[Serialize] Serialize uniqued generic signatures.

Rather than inlining generic signatures in a half dozen places throughout
the serialization format, serialize (uniqued) generic signatures with their
own GenericSignatureID. Update various layouts (generic function types,
SIL function types, generic environments, extension cross-references) to
use GenericSignatureID.

Shaves ~187k off the size of Swift.swiftmodule.

Author: DougGregor

include/swift/Serialization/DeclTypeRecordNodes.def

@@ -162,7 +162,7 @@ OTHER(GENERIC_PARAM_LIST, 240)
 TRAILING_INFO(GENERIC_PARAM)
 TRAILING_INFO(GENERIC_REQUIREMENT)
 TRAILING_INFO(LAYOUT_REQUIREMENT)
-OTHER(GENERIC_ENVIRONMENT, 244)
+OTHER(GENERIC_SIGNATURE, 244)
 OTHER(SIL_GENERIC_ENVIRONMENT, 245)
 
 OTHER(LOCAL_DISCRIMINATOR, 248)

include/swift/Serialization/ModuleFile.h

@@ -299,6 +299,9 @@ class ModuleFile
   /// Types referenced by this module.
   std::vector<Serialized<Type>> Types;
 
+  /// Generic signatures referenced by this module.
+  std::vector<Serialized<GenericSignature *>> GenericSignatures;
+
   /// Generic environments referenced by this module.
   std::vector<Serialized<GenericEnvironment *>> GenericEnvironments;
 
@@ -818,6 +821,9 @@ class ModuleFile
   /// is loaded instead.
   ModuleDecl *getModule(ArrayRef<Identifier> name);
 
+  /// Returns the generic signature for the given ID.
+  GenericSignature *getGenericSignature(serialization::GenericSignatureID ID);
+
   /// Returns the generic signature or environment for the given ID,
   /// deserializing it if needed.
   ///

include/swift/Serialization/ModuleFormat.h

@@ -54,7 +54,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// in source control, you should also update the comment to briefly
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
-const uint16_t VERSION_MINOR = 380; // Last change: IUO decl attribute
+const uint16_t VERSION_MINOR = 381; // Last change: generic signature
 
 using DeclIDField = BCFixed<31>;
 
@@ -77,6 +77,11 @@ using DeclContextIDField = DeclIDField;
 using NormalConformanceID = DeclID;
 using NormalConformanceIDField = DeclIDField;
 
+// GenericSignatureID must be the same as DeclID because it is stored in the
+// same way.
+using GenericSignatureID = DeclID;
+using GenericSignatureIDField = DeclIDField;
+
 // GenericEnvironmentID must be the same as DeclID because it is stored in the
 // same way.
 using GenericEnvironmentID = DeclID;
@@ -738,8 +743,7 @@ namespace decls_block {
     TypeIDField,         // output
     FunctionTypeRepresentationField, // representation
     BCFixed<1>,          // throws?
-    BCArray<TypeIDField> // generic parameters
-                         // followed by requirements
+    GenericSignatureIDField // generic signture
   >;
 
   using SILFunctionTypeLayout = BCRecordLayout<
@@ -753,12 +757,11 @@ namespace decls_block {
     BCFixed<30>,           // number of parameters
     BCFixed<30>,           // number of yields
     BCFixed<30>,           // number of results
+    GenericSignatureIDField, // generic signature
     BCArray<TypeIDField>   // parameter types/conventions, alternating
                            // followed by result types/conventions, alternating
                            // followed by error result type/convention
-                           // followed by generic parameter types
     // Optionally a protocol conformance (for witness_methods)
-    // Trailed by its generic requirements, if any.
   >;
 
   using SILBlockStorageTypeLayout = BCRecordLayout<
@@ -768,10 +771,9 @@ namespace decls_block {
 
   using SILLayoutLayout = BCRecordLayout<
     SIL_LAYOUT,
+    GenericSignatureIDField,    // generic signature
     BCFixed<31>,                // number of fields
     BCArray<TypeIDWithBitField> // field types with mutability
-                                // followed by generic parameters
-                                // trailed by generic requirements, if any
   >;
 
   using SILBoxTypeLayout = BCRecordLayout<
@@ -1165,9 +1167,9 @@ namespace decls_block {
     DeclIDField // Typealias
   >;
 
-  using GenericEnvironmentLayout = BCRecordLayout<
-    GENERIC_ENVIRONMENT,
-    BCArray<TypeIDField>         // sugared interface types
+  using GenericSignatureLayout = BCRecordLayout<
+    GENERIC_SIGNATURE,
+    BCArray<TypeIDField>         // generic parameter types
   >;
 
   using SILGenericEnvironmentLayout = BCRecordLayout<
@@ -1282,8 +1284,8 @@ namespace decls_block {
   using XRefExtensionPathPieceLayout = BCRecordLayout<
     XREF_EXTENSION_PATH_PIECE,
     ModuleIDField,       // module ID
-    BCArray<TypeIDField> // for a constrained extension, the type parameters
-    // for a constrained extension, requirements follow
+    GenericSignatureIDField  // for a constrained extension,
+                             // the generic signature
   >;
 
   using XRefOperatorOrAccessorPathPieceLayout = BCRecordLayout<
@@ -1535,7 +1537,8 @@ namespace index_block {
     NESTED_TYPE_DECLS,
     DECL_MEMBER_NAMES,
 
-    LastRecordKind = DECL_MEMBER_NAMES,
+    GENERIC_SIGNATURE_OFFSETS,
+    LastRecordKind = GENERIC_SIGNATURE_OFFSETS,
   };
 
   constexpr const unsigned RecordIDFieldWidth = 5;