[Runtime] Improve representation of protocol conformance records

docs/ABI/TypeMetadata.rst

@@ -417,21 +417,24 @@ section, which is scanned by the Swift runtime when needed (e.g., in response to
 a `swift_conformsToProtocol()` query). Each protocol conformance record
 contains:
 
-- The `protocol descriptor`_ describing the protocol of the conformance.
-- A reference to the metadata for the **conforming type**, whose form is
-  determined by the **protocol conformance flags** described below.
+- The `protocol descriptor`_ describing the protocol of the conformance,
+  represented as an (possibly indirect) 32-bit offset relative to the field.
+  The low bit indicates whether it is an indirect offset; the second lowest
+  bit is reserved for future use.
+- A reference to the **conforming type**, represented as a 32-bit offset
+  relative to the field. The lower two bits indicate how the conforming
+  type is represented:
+    0. A direct reference to a nominal type descriptor.
+    1. An indirect reference to a nominal type descriptor.
+    2. A reference to nonunique, foreign type metadata.
+    3. A reference to a pointer to an Objective-C class object.
 - The **witness table field** that provides access to the witness table
-  describing the conformance itself; the form of this field is determined by the
-  **protocol conformance flags** described below.
-- The **protocol conformance flags** is a 32-bit field comprised of:
-
-  * **Bits 0-3** contain the type metadata record kind, which indicates how
-    the **conforming type** field is encoded.
-  * **Bits 4-5** contain the kind of witness table. The value can be one of:
-
+  describing the conformance itself, represented as a direct 32-bit relative
+  offset. The lower two bits indicate how the witness table is represented:
     0. The **witness table field** is a reference to a witness table.
     1. The **witness table field** is a reference to a **witness table
        accessor** function for an unconditional conformance.
     2. The **witness table field** is a reference to a **witness table
        accessor** function for a conditional conformance.
-
+    3. Reserved for future use.
+- A 32-bit value reserved for future use.

include/swift/ABI/MetadataValues.h

@@ -173,40 +173,27 @@ enum : unsigned {
 
 /// Kinds of type metadata/protocol conformance records.
 enum class TypeMetadataRecordKind : unsigned {
-  /// The conformance is universal and might apply to any type.
-  /// getDirectType() is nil.
-  Universal,
+  /// The conformance is for a nominal type referenced directly;
+  /// getNominalTypeDescriptor() points to the nominal type descriptor.
+  DirectNominalTypeDescriptor = 0x00,
 
-  /// The conformance is for a nongeneric native struct or enum type.
-  /// getDirectType() points to the canonical metadata for the type.
-  UniqueDirectType,
-  
-  /// The conformance is for a nongeneric foreign struct or enum type.
+  /// The conformance is for a nominal type referenced indirectly;
+  /// getNominalTypeDescriptor() points to the nominal type descriptor.
+  IndirectNominalTypeDescriptor = 0x01,
+
+  /// The conformance is for a foreign type described by its type metadata.
   /// getDirectType() points to a nonunique metadata record for the type, which
   /// needs to be uniqued by the runtime.
-  NonuniqueDirectType,
-
-  /// The conformance is for a nongeneric class type.
-  /// getIndirectClass() points to a variable that contains the pointer to the
-  /// class object, which may be ObjC and thus require a runtime call to get
-  /// metadata.
-  ///
-  /// On platforms without ObjC interop, this indirection isn't necessary,
-  /// and classes could be emitted as UniqueDirectType.
-  UniqueIndirectClass,
+  NonuniqueDirectType = 0x02,
 
-  /// The conformance is for a generic or resilient type.
-  /// getNominalTypeDescriptor() points to the nominal type descriptor shared
-  /// by all metadata instantiations of this type.
-  UniqueNominalTypeDescriptor,
-
-  /// The conformance is for a nongeneric class type.
-  /// getDirectType() points to the unique class object.
+  /// The conformance is for an Objective-C class that has no nominal type
+  /// descriptor.
+  /// getIndirectObjCClass() points to a variable that contains the pointer to
+  /// the class object, which then requires a runtime call to get metadata.
   ///
-  /// FIXME: This shouldn't exist. On ObjC interop platforms, class references
-  /// must be indirected (using UniqueIndirectClass). On non-ObjC interop
-  /// platforms, the class object always is the type metadata.
-  UniqueDirectClass = 0xF,
+  /// On platforms without Objective-C interoperability, this case is
+  /// unused.
+  IndirectObjCClass = 0x03,
 };
 
 /// Kinds of reference to protocol conformance.
@@ -220,6 +207,8 @@ enum class ProtocolConformanceReferenceKind : unsigned {
   /// table whose conformance is conditional on additional requirements that
   /// must first be evaluated and then provided to the accessor function.
   ConditionalWitnessTableAccessor,
+  /// Reserved for future use.
+  Reserved,
 };
 
 // Type metadata record discriminant
@@ -251,13 +240,6 @@ struct TypeMetadataRecordFlags {
 
 // Protocol conformance discriminant
 struct ProtocolConformanceFlags : public TypeMetadataRecordFlags {
-private:
-  enum : int_type {
-    ConformanceKindMask = 0x00000030U,
-    ConformanceKindShift = 4,
-  };
-
-public:
   constexpr ProtocolConformanceFlags() : TypeMetadataRecordFlags(0) {}
   constexpr ProtocolConformanceFlags(int_type Data) : TypeMetadataRecordFlags(Data) {}
 
@@ -266,15 +248,6 @@ struct ProtocolConformanceFlags : public TypeMetadataRecordFlags {
     return ProtocolConformanceFlags(
                      (Data & ~TypeKindMask) | (int_type(ptk) << TypeKindShift));
   }
-  constexpr ProtocolConformanceReferenceKind getConformanceKind() const {
-    return ProtocolConformanceReferenceKind((Data & ConformanceKindMask)
-                                     >> ConformanceKindShift);
-  }
-  constexpr ProtocolConformanceFlags withConformanceKind(
-                                  ProtocolConformanceReferenceKind pck) const {
-    return ProtocolConformanceFlags(
-       (Data & ~ConformanceKindMask) | (int_type(pck) << ConformanceKindShift));
-  }
 };
 
 /// Flag that indicates whether an existential type is class-constrained or not.

include/swift/Basic/RelativePointer.h

@@ -260,6 +260,76 @@ class RelativeIndirectablePointer {
   }
 };
 
+/// A relative reference to an aligned object stored in memory. The reference
+/// may be direct or indirect, and uses the low bit of the (assumed at least
+/// 2-byte-aligned) pointer to differentiate. The remaining low bits store
+/// an additional tiny integer value.
+template<typename ValueTy, typename IntTy, bool Nullable = false,
+         typename Offset = int32_t>
+class RelativeIndirectablePointerIntPair {
+private:
+  static_assert(std::is_integral<Offset>::value &&
+                std::is_signed<Offset>::value,
+                "offset type should be signed integer");
+
+  /// The relative offset of the pointer's memory from the `this` pointer.
+  /// If the low bit is clear, this is a direct reference; otherwise, it is
+  /// an indirect reference.
+  Offset RelativeOffsetPlusIndirectAndInt;
+
+  /// RelativePointers should appear in statically-generated metadata. They
+  /// shouldn't be constructed or copied.
+  RelativeIndirectablePointerIntPair() = delete;
+  RelativeIndirectablePointerIntPair(
+                           RelativeIndirectablePointerIntPair &&) = delete;
+  RelativeIndirectablePointerIntPair(
+                      const RelativeIndirectablePointerIntPair &) = delete;
+  RelativeIndirectablePointerIntPair& operator=(
+                           RelativeIndirectablePointerIntPair &&) = delete;
+  RelativeIndirectablePointerIntPair &operator=(
+                      const RelativeIndirectablePointerIntPair &) = delete;
+
+  // Retrieve the mask for the stored integer value.
+  static Offset getIntMask() {
+    return (alignof(Offset) - 1) & ~(Offset)0x01;
+  }
+
+public:
+  const ValueTy *getPointer() const & {
+    static_assert(alignof(ValueTy) >= 2 && alignof(Offset) >= 2,
+                  "alignment of value and offset must be at least 2 to "
+                  "make room for indirectable flag");
+
+    Offset offset = (RelativeOffsetPlusIndirectAndInt & ~getIntMask());
+
+    // Check for null.
+    if (Nullable && offset == 0)
+      return nullptr;
+
+    Offset offsetPlusIndirect = offset;
+    uintptr_t address = detail::applyRelativeOffset(this,
+                                                    offsetPlusIndirect & ~1);
+
+    // If the low bit is set, then this is an indirect address. Otherwise,
+    // it's direct.
+    if (offsetPlusIndirect & 1) {
+      return *reinterpret_cast<const ValueTy * const *>(address);
+    } else {
+      return reinterpret_cast<const ValueTy *>(address);
+    }
+  }
+
+  /// A zero relative offset encodes a null reference.
+  bool isNull() const & {
+    Offset offset = (RelativeOffsetPlusIndirectAndInt & ~getIntMask());
+    return offset == 0;
+  }
+
+  IntTy getInt() const & {
+    return IntTy((RelativeOffsetPlusIndirectAndInt & getIntMask()) >> 1);
+  }
+};
+
 /// A relative reference to a function, intended to reference private metadata
 /// functions for the current executable or dynamic library image from
 /// position-independent constant data.
@@ -398,9 +468,6 @@ class RelativeDirectPointerIntPair {
     = delete;
 
   static Offset getMask() {
-    static_assert(alignof(PointeeTy) >= alignof(Offset),
-                 "pointee alignment must be at least as strict as offset type");
-
     return alignof(Offset) - 1;
   }