDescriptor>(anyTypeDecl);
    if (!typeDecl) {
      if (auto protocol = dyn_cast<ProtocolDescriptor>(anyTypeDecl))
        return _getSimpleProtocolTypeMetadata(protocol);

      return BuiltType();
    }

    // Figure out the various levels of generic parameters we have in
    // this type.
    llvm::SmallVector<unsigned, 8> genericParamCounts;
    llvm::SmallVector<const void *, 8> allGenericArgsVec;

    if (auto error = _gatherGenericParameters(typeDecl, genericArgs, parent,
                                              genericParamCounts,
                                              allGenericArgsVec, demangler))
      return *error;

    // Call the access function.
    auto accessFunction = typeDecl->getAccessFunction();
    if (!accessFunction) return BuiltType();

    return accessFunction(MetadataState::Abstract, allGenericArgsVec).Value;
  }

  TypeLookupErrorOr<BuiltType> createBuiltinType(StringRef builtinName,
                                                 StringRef mangledName) const {
#define BUILTIN_TYPE(Symbol, _) \
    if (mangledName.equals(#Symbol)) \
      return &METADATA_SYM(Symbol).base;
#include "swift/Runtime/BuiltinTypes.def"
    return BuiltType();
  }

  TypeLookupErrorOr<BuiltType> createMetatypeType(
      BuiltType instance,
      llvm::Optional<Demangle::ImplMetatypeRepresentation> repr = None) const {
    return swift_getMetatypeMetadata(instance);
  }

  TypeLookupErrorOr<BuiltType> createExistentialMetatypeType(
      BuiltType instance,
      llvm::Optional<Demangle::ImplMetatypeRepresentation> repr = None) const {
    return swift_getExistentialMetatypeMetadata(instance);
  }

  TypeLookupErrorOr<BuiltType>
  createProtocolCompositionType(llvm::ArrayRef<BuiltProtocolDecl> protocols,
                                BuiltType superclass, bool isClassBound) const {
    // Determine whether we have a class bound.
    ProtocolClassConstraint classConstraint = ProtocolClassConstraint::Any;
    if (isClassBound || superclass) {
      classConstraint = ProtocolClassConstraint::Class;
    } else {
      for (auto protocol : protocols) {
        if (protocol.getClassConstraint() == ProtocolClassConstraint::Class) {
          classConstraint = ProtocolClassConstraint::Class;
          break;
        }
      }
    }

    return swift_getExistentialTypeMetadata(classConstraint, superclass,
                                            protocols.size(), protocols.data());
  }

  TypeLookupErrorOr<BuiltType> createDynamicSelfType(BuiltType selfType) const {
    // Free-standing mangled type strings should not contain DynamicSelfType.
    return BuiltType();
  }

  TypeLookupErrorOr<BuiltType>
  createGenericTypeParameterType(unsigned depth, unsigned index) const {
    // Use the callback, when provided.
    if (substGenericParameter)
      return substGenericParameter(depth, index);

    return BuiltType();
  }

  TypeLookupErrorOr<BuiltType>
  createFunctionType(llvm::ArrayRef<Demangle::FunctionParam<BuiltType>> params,
                     BuiltType result, FunctionTypeFlags flags) const {
    llvm::SmallVector<BuiltType, 8> paramTypes;
    llvm::SmallVector<uint32_t, 8> paramFlags;

    // Fill in the parameters.
    paramTypes.reserve(params.size());
    if (flags.hasParameterFlags())
      paramFlags.reserve(params.size());
    for (const auto &param : params) {
      paramTypes.push_back(param.getType());
      if (flags.hasParameterFlags())
        paramFlags.push_back(param.getFlags().getIntValue());
    }

    return swift_getFunctionTypeMetadata(flags, paramTypes.data(),
                                         flags.hasParameterFlags()
                                           ? paramFlags.data()
                                           : nullptr,
                                         result);
  }

  TypeLookupErrorOr<BuiltType> createImplFunctionType(
      Demangle::ImplParameterConvention calleeConvention,
      llvm::ArrayRef<Demangle::ImplFunctionParam<BuiltType>> params,
      llvm::ArrayRef<Demangle::ImplFunctionResult<BuiltType>> results,
      llvm::Optional<Demangle::ImplFunctionResult<BuiltType>> errorResult,
      ImplFunctionTypeFlags flags) {
    // We can't realize the metadata for a SILFunctionType.
    return BuiltType();
  }

  TypeLookupErrorOr<BuiltType>
  createTupleType(llvm::ArrayRef<BuiltType> elements,
                  std::string labels) const {
    auto flags = TupleTypeFlags().withNumElements(elements.size());
    if (!labels.empty())
      flags = flags.withNonConstantLabels(true);
    return MetadataResponse(swift_getTupleTypeMetadata(
                                MetadataState::Abstract, flags, elements.data(),
                                labels.empty() ? nullptr : labels.c_str(),
                                /*proposedWitnesses=*/nullptr))
        .Value;
  }

  TypeLookupErrorOr<BuiltType> createDependentMemberType(StringRef name,
                                                         BuiltType base) const {
    // Should not have unresolved dependent member types here.
    return BuiltType();
  }

  TypeLookupErrorOr<BuiltType>
  createDependentMemberType(StringRef name, BuiltType base,
                            BuiltProtocolDecl protocol) const {
#if SWIFT_OBJC_INTEROP
    if (protocol.isObjC())
      return BuiltType();
#endif

    auto swiftProtocol = protocol.getSwiftProtocol();
    auto witnessTable = swift_conformsToProtocol(base, swiftProtocol);
    if (!witnessTable)
      return BuiltType();

    // Look for the named associated type within the protocol.
    auto assocType = findAssociatedTypeByName(swiftProtocol, name);
    if (!assocType) return nullptr;

    // Call the associated type access function.
    return swift_getAssociatedTypeWitness(
                                 MetadataState::Abstract,
                                 const_cast<WitnessTable *>(witnessTable),
                                 base,
                                 swiftProtocol->getRequirementBaseDescriptor(),
                                 *assocType).Value;
  }

#define REF_STORAGE(Name, ...)                                                 \
  TypeLookupErrorOr<BuiltType> create##Name##StorageType(BuiltType base) {     \
    ReferenceOwnership.set##Name();                                            \
    return base;                                                               \
  }
#include "swift/AST/ReferenceStorage.def"

  TypeLookupErrorOr<BuiltType> createSILBoxType(BuiltType base) const {
    // FIXME: Implement.
    return BuiltType();
  }

  TypeReferenceOwnership getReferenceOwnership() const {
    return ReferenceOwnership;
  }

  TypeLookupErrorOr<BuiltType> createOptionalType(BuiltType base) {
    // Mangled types for building metadata don't contain sugared types
    return BuiltType();
  }

  TypeLookupErrorOr<BuiltType> createArrayType(BuiltType base) {
    // Mangled types for building metadata don't contain sugared types
    return BuiltType();
  }

  TypeLookupErrorOr<BuiltType> createDictionaryType(BuiltType key,
                                                    BuiltType value) {
    // Mangled types for building metadata don't contain sugared types
    return BuiltType();
  }

  TypeLookupErrorOr<BuiltType> createParenType(BuiltType base) {
    // Mangled types for building metadata don't contain sugared types
    return BuiltType();
  }
};

}

SWIFT_CC(swift)
static TypeLookupErrorOr<TypeInfo>
swift_getTypeByMangledNodeImpl(MetadataRequest request, Demangler &demangler,
                               Demangle::NodePointer node,
                               const void *const *origArgumentVector,
                               SubstGenericParameterFn substGenericParam,
                               SubstDependentWitnessTableFn substWitnessTable) {
  // Simply call an accessor function if that's all we got.
  if (node->getKind() == Node::Kind::AccessorFunctionReference) {
    // The accessor function is passed the pointer to the original argument
    // buffer. It's assumed to match the generic context.
    auto accessorFn =
        (const Metadata *(*)(const void * const *))node->getIndex();
    auto type = accessorFn(origArgumentVector);
    // We don't call checkMetadataState here since the result may not really
    // *be* type metadata. If the accessor returns a type, it is responsible
    // for completing the metadata.
    return TypeInfo{MetadataResponse{type, MetadataState::Complete},
                    TypeReferenceOwnership()};
  }
  
  // TODO: propagate the request down to the builder instead of calling
  // swift_checkMetadataState after the fact.
  DecodedMetadataBuilder builder(demangler, substGenericParam,
                                 substWitnessTable);
  auto type = Demangle::decodeMangledType(builder, node);
  if (type.isError()) {
    return *type.getError();
  }
  if (!type.getType()) {
    return TypeLookupError("NULL type but no error provided");
  }

  return TypeInfo{swift_checkMetadataState(request, type.getType()),
                  builder.getReferenceOwnership()};
}

SWIFT_CC(swift)
static TypeLookupErrorOr<TypeInfo>
swift_getTypeByMangledNameImpl(MetadataRequest request, StringRef typeName,
                               const void *const *origArgumentVector,
                               SubstGenericParameterFn substGenericParam,
                               SubstDependentWitnessTableFn substWitnessTable) {
  DemanglerForRuntimeTypeResolution<StackAllocatedDemangler<2048>> demangler;

  NodePointer node;

  // Check whether this is the convenience syntax "ModuleName.ClassName".
  auto getDotPosForConvenienceSyntax = [&]() -> size_t {
    size_t dotPos = llvm::StringRef::npos;
    for (unsigned i = 0; i < typeName.size(); ++i) {
      // Should only contain one dot.
      if (typeName[i] == '.') {
        if (dotPos == llvm::StringRef::npos) {
          dotPos = i;
          continue;
        } else {
          return llvm::StringRef::npos;
        }
      }
      
      // Should not contain symbolic references.
      if ((unsigned char)typeName[i] <= '\x1F') {
        return llvm::StringRef::npos;
      }
    }
    return dotPos;
  };

  auto dotPos = getDotPosForConvenienceSyntax();
  if (dotPos != llvm::StringRef::npos) {
    // Form a demangle tree for this class.
    NodePointer classNode = demangler.createNode(Node::Kind::Class);
    NodePointer moduleNode = demangler.createNode(Node::Kind::Module,
                                                  typeName.substr(0, dotPos));
    NodePointer nameNode = demangler.createNode(Node::Kind::Identifier,
                                                typeName.substr(dotPos + 1));
    classNode->addChild(moduleNode, demangler);
    classNode->addChild(nameNode, demangler);

    node = classNode;
  } else {
    // Demangle the type name.
    node = demangler.demangleTypeRef(typeName);
    if (!node) {
      return TypeInfo();
    }
  }

  return swift_getTypeByMangledNode(request, demangler, node,
                                    origArgumentVector,
                                    substGenericParam, substWitnessTable);
}

SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
const Metadata * _Nullable
swift_getTypeByMangledNameInEnvironment(
                        const char *typeNameStart,
                        size_t typeNameLength,
                        const TargetGenericEnvironment<InProcess> *environment,
                        const void * const *genericArgs) {
  llvm::StringRef typeName(typeNameStart, typeNameLength);
  SubstGenericParametersFromMetadata substitutions(environment, genericArgs);
  return swift_getTypeByMangledName(MetadataState::Complete, typeName,
    genericArgs,
    [&substitutions](unsigned depth, unsigned index) {
      return substitutions.getMetadata(depth, index);
    },
    [&substitutions](const Metadata *type, unsigned index) {
      return substitutions.getWitnessTable(type, index);
    }).getType().getMetadata();
}

SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
const Metadata * _Nullable
swift_getTypeByMangledNameInEnvironmentInMetadataState(
                        size_t metadataState,
                        const char *typeNameStart,
                        size_t typeNameLength,
                        const TargetGenericEnvironment<InProcess> *environment,
                        const void * const *genericArgs) {
  llvm::StringRef typeName(typeNameStart, typeNameLength);
  SubstGenericParametersFromMetadata substitutions(environment, genericArgs);
  return swift_getTypeByMangledName((MetadataState)metadataState, typeName,
    genericArgs,
    [&substitutions](unsigned depth, unsigned index) {
      return substitutions.getMetadata(depth, index);
    },
    [&substitutions](const Metadata *type, unsigned index) {
      return substitutions.getWitnessTable(type, index);
    }).getType().getMetadata();
}

SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
const Metadata * _Nullable
swift_getTypeByMangledNameInContext(
                        const char *typeNameStart,
                        size_t typeNameLength,
                        const TargetContextDescriptor<InProcess> *context,
                        const void * const *genericArgs) {
  llvm::StringRef typeName(typeNameStart, typeNameLength);
  SubstGenericParametersFromMetadata substitutions(context, genericArgs);
  return swift_getTypeByMangledName(MetadataState::Complete, typeName,
    genericArgs,
    [&substitutions](unsigned depth, unsigned index) {
      return substitutions.getMetadata(depth, index);
    },
    [&substitutions](const Metadata *type, unsigned index) {
      return substitutions.getWitnessTable(type, index);
    }).getType().getMetadata();
}

SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
const Metadata * _Nullable
swift_getTypeByMangledNameInContextInMetadataState(
                        size_t metadataState,
                        const char *typeNameStart,
                        size_t typeNameLength,
                        const TargetContextDescriptor<InProcess> *context,
                        const void * const *genericArgs) {
  llvm::StringRef typeName(typeNameStart, typeNameLength);
  SubstGenericParametersFromMetadata substitutions(context, genericArgs);
  return swift_getTypeByMangledName((MetadataState)metadataState, typeName,
    genericArgs,
    [&substitutions](unsigned depth, unsigned index) {
      return substitutions.getMetadata(depth, index);
    },
    [&substitutions](const Metadata *type, unsigned index) {
      return substitutions.getWitnessTable(type, index);
    }).getType().getMetadata();
}

/// Demangle a mangled name, but don't allow symbolic references.
SWIFT_CC(swift) SWIFT_RUNTIME_STDLIB_INTERNAL
const Metadata *_Nullable
swift_stdlib_getTypeByMangledNameUntrusted(const char *typeNameStart,
                                           size_t typeNameLength) {
  llvm::StringRef typeName(typeNameStart, typeNameLength);
  for (char c : typeName) {
    if (c >= '\x01' && c <= '\x1F')
      return nullptr;
  }
  
  return swift_getTypeByMangledName(MetadataState::Complete, typeName, nullptr,
                                    {}, {}).getType().getMetadata();
}

SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
MetadataResponse
swift_getOpaqueTypeMetadata(MetadataRequest request,
                            const void * const *arguments,
                            const OpaqueTypeDescriptor *descriptor,
                            unsigned index) {
  auto mangledName = descriptor->getUnderlyingTypeArgument(index);
  SubstGenericParametersFromMetadata substitutions(descriptor, arguments);

  return swift_getTypeByMangledName(request.getState(),
                                    mangledName, arguments,
    [&substitutions](unsigned depth, unsigned index) {
      return substitutions.getMetadata(depth, index);
    },
    [&substitutions](const Metadata *type, unsigned index) {
      return substitutions.getWitnessTable(type, index);
    }).getType().getResponse();
}

SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
const WitnessTable *
swift_getOpaqueTypeConformance(const void * const *arguments,
                               const OpaqueTypeDescriptor *descriptor,
                               unsigned index) {
  auto response = swift_getOpaqueTypeMetadata(
                                    MetadataRequest(MetadataState::Complete),
                                    arguments, descriptor, index);
  return (const WitnessTable *)response.Value;
}

#if SWIFT_OBJC_INTEROP

// Return the ObjC class for the given type name.
// This gets installed as a callback from libobjc.

// FIXME: delete this #if and dlsym once we don't
// need to build with older libobjc headers
#if !OBJC_GETCLASSHOOK_DEFINED
using objc_hook_getClass =  BOOL(*)(const char * _Nonnull name,
                                    Class _Nullable * _Nonnull outClass);
#endif
static objc_hook_getClass OldGetClassHook;

static BOOL
getObjCClassByMangledName(const char * _Nonnull typeName,
                          Class _Nullable * _Nonnull outClass) {
  // Demangle old-style class and protocol names, which are still used in the
  // ObjC metadata.
  StringRef typeStr(typeName);
  const Metadata *metadata = nullptr;
  if (typeStr.startswith("_Tt")) {
    Demangler demangler;
    auto node = demangler.demangleSymbol(typeName);
    if (!node)
      return NO;

    // If we successfully demangled but there is a suffix, then we did NOT use
    // the entire name, and this is NOT a match. Reject it.
    if (node->hasChildren() &&
        node->getLastChild()->getKind() == Node::Kind::Suffix)
      return NO;

    metadata = swift_getTypeByMangledNode(
      MetadataState::Complete, demangler, node,
      nullptr,
      /* no substitutions */
      [&](unsigned depth, unsigned index) {
        return nullptr;
      },
      [&](const Metadata *type, unsigned index) {
        return nullptr;
      }).getType().getMetadata();
  } else {
    metadata = swift_stdlib_getTypeByMangledNameUntrusted(typeStr.data(),
                                                          typeStr.size());
  }
  if (metadata) {
    auto objcClass =
      reinterpret_cast<Class>(
        const_cast<ClassMetadata *>(
          swift_getObjCClassFromMetadataConditional(metadata)));

    if (objcClass) {
      *outClass = objcClass;
      return YES;
    }
  }

  return OldGetClassHook(typeName, outClass);
}

__attribute__((constructor))
static void installGetClassHook() {
  // FIXME: delete this #if and dlsym once we don't
  // need to build with older libobjc headers
#if !OBJC_GETCLASSHOOK_DEFINED
  using objc_hook_getClass =  BOOL(*)(const char * _Nonnull name,
                                      Class _Nullable * _Nonnull outClass);
  auto objc_setHook_getClass =
    (void(*)(objc_hook_getClass _Nonnull,
             objc_hook_getClass _Nullable * _Nonnull))
    dlsym(RTLD_DEFAULT, "objc_setHook_getClass");
#endif

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunguarded-availability"
  if (objc_setHook_getClass) {
    objc_setHook_getClass(getObjCClassByMangledName, &OldGetClassHook);
  }
#pragma clang diagnostic pop
}

#endif

unsigned SubstGenericParametersFromMetadata::
buildDescriptorPath(const ContextDescriptor *context,
                    Demangler &borrowFrom) const {
  assert(sourceIsMetadata);

  // Terminating condition: we don't have a context.
  if (!context)
    return 0;

  DemanglerForRuntimeTypeResolution<> demangler;
  demangler.providePreallocatedMemory(borrowFrom);

  if (auto extension = _findExtendedTypeContextDescriptor(context, demangler)) {
    // If we have a nominal type extension descriptor, extract the extended type
    // and use that. If the extension is not nominal, then we can use the
    // extension's own signature.
    context = extension;
  }

  // Add the parent's contribution to the descriptor path.
  const ContextDescriptor *parent = context->Parent.get();
  unsigned numKeyGenericParamsInParent = buildDescriptorPath(parent, demangler);

  // If this context is non-generic, we're done.
  if (!context->isGeneric())
    return numKeyGenericParamsInParent;

  // Count the number of key generic params at this level.
  auto allGenericParams = baseContext->getGenericContext()->getGenericParams();
  unsigned parentCount = parent->getNumGenericParams();
  unsigned localCount = context->getNumGenericParams();
  auto localGenericParams = allGenericParams.slice(parentCount,
                                                   localCount - parentCount);

  unsigned numKeyGenericParamsHere = 0;
  bool hasNonKeyGenericParams = false;
  for (const auto &genericParam : localGenericParams) {
    if (genericParam.hasKeyArgument())
      ++numKeyGenericParamsHere;
    else
      hasNonKeyGenericParams = true;
  }

  // Form the path element if there are any new generic parameters.
  if (localCount > parentCount)
    descriptorPath.push_back(PathElement{localGenericParams,
                                         context->getNumGenericParams(),
                                         numKeyGenericParamsInParent,
                                         numKeyGenericParamsHere,
                                         hasNonKeyGenericParams});
  return numKeyGenericParamsInParent + numKeyGenericParamsHere;
}

  /// Builds a path from the generic environment.
unsigned SubstGenericParametersFromMetadata::
buildEnvironmentPath(
    const TargetGenericEnvironment<InProcess> *environment) const {
  unsigned totalParamCount = 0;
  unsigned totalKeyParamCount = 0;
  auto genericParams = environment->getGenericParameters();
  for (unsigned numLocalParams : environment->getGenericParameterCounts()) {
    // Adkjust totalParamCount so we have the # of local parameters.
    numLocalParams -= totalParamCount;

    // Get the local generic parameters.
    auto localGenericParams = genericParams.slice(0, numLocalParams);
    genericParams = genericParams.slice(numLocalParams);

    // Count the parameters.
    unsigned numKeyGenericParamsInParent = totalKeyParamCount;
    unsigned numKeyGenericParamsHere = 0;
    bool hasNonKeyGenericParams = false;
    for (const auto &genericParam : localGenericParams) {
      if (genericParam.hasKeyArgument())
        ++numKeyGenericParamsHere;
      else
        hasNonKeyGenericParams = true;
    }

    // Update totals.
    totalParamCount += numLocalParams;
    totalKeyParamCount += numKeyGenericParamsHere;

    // Add to the descriptor path.
    descriptorPath.push_back(PathElement{localGenericParams,
                                         totalParamCount,
                                         numKeyGenericParamsInParent,
                                         numKeyGenericParamsHere,
                                         hasNonKeyGenericParams});
  }

  return totalKeyParamCount;
}

void SubstGenericParametersFromMetadata::setup() const {
  if (!descriptorPath.empty())
    return;

  if (sourceIsMetadata && baseContext) {
    DemanglerForRuntimeTypeResolution<StackAllocatedDemangler<2048>> demangler;
    numKeyGenericParameters = buildDescriptorPath(baseContext, demangler);
    return;
  }

  if (!sourceIsMetadata && environment) {
    numKeyGenericParameters = buildEnvironmentPath(environment);
    return;
  }
}

const Metadata *
SubstGenericParametersFromMetadata::getMetadata(
                                        unsigned depth, unsigned index) const {
  // On first access, compute the descriptor path.
  setup();

  // If the depth is too great, there is nothing to do.
  if (depth >= descriptorPath.size())
    return nullptr;

  /// Retrieve the descriptor path element at this depth.
  auto &pathElement = descriptorPath[depth];

  // Check whether the index is clearly out of bounds.
  if (index >= pathElement.numTotalGenericParams)
    return nullptr;

  // Compute the flat index.
  unsigned flatIndex = pathElement.numKeyGenericParamsInParent;
  if (pathElement.hasNonKeyGenericParams > 0) {
    // We have non-key generic parameters at this level, so the index needs to
    // be checked more carefully.
    auto genericParams = pathElement.localGenericParams;

    // Make sure that the requested parameter itself has a key argument.
    if (!genericParams[index].hasKeyArgument())
      return nullptr;

    // Increase the flat index for each parameter with a key argument, up to
    // the given index.
    for (const auto &genericParam : genericParams.slice(0, index)) {
      if (genericParam.hasKeyArgument())
        ++flatIndex;
    }
  } else {
    flatIndex += index;
  }

  return (const Metadata *)genericArgs[flatIndex];
}

const WitnessTable *
SubstGenericParametersFromMetadata::getWitnessTable(const Metadata *type,
                                                    unsigned index) const {
  // On first access, compute the descriptor path.
  setup();

  return (const WitnessTable *)genericArgs[index + numKeyGenericParameters];
}

const Metadata *SubstGenericParametersFromWrittenArgs::getMetadata(
                                        unsigned depth, unsigned index) const {
  if (auto flatIndex =
          _depthIndexToFlatIndex(depth, index, genericParamCounts)) {
    if (*flatIndex < allGenericArgs.size())
      return allGenericArgs[*flatIndex];
  }

  return nullptr;
}

const WitnessTable *
SubstGenericParametersFromWrittenArgs::getWitnessTable(const Metadata *type,
                                                       unsigned index) const {
  return nullptr;
}

/// Demangle the given type name to a generic parameter reference, which
/// will be returned as (depth, index).
static llvm::Optional<std::pair<unsigned, unsigned>>
demangleToGenericParamRef(StringRef typeName) {
  StackAllocatedDemangler<1024> demangler;
  NodePointer node = demangler.demangleType(typeName);
  if (!node)
    return None;

  // Find the flat index that the right-hand side refers to.
  if (node->getKind() == Demangle::Node::Kind::Type)
    node = node->getChild(0);
  if (node->getKind() != Demangle::Node::Kind::DependentGenericParamType)
    return None;

  return std::pair<unsigned, unsigned>(node->getChild(0)->getIndex(),
                                       node->getChild(1)->getIndex());
}

void swift::gatherWrittenGenericArgs(
    const Metadata *metadata, const TypeContextDescriptor *description,
    llvm::SmallVectorImpl<const Metadata *> &allGenericArgs,
    Demangler &BorrowFrom) {
  if (!description)
    return;
  auto generics = description->getGenericContext();
  if (!generics)
    return;

  bool missingWrittenArguments = false;
  auto genericArgs = description->getGenericArguments(metadata);
  for (auto param : generics->getGenericParams()) {
    switch (param.getKind()) {
    case GenericParamKind::Type:
      // The type should have a key argument unless it's been same-typed to
      // another type.
      if (param.hasKeyArgument()) {
        auto genericArg = *genericArgs++;
        allGenericArgs.push_back(genericArg);
      } else {
        // Leave a gap for us to fill in by looking at same type info.
        allGenericArgs.push_back(nullptr);
        missingWrittenArguments = true;
      }

      // We don't know about type parameters with extra arguments. Leave
      // a hole for it.
      if (param.hasExtraArgument()) {
        allGenericArgs.push_back(nullptr);
        ++genericArgs;
      }
      break;

    default:
      // We don't know about this kind of parameter. Create placeholders where
      // needed.
      if (param.hasKeyArgument()) {
        allGenericArgs.push_back(nullptr);
        ++genericArgs;
      }

      if (param.hasExtraArgument()) {
        allGenericArgs.push_back(nullptr);
        ++genericArgs;
      }
      break;
    }
  }

  // If there is no follow-up work to do, we're done.
  if (!missingWrittenArguments)
    return;

  // We have generic arguments that would be written, but have been
  // canonicalized away. Use same-type requirements to reconstitute them.

  // Retrieve the mapping information needed for depth/index -> flat index.
  llvm::SmallVector<unsigned, 8> genericParamCounts;
  (void)_gatherGenericParameterCounts(description, genericParamCounts,
                                      BorrowFrom);

  // Walk through the generic requirements to evaluate same-type
  // constraints that are needed to fill in missing generic arguments.
  for (const auto &req : generics->getGenericRequirements()) {
    // We only care about same-type constraints.
    if (req.Flags.getKind() != GenericRequirementKind::SameType)
      continue;

    auto lhsParam = demangleToGenericParamRef(req.getParam());
    if (!lhsParam)
      continue;

    // If we don't yet have an argument for this parameter, it's a
    // same-type-to-concrete constraint.
    auto lhsFlatIndex =
      _depthIndexToFlatIndex(lhsParam->first, lhsParam->second,
                             genericParamCounts);
    if (!lhsFlatIndex || *lhsFlatIndex >= allGenericArgs.size())
      continue;

    if (!allGenericArgs[*lhsFlatIndex]) {
      // Substitute into the right-hand side.
      SubstGenericParametersFromWrittenArgs substitutions(allGenericArgs,
                                                          genericParamCounts);
      allGenericArgs[*lhsFlatIndex] =
          swift_getTypeByMangledName(MetadataState::Abstract,
            req.getMangledTypeName(),
            (const void * const *)allGenericArgs.data(),
            [&substitutions](unsigned depth, unsigned index) {
              return substitutions.getMetadata(depth, index);
            },
            [&substitutions](const Metadata *type, unsigned index) {
              return substitutions.getWitnessTable(type, index);
            }).getType().getMetadata();
      continue;
    }

    // If we do have an argument for this parameter, it might be that
    // the right-hand side is itself a generic parameter, which means
    // we have a same-type constraint A == B where A is already filled in.
    auto rhsParam = demangleToGenericParamRef(req.getMangledTypeName());
    if (!rhsParam)
      continue;

    auto rhsFlatIndex =
      _depthIndexToFlatIndex(rhsParam->first, rhsParam->second,
                             genericParamCounts);
    if (!rhsFlatIndex || *rhsFlatIndex >= allGenericArgs.size())
      continue;

    if (allGenericArgs[*rhsFlatIndex] || !allGenericArgs[*lhsFlatIndex])
      continue;

    allGenericArgs[*rhsFlatIndex] = allGenericArgs[*lhsFlatIndex];
  }
}

struct InitializeDynamicReplacementLookup {
  InitializeDynamicReplacementLookup() {
    initializeDynamicReplacementLookup();
  }
};

SWIFT_ALLOWED_RUNTIME_GLOBAL_CTOR_BEGIN
static InitializeDynamicReplacementLookup initDynamicReplacements;
SWIFT_ALLOWED_RUNTIME_GLOBAL_CTOR_END

void DynamicReplacementDescriptor::enableReplacement() const {
  auto *chainRoot = const_cast<DynamicReplacementChainEntry *>(
      replacedFunctionKey->root.get());

  // Make sure this entry is not already enabled.
  // This does not work until we make sure that when a dynamic library is
  // unloaded all descriptors are removed.
#if 0
  for (auto *curr = chainRoot; curr != nullptr; curr = curr->next) {
    if (curr == chainEntry.get()) {
      swift::swift_abortDynamicReplacementEnabling();
    }
  }
#endif

  // Unlink the previous entry if we are not chaining.
  if (!shouldChain() && chainRoot->next) {
    auto *previous = chainRoot->next;
    chainRoot->next = previous->next;
    //chainRoot->implementationFunction = previous->implementationFunction;
    swift_ptrauth_copy(
      reinterpret_cast<void **>(&chainRoot->implementationFunction),
      reinterpret_cast<void *const *>(&previous->implementationFunction),
      replacedFunctionKey->getExtraDiscriminator());
  }

  // First populate the current replacement's chain entry.
  auto *currentEntry =
      const_cast<DynamicReplacementChainEntry *>(chainEntry.get());
  // currentEntry->implementationFunction = chainRoot->implementationFunction;
  swift_ptrauth_copy(
      reinterpret_cast<void **>(&currentEntry->implementationFunction),
      reinterpret_cast<void *const *>(&chainRoot->implementationFunction),
      replacedFunctionKey->getExtraDiscriminator());

  currentEntry->next = chainRoot->next;

  // Link the replacement entry.
  chainRoot->next = chainEntry.get();
  // chainRoot->implementationFunction = replacementFunction.get();
  swift_ptrauth_init(
      reinterpret_cast<void **>(&chainRoot->implementationFunction),
      reinterpret_cast<void *>(replacementFunction.get()),
      replacedFunctionKey->getExtraDiscriminator());
}

void DynamicReplacementDescriptor::disableReplacement() const {
  const auto *chainRoot = replacedFunctionKey->root.get();
  auto *thisEntry =
      const_cast<DynamicReplacementChainEntry *>(chainEntry.get());

  // Find the entry previous to this one.
  auto *prev = chainRoot;
  while (prev && prev->next != thisEntry)
    prev = prev->next;
  if (!prev) {
    swift::swift_abortDynamicReplacementDisabling();
    return;
  }

  // Unlink this entry.
  auto *previous = const_cast<DynamicReplacementChainEntry *>(prev);
  previous->next = thisEntry->next;
  // previous->implementationFunction = thisEntry->implementationFunction;
  swift_ptrauth_copy(
      reinterpret_cast<void **>(&previous->implementationFunction),
      reinterpret_cast<void *const *>(&thisEntry->implementationFunction),
      replacedFunctionKey->getExtraDiscriminator());
}

/// An automatic dymamic replacement entry.
namespace {
class AutomaticDynamicReplacementEntry {
  RelativeDirectPointer<DynamicReplacementScope, false> replacementScope;
  uint32_t flags;

public:
  void enable() const { replacementScope->enable(); }

  uint32_t getFlags() { return flags; }
};

/// A list of automatic dynamic replacement scopes.
class AutomaticDynamicReplacements
    : private swift::ABI::TrailingObjects<AutomaticDynamicReplacements,
                                          AutomaticDynamicReplacementEntry> {
  uint32_t flags;
  uint32_t numScopes;

  using TrailingObjects =
      swift::ABI::TrailingObjects<AutomaticDynamicReplacements,
                                  AutomaticDynamicReplacementEntry>;
  friend TrailingObjects;

  llvm::ArrayRef<AutomaticDynamicReplacementEntry>
  getReplacementEntries() const {
    return {
        this->template getTrailingObjects<AutomaticDynamicReplacementEntry>(),
        numScopes};
  }

public:
  void enableReplacements() const {
    for (auto &replacementEntry : getReplacementEntries())
      replacementEntry.enable();
  }

  uint32_t getNumScopes() const { return numScopes; }
};

/// A map from original to replaced opaque type descriptor of a some type.
class DynamicReplacementSomeDescriptor {
  RelativeIndirectablePointer<
      const OpaqueTypeDescriptor, false, int32_t,
      TargetSignedPointer<InProcess, OpaqueTypeDescriptor *
                                         __ptrauth_swift_type_descriptor>>
      originalOpaqueTypeDesc;
  RelativeDirectPointer<const OpaqueTypeDescriptor, false>
      replacementOpaqueTypeDesc;

public:
  void enable(const Mutex &lock) const {
    opaqueTypeMappings.get().insert(originalOpaqueTypeDesc.get(),
                                    replacementOpaqueTypeDesc.get(), lock);
  }
};

/// A list of dynamic replacements of some types.
class AutomaticDynamicReplacementsSome
    : private swift::ABI::TrailingObjects<AutomaticDynamicReplacementsSome,
                                          DynamicReplacementSomeDescriptor> {
  uint32_t flags;
  uint32_t numEntries;
  using TrailingObjects =
      swift::ABI::TrailingObjects<AutomaticDynamicReplacementsSome,
                                  DynamicReplacementSomeDescriptor>;
  friend TrailingObjects;

  llvm::ArrayRef<DynamicReplacementSomeDescriptor>
  getReplacementEntries() const {
    return {
        this->template getTrailingObjects<DynamicReplacementSomeDescriptor>(),
        numEntries};
  }

public:
  void enableReplacements(const Mutex &lock) const {
    for (auto &replacementEntry : getReplacementEntries())
      replacementEntry.enable(lock);
  }
  uint32_t getNumEntries() const { return numEntries; }
};

} // anonymous namespace

void swift::addImageDynamicReplacementBlockCallback(
    const void *replacements, uintptr_t replacementsSize,
    const void *replacementsSome, uintptr_t replacementsSomeSize) {

  auto *automaticReplacements =
      reinterpret_cast<const AutomaticDynamicReplacements *>(replacements);

  const AutomaticDynamicReplacementsSome *someReplacements = nullptr;
  if (replacementsSomeSize) {
    someReplacements =
        reinterpret_cast<const AutomaticDynamicReplacementsSome *>(
            replacementsSome);
  }

  auto sizeOfCurrentEntry = sizeof(AutomaticDynamicReplacements) +
                            (automaticReplacements->getNumScopes() *
                             sizeof(AutomaticDynamicReplacementEntry));
  auto sizeOfCurrentSomeEntry =
      replacementsSomeSize == 0
          ? 0
          : sizeof(AutomaticDynamicReplacementsSome) +
                (someReplacements->getNumEntries() *
                 sizeof(DynamicReplacementSomeDescriptor));

  auto &lock = DynamicReplacementLock.get();
  lock.withLock([&] {
    auto endOfAutomaticReplacements =
        ((const char *)automaticReplacements) + replacementsSize;
    while (((const char *)automaticReplacements) < endOfAutomaticReplacements) {
      automaticReplacements->enableReplacements();
      automaticReplacements =
          reinterpret_cast<const AutomaticDynamicReplacements *>(
              ((const char *)automaticReplacements) + sizeOfCurrentEntry);
      if ((const char*)automaticReplacements <  endOfAutomaticReplacements)
        sizeOfCurrentEntry = sizeof(AutomaticDynamicReplacements) +
                             (automaticReplacements->getNumScopes() *
                              sizeof(AutomaticDynamicReplacementEntry));
    }
    if (!replacementsSomeSize)
      return;
    auto endOfSomeReplacements =
        ((const char *)someReplacements) + replacementsSomeSize;
    while (((const char *)someReplacements) < endOfSomeReplacements) {
      someReplacements->enableReplacements(lock);
      someReplacements =
          reinterpret_cast<const AutomaticDynamicReplacementsSome *>(
              ((const char *)someReplacements) + sizeOfCurrentSomeEntry);
      if  ((const char*) someReplacements < endOfSomeReplacements)
        sizeOfCurrentSomeEntry = sizeof(AutomaticDynamicReplacementsSome) +
                                 (someReplacements->getNumEntries() *
                                  sizeof(DynamicReplacementSomeDescriptor));
    }
  });
}

void swift::swift_enableDynamicReplacementScope(
    const DynamicReplacementScope *scope) {
  scope = swift_auth_data_non_address(
      scope, SpecialPointerAuthDiscriminators::DynamicReplacementScope);
  DynamicReplacementLock.get().withLock([=] { scope->enable(); });
}

void swift::swift_disableDynamicReplacementScope(
    const DynamicReplacementScope *scope) {
  scope = swift_auth_data_non_address(
      scope, SpecialPointerAuthDiscriminators::DynamicReplacementScope);
  DynamicReplacementLock.get().withLock([=] { scope->disable(); });
}
#define OVERRIDE_METADATALOOKUP COMPATIBILITY_OVERRIDE
#include "CompatibilityOverride.def"