Only store the minimal requirements in generic metadata, where

"minimal" is defined as the set of requirements that would be
passed to a function with the type's generic signature that
takes the thick metadata of the parent type as its only argument.

include/swift/ABI/MetadataValues.h

@@ -237,6 +237,63 @@ enum class ProtocolDispatchStrategy: uint8_t {
   Empty = 2,
 };
 
+/// Flags in a generic nominal type descriptor.
+class GenericParameterDescriptorFlags {
+  typedef uint32_t int_type;
+  enum : int_type {
+    HasParent        = 0x01,
+    HasGenericParent = 0x02,
+  };
+  int_type Data;
+
+  constexpr GenericParameterDescriptorFlags(int_type data) : Data(data) {}
+public:
+  constexpr GenericParameterDescriptorFlags() : Data(0) {}
+
+  constexpr GenericParameterDescriptorFlags withHasParent(bool b) const {
+    return GenericParameterDescriptorFlags(b ? (Data | HasParent)
+                                             : (Data & ~HasParent));
+  }
+
+  constexpr GenericParameterDescriptorFlags withHasGenericParent(bool b) const {
+    return GenericParameterDescriptorFlags(b ? (Data | HasGenericParent)
+                                             : (Data & ~HasGenericParent));
+  }
+
+  /// Does this type have a lexical parent type?
+  ///
+  /// For class metadata, if this is true, the storage for the parent type
+  /// appears immediately prior to the first generic argument.  Other
+  /// metadata always have a slot for their parent type.
+  bool hasParent() const {
+    return Data & HasParent;
+  }
+
+  /// Given that this type has a parent type, is that type generic?  If so,
+  /// it forms part of the key distinguishing this metadata from other
+  /// metadata, and the parent metadata will be the first argument to
+  /// the generic metadata access function.
+  bool hasGenericParent() const {
+    return Data & HasGenericParent;
+  }
+
+  int_type getIntValue() const {
+    return Data;
+  }
+
+  static GenericParameterDescriptorFlags fromIntValue(int_type Data) {
+    return GenericParameterDescriptorFlags(Data);
+  }
+
+  bool operator==(GenericParameterDescriptorFlags other) const {
+    return Data == other.Data;
+  }
+  bool operator!=(GenericParameterDescriptorFlags other) const {
+    return Data != other.Data;
+  }
+};
+
+
 /// Flags for protocol descriptors.
 class ProtocolDescriptorFlags {
   typedef uint32_t int_type;

include/swift/Runtime/Metadata.h

@@ -1208,34 +1208,37 @@ struct HeapMetadata : Metadata {
 /// structure that describes how to find and parse a generic parameter vector
 /// within the type metadata for an instance of a nominal type.
 struct GenericParameterDescriptor {
-  /// The offset of the descriptor in the metadata record. If NumParams is zero,
-  /// this value is meaningless.
+  /// The offset of the descriptor in the metadata record.  This is
+  /// meaningful if either NumGenericRequirements is nonzero or 
+  /// (for classes) if Flags.hasParent() is true.
   uint32_t Offset;
-  /// The number of type parameters. A value of zero means there is no generic
-  /// parameter vector. This includes associated types of the primary type
-  /// parameters.
-  uint32_t NumParams;
+
+  /// The amount of generic requirement data in the metadata record, in
+  /// words, excluding the lexical parent type.  A value of zero means
+  /// there is no generic requirement data.
+  ///
+  /// This may include protocol witness tables for type parameters or
+  /// their associated types.
+  uint32_t NumGenericRequirements;
+
   /// The number of primary type parameters. This is always less than or equal
-  /// to NumParams; it counts only the primary type parameters and not their
-  /// associated types.
+  /// to NumGenericRequirements; it counts only the type parameters
+  /// and not any required witness tables.
   uint32_t NumPrimaryParams;
-
-  /// True if the nominal type has generic parameters.
-  bool hasGenericParams() const { return NumParams > 0; }
-
-  /// A type parameter.
-  struct Parameter {
-    /// The number of protocol witness tables required by this type parameter.
-    uint32_t NumWitnessTables;
-
-    // TODO: This is the bare minimum to be able to parse an opaque generic
-    // parameter vector. Should we include additional info, such as the
-    // required protocols?
-  };
 
-  /// The parameter descriptors are in a tail-emplaced array of NumParams
-  /// elements.
-  Parameter Parameters[1];
+  /// Flags for this generic parameter descriptor.
+  GenericParameterDescriptorFlags Flags;
+
+  /// True if the nominal type has generic requirements other than its
+  /// parent metadata.
+  bool hasGenericRequirements() const { return NumGenericRequirements > 0; }
+
+  /// True if the nominal type is generic in any way.
+  bool isGeneric() const {
+    return hasGenericRequirements() || Flags.hasGenericParent();
+  }
+
+  // TODO: add meaningful descriptions of the generic requirements.
 };
 
 struct ClassTypeDescriptor;
@@ -1566,6 +1569,16 @@ struct ClassMetadata : public HeapMetadata {
     return getter(this);
   }
 
+  /// Return the parent type for a given level in the class hierarchy, or
+  /// null if that level does not have a parent type.
+  const Metadata *getParentType(const NominalTypeDescriptor *theClass) const {
+    if (!theClass->GenericParams.Flags.hasParent())
+      return nullptr;
+
+    auto metadataAsWords = reinterpret_cast<const Metadata * const *>(this);
+    return metadataAsWords[theClass->GenericParams.Offset - 1];
+  }
+
   static bool classof(const Metadata *metadata) {
     return metadata->getKind() == MetadataKind::Class;
   }
@@ -1739,12 +1752,11 @@ struct ValueMetadata : public Metadata {
 
   /// Retrieve the generic arguments of this type.
   const Metadata * const *getGenericArgs() const {
-    if (Description->GenericParams.NumParams == 0)
+    if (!Description->GenericParams.hasGenericRequirements())
       return nullptr;
 
-    const void* const *asWords = reinterpret_cast<const void * const *>(this);
-    asWords += Description->GenericParams.Offset;
-    return reinterpret_cast<const Metadata * const *>(asWords);
+    auto asWords = reinterpret_cast<const Metadata * const *>(this);
+    return (asWords + Description->GenericParams.Offset);
   }
 };
 

lib/IRGen/ClassMetadataLayout.h

@@ -73,23 +73,23 @@ template <class Impl> class ClassMetadataLayout : public MetadataLayout<Impl> {
     asImpl().addIVarDestroyer();
 
     // Class members.
-    addClassMembers(Target);
+    addClassMembers(Target, Target->getDeclaredTypeInContext());
   }
 
 private:
   /// Add fields associated with the given class and its bases.
-  void addClassMembers(ClassDecl *theClass) {
+  void addClassMembers(ClassDecl *theClass, Type type) {
     // Add any fields associated with the superclass.
     // NB: We don't apply superclass substitutions to members because we want
     // consistent metadata layout between generic superclasses and concrete
     // subclasses.
-    if (Type superclass = theClass->getSuperclass()) {
+    if (Type superclass = type->getSuperclass(nullptr)) {
       ClassDecl *superclassDecl = superclass->getClassOrBoundGenericClass();
       // Skip superclass fields if superclass is resilient.
       // FIXME: Needs runtime support to ensure the field offset vector is
       // populated correctly.
       if (!IGM.isResilient(superclassDecl, ResilienceExpansion::Maximal)) {
-        addClassMembers(superclass->getClassOrBoundGenericClass());
+        addClassMembers(superclassDecl, superclass);
       }
     }
 
@@ -101,9 +101,7 @@ template <class Impl> class ClassMetadataLayout : public MetadataLayout<Impl> {
     // Add space for the generic parameters, if applicable.
     // Note that we only add references for the immediate parameters;
     // parameters for the parent context are handled by the parent.
-    if (auto generics = theClass->getGenericParams()) {
-      addGenericClassFields(theClass, *generics);
-    }
+    asImpl().addGenericFields(theClass, type, theClass);
 
     // Add entries for the methods.
     for (auto member : theClass->getMembers()) {
@@ -177,15 +175,6 @@ template <class Impl> class ClassMetadataLayout : public MetadataLayout<Impl> {
   void noteEndOfFieldOffsets(ClassDecl *whichClass) {}
 
 private:
-  /// Add fields related to the generics of this class declaration.
-  /// TODO: don't add new fields that are implied by the superclass.
-  /// fields.  e.g., if B<T> extends A<T>, the witness for T in A's
-  /// section should be enough.
-  void addGenericClassFields(ClassDecl *theClass,
-                             const GenericParamList &generics) {
-    asImpl().addGenericFields(generics, theClass);
-  }
-
   void addFieldEntries(VarDecl *field) {
     asImpl().addFieldOffset(field);
   }
@@ -258,11 +247,10 @@ class ClassMetadataScanner : public ClassMetadataLayout<Impl> {
     addPointer();
   }
   void addFieldOffset(VarDecl *var) { addPointer(); }
-  void addGenericArgument(ArchetypeType *argument, ClassDecl *forClass) {
+  void addGenericArgument(CanType argument, ClassDecl *forClass) {
     addPointer();
   }
-  void addGenericWitnessTable(ArchetypeType *argument,
-                              ProtocolDecl *protocol,
+  void addGenericWitnessTable(CanType argument, ProtocolConformanceRef conf,
                               ClassDecl *forClass) {
     addPointer();
   }

lib/IRGen/EnumMetadataLayout.h

@@ -60,8 +60,7 @@ template <class Impl> class EnumMetadataLayout : public MetadataLayout<Impl> {
       asImpl().addPayloadSize();
 
     // Add fields for generic cases.
-    if (auto generics = Target->getGenericParamsOfContext())
-      asImpl().addGenericFields(*generics);
+    asImpl().addGenericFields(Target, Target->getDeclaredTypeInContext());
   }
 };
 
@@ -82,9 +81,8 @@ class EnumMetadataScanner : public EnumMetadataLayout<Impl> {
   void addValueWitnessTable() { addPointer(); }
   void addNominalTypeDescriptor() { addPointer(); }
   void addParentMetadataRef() { addPointer(); }
-  void addGenericArgument(ArchetypeType *argument) { addPointer(); }
-  void addGenericWitnessTable(ArchetypeType *argument,
-                              ProtocolDecl *protocol) {
+  void addGenericArgument(CanType argument) { addPointer(); }
+  void addGenericWitnessTable(CanType argument, ProtocolConformanceRef conf) {
     addPointer();
   }
   void addPayloadSize() { addPointer(); }