[ABI] Collapse generic witness table into protocol conformance record.

Collapse the generic witness table, which was used only as a uniquing
data structure during witness table instantiation, into the protocol
conformance record. This colocates all of the constant protocol conformance
metadata and makes it possible for us to recover the generic witness table
from the conformance descriptor (including looking at the pattern itself).

Rename swift_getGenericWitnessTable() to swift_instantiateWitnessTable()
to make it clearer what its purpose is, and take the conformance descriptor
directly.

Author: DougGregor

docs/ABI/Mangling.rst

@@ -84,7 +84,7 @@ Globals
   global ::= protocol-conformance 'WP'   // protocol witness table
   global ::= protocol-conformance 'Wa'   // protocol witness table accessor
 
-  global ::= protocol-conformance 'WG'   // generic protocol witness table
+  global ::= protocol-conformance 'WG'   // generic protocol witness table (HISTORICAL)
   global ::= protocol-conformance 'Wp'   // protocol witness table pattern
   global ::= protocol-conformance 'Wr'   // resilient witness table (HISTORICAL)
   global ::= protocol-conformance 'WI'   // generic protocol witness table instantiation function

include/swift/ABI/Metadata.h

@@ -1989,7 +1989,7 @@ struct TargetResilientWitnessTable final
 using ResilientWitnessTable = TargetResilientWitnessTable<InProcess>;
 
 /// \brief The control structure of a generic or resilient protocol
-/// conformance.
+/// conformance, which is embedded in the protocol conformance descriptor.
 ///
 /// Witness tables need to be instantiated at runtime in these cases:
 /// - For a generic conforming type, associated type requirements might be
@@ -2245,14 +2245,16 @@ struct TargetProtocolConformanceDescriptor final
              RelativeContextPointer<Runtime>,
              TargetGenericRequirementDescriptor<Runtime>,
              TargetResilientWitnessesHeader<Runtime>,
-             TargetResilientWitness<Runtime>> {
+             TargetResilientWitness<Runtime>,
+             TargetGenericWitnessTable<Runtime>> {
 
   using TrailingObjects = swift::ABI::TrailingObjects<
                              TargetProtocolConformanceDescriptor<Runtime>,
                              RelativeContextPointer<Runtime>,
                              TargetGenericRequirementDescriptor<Runtime>,
                              TargetResilientWitnessesHeader<Runtime>,
-                             TargetResilientWitness<Runtime>>;
+                             TargetResilientWitness<Runtime>,
+                             TargetGenericWitnessTable<Runtime>>;
   friend TrailingObjects;
 
   template<typename T>
@@ -2268,6 +2270,7 @@ struct TargetProtocolConformanceDescriptor final
 
   using ResilientWitnessesHeader = TargetResilientWitnessesHeader<Runtime>;
   using ResilientWitness = TargetResilientWitness<Runtime>;
+  using GenericWitnessTable = TargetGenericWitnessTable<Runtime>;
 
 private:
   /// The protocol being conformed to.
@@ -2375,6 +2378,14 @@ struct TargetProtocolConformanceDescriptor final
              numTrailingObjects(OverloadToken<ResilientWitness>()));
   }
 
+  ConstTargetPointer<Runtime, GenericWitnessTable>
+  getGenericWitnessTable() const {
+    if (!Flags.hasGenericWitnessTable())
+      return nullptr;
+
+    return this->template getTrailingObjects<GenericWitnessTable>();
+  }
+
 #if !defined(NDEBUG) && SWIFT_OBJC_INTEROP
   void dump() const;
 #endif
@@ -2409,6 +2420,10 @@ struct TargetProtocolConformanceDescriptor final
           ->NumWitnesses
       : 0;
   }
+
+  size_t numTrailingObjects(OverloadToken<GenericWitnessTable>) const {
+    return Flags.hasGenericWitnessTable() ? 1 : 0;
+  }
 };
 using ProtocolConformanceDescriptor
   = TargetProtocolConformanceDescriptor<InProcess>;

include/swift/ABI/MetadataValues.h

@@ -629,6 +629,7 @@ class ConformanceFlags {
     NumConditionalRequirementsShift = 8,
 
     HasResilientWitnessesMask = 0x01 << 16,
+    HasGenericWitnessTableMask = 0x01 << 17,
   };
 
   int_type Value;
@@ -668,6 +669,14 @@ class ConformanceFlags {
                                                     : 0));
   }
 
+  ConformanceFlags withHasGenericWitnessTable(
+                                           bool hasGenericWitnessTable) const {
+    return ConformanceFlags((Value & ~HasGenericWitnessTableMask)
+                            | (hasGenericWitnessTable
+                                 ? HasGenericWitnessTableMask
+                                 : 0));
+  }
+
   /// Retrieve the conformance kind.
   ConformanceKind getConformanceKind() const {
     return ConformanceKind(Value & ConformanceKindMask);
@@ -709,6 +718,12 @@ class ConformanceFlags {
     return Value & HasResilientWitnessesMask;
   }
 
+  /// Whether this conformance has a generic witness table that may need to
+  /// be instantiated.
+  bool hasGenericWitnessTable() const {
+    return Value & HasGenericWitnessTableMask;
+  }
+
   int_type getIntValue() const { return Value; }
 };
 

include/swift/IRGen/Linking.h

@@ -247,10 +247,6 @@ class LinkEntity {
     /// ProtocolConformance*.
     ProtocolWitnessTableAccessFunction,
 
-    /// A generic protocol witness table cache.  The secondary pointer is a
-    /// ProtocolConformance*.
-    GenericProtocolWitnessTableCache,
-
     /// The instantiation function for a generic protocol witness table.
     /// The secondary pointer is a ProtocolConformance*.
     GenericProtocolWitnessTableInstantiationFunction,
@@ -738,13 +734,6 @@ class LinkEntity {
     return entity;
   }
 
-  static LinkEntity
-  forGenericProtocolWitnessTableCache(const ProtocolConformance *C) {
-    LinkEntity entity;
-    entity.setForProtocolConformance(Kind::GenericProtocolWitnessTableCache, C);
-    return entity;
-  }
-
   static LinkEntity
   forGenericProtocolWitnessTableInstantiationFunction(
                                       const ProtocolConformance *C) {

include/swift/Runtime/Metadata.h

@@ -387,10 +387,10 @@ MetadataResponse swift_checkMetadataState(MetadataRequest request,
 
 /// Instantiate a resilient or generic protocol witness table.
 ///
-/// \param genericTable - The witness table template for the
-///   conformance. It may either have fields that require runtime
-///   initialization, or be missing requirements at the end for
-///   which default witnesses are available.
+/// \param conformance - The protocol conformance descriptor, which
+///   contains the generic witness table record. It may either have fields
+///   that require runtime initialization, or be missing requirements at the
+///   end for which default witnesses are available.
 ///
 /// \param type - The conforming type, used to form a uniquing key
 ///   for the conformance. If the witness table is not dependent on
@@ -406,9 +406,9 @@ MetadataResponse swift_checkMetadataState(MetadataRequest request,
 ///   conformances.
 SWIFT_RUNTIME_EXPORT
 const WitnessTable *
-swift_getGenericWitnessTable(GenericWitnessTable *genericTable,
-                             const Metadata *type,
-                             void **const *instantiationArgs);
+swift_instantiateWitnessTable(ProtocolConformanceDescriptor *conformance,
+                              const Metadata *type,
+                              void **const *instantiationArgs);
 
 /// Retrieve an associated type witness from the given witness table.
 ///

include/swift/Runtime/RuntimeFunctions.def

@@ -635,12 +635,12 @@ FUNCTION(CheckMetadataState, swift_checkMetadataState, SwiftCC,
          ATTRS(NoUnwind, ReadOnly))
 
 // const ProtocolWitnessTable *
-// swift_getGenericWitnessTable(GenericProtocolWitnessTable *genericTable,
-//                              const Metadata *type,
-//                              void ** const *instantiationArgs);
-FUNCTION(GetGenericWitnessTable, swift_getGenericWitnessTable, C_CC,
+// swift_instantiateWitnessTable(const ProtocolConformanceDescriptor *conf,
+//                               const Metadata *type,
+//                               void ** const *instantiationArgs);
+FUNCTION(InstantiateWitnessTable, swift_instantiateWitnessTable, C_CC,
          RETURNS(WitnessTablePtrTy),
-         ARGS(getGenericWitnessTableCacheTy()->getPointerTo(),
+         ARGS(ProtocolConformanceDescriptorPtrTy,
               TypeMetadataPtrTy,
               WitnessTablePtrPtrTy),
          ATTRS(NoUnwind, ReadOnly))

lib/IRGen/GenDecl.cpp

@@ -3644,15 +3644,6 @@ IRGenModule::getResilienceExpansionForLayout(SILGlobalVariable *global) {
   return ResilienceExpansion::Maximal;
 }
 
-llvm::Constant *IRGenModule::
-getAddrOfGenericWitnessTableCache(const NormalProtocolConformance *conf,
-                                  ForDefinition_t forDefinition) {
-  auto entity = LinkEntity::forGenericProtocolWitnessTableCache(conf);
-  auto expectedTy = getGenericWitnessTableCacheTy();
-  return getAddrOfLLVMVariable(entity, getPointerAlignment(), forDefinition,
-                               expectedTy, DebugTypeInfo());
-}
-
 llvm::Function *
 IRGenModule::getAddrOfGenericWitnessTableInstantiationFunction(
                                       const NormalProtocolConformance *conf) {
@@ -3675,25 +3666,6 @@ IRGenModule::getAddrOfGenericWitnessTableInstantiationFunction(
   return entry;
 }
 
-llvm::StructType *IRGenModule::getGenericWitnessTableCacheTy() {
-  if (auto ty = GenericWitnessTableCacheTy) return ty;
-
-  GenericWitnessTableCacheTy = llvm::StructType::create(getLLVMContext(),
-    {
-      // WitnessTableSizeInWords
-      Int16Ty,
-      // WitnessTablePrivateSizeInWords + RequiresInstantiation bit
-      Int16Ty,
-      // Pattern
-      RelativeAddressTy,
-      // Instantiator
-      RelativeAddressTy,
-      // PrivateData
-      RelativeAddressTy
-    }, "swift.generic_witness_table_cache");
-  return GenericWitnessTableCacheTy;
-}
-
 /// Fetch the witness table access function for a protocol conformance.
 llvm::Function *
 IRGenModule::getAddrOfWitnessTableAccessFunction(