[Serialization] Drop GenericEnvironmentID for GenericSignatureID (swiftlang#26862)

A generic environment is always serialized as a GenericSignature with
a lazily-recreated environment, though sometimes it has to include
extra info specifically for generic environments used by SIL. The code
that was doing this claimed a bit for disambiguating between the two,
shrinking the permitted size of a compiled module from 2^31 bits to
2^30. (The code isn't just needlessly complicated; GenericEnvironments
used to be serialized with more information.)

Rather than have two representations for GenericEnvironmentID, this
commit just drops it altogether in favor of referencing
GenericSignatures directly.  This causes a negligible file size
shrinkage for swiftmodules in addition to eliminating the problematic
disambiguation bit.

For now, the Deserialization logic will continue to cache
GenericEnvironments that are used directly by Deserialization, but
really that should probably be done at the AST level. Then we can
simplify further to ModuleFile tracking a plain list of
GenericSignatures.

Author: jrose-apple

include/swift/Serialization/DeclTypeRecordNodes.def

@@ -167,7 +167,7 @@ OTHER(GENERIC_SIGNATURE, 231)
 TRAILING_INFO(GENERIC_REQUIREMENT)
 TRAILING_INFO(LAYOUT_REQUIREMENT)
 // 234 is unused
-OTHER(SIL_GENERIC_ENVIRONMENT, 235)
+OTHER(SIL_GENERIC_SIGNATURE, 235)
 OTHER(SUBSTITUTION_MAP, 236)
 
 OTHER(LOCAL_DISCRIMINATOR, 237)

include/swift/Serialization/ModuleFile.h

@@ -210,14 +210,13 @@ class ModuleFile
       return Value.template get<serialization::BitOffset>();
     }
 
-    template <typename Derived>
-    Serialized &operator=(Derived deserialized) {
+    Serialized &operator=(T deserialized) {
       assert(!isComplete() || ImplTy(deserialized) == Value);
       Value = deserialized;
       return *this;
     }
 
-    void unsafeOverwrite(T t) {
+    void uncheckedOverwrite(T t) {
       Value = t;
     }
   };
@@ -317,11 +316,17 @@ class ModuleFile
   /// Types referenced by this module.
   MutableArrayRef<Serialized<Type>> Types;
 
-  /// Generic signatures referenced by this module.
-  MutableArrayRef<Serialized<GenericSignature *>> GenericSignatures;
+  using GenericSignatureOrEnvironment =
+      llvm::PointerUnion<GenericSignature *, GenericEnvironment *>;
 
-  /// Generic environments referenced by this module.
-  MutableArrayRef<Serialized<GenericEnvironment *>> GenericEnvironments;
+  /// Generic signatures and environments referenced by this module.
+  ///
+  /// Technically only the GenericSignatures are encoded, but storing the
+  /// environment here too allows caching them.
+  // FIXME: That caching should be done at the AST level; it's not specific to
+  // Serialization.
+  MutableArrayRef<Serialized<GenericSignatureOrEnvironment>>
+      GenericSignaturesAndEnvironments;
 
   /// Substitution maps referenced by this module.
   MutableArrayRef<Serialized<SubstitutionMap>> SubstitutionMaps;
@@ -560,9 +565,8 @@ class ModuleFile
 
   /// Set up a (potentially lazy) generic environment for the given type,
   /// function or extension.
-  void configureGenericEnvironment(
-                   GenericContext *genericDecl,
-                   serialization::GenericEnvironmentID envID);
+  void configureGenericEnvironment(GenericContext *genericDecl,
+                                   serialization::GenericSignatureID envID);
 
   /// Populates the protocol's default witness table.
   ///
@@ -879,14 +883,14 @@ class ModuleFile
   /// \param wantEnvironment If true, always return the full generic
   /// environment. Otherwise, only return the generic environment if it's
   /// already been constructed, and the signature in other cases.
-  llvm::PointerUnion<GenericSignature *, GenericEnvironment *>
-  getGenericSignatureOrEnvironment(serialization::GenericEnvironmentID ID,
+  GenericSignatureOrEnvironment
+  getGenericSignatureOrEnvironment(serialization::GenericSignatureID ID,
                                    bool wantEnvironment = false);
 
   /// Returns the generic environment for the given ID, deserializing it if
   /// needed.
-  GenericEnvironment *getGenericEnvironment(
-                                        serialization::GenericEnvironmentID ID);
+  GenericEnvironment *
+  getGenericEnvironment(serialization::GenericSignatureID ID);
 
   /// Returns the substitution map for the given ID, deserializing it if
   /// needed.

include/swift/Serialization/ModuleFormat.h

@@ -52,7 +52,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// 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.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 515; // remove DeclContext indirection
+const uint16_t SWIFTMODULE_VERSION_MINOR = 516; // encode GenericSignature and GenericEnvironment together
 
 using DeclIDField = BCFixed<31>;
 
@@ -129,11 +129,6 @@ using NormalConformanceIDField = DeclIDField;
 using GenericSignatureID = DeclID;
 using GenericSignatureIDField = DeclIDField;
 
-// GenericEnvironmentID must be the same as DeclID because it is stored in the
-// same way.
-using GenericEnvironmentID = DeclID;
-using GenericEnvironmentIDField = DeclIDField;
-
 // SubstitutionMapID must be the same as DeclID because it is stored in the
 // same way.
 using SubstitutionMapID = DeclID;
@@ -843,7 +838,7 @@ namespace decls_block {
 
   using PrimaryArchetypeTypeLayout = BCRecordLayout<
     PRIMARY_ARCHETYPE_TYPE,
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     BCVBR<4>, // generic type parameter depth
     BCVBR<4>  // index + 1, or zero if we have a generic type parameter decl
   >;
@@ -963,7 +958,7 @@ namespace decls_block {
     TypeIDField, // underlying type
     TypeIDField, // interface type (no longer used)
     BCFixed<1>,  // implicit flag
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     AccessLevelField, // access level
     BCArray<TypeIDField> // dependency types
     // Trailed by generic parameters (if any).
@@ -992,7 +987,7 @@ namespace decls_block {
     DeclContextIDField,     // context decl
     BCFixed<1>,             // implicit flag
     BCFixed<1>,             // isObjC
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     AccessLevelField,       // access level
     BCVBR<4>,               // number of conformances
     BCVBR<4>,               // number of inherited types
@@ -1007,7 +1002,7 @@ namespace decls_block {
     DeclContextIDField,     // context decl
     BCFixed<1>,             // implicit flag
     BCFixed<1>,             // isObjC
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     TypeIDField,            // raw type
     AccessLevelField,       // access level
     BCVBR<4>,               // number of conformances
@@ -1024,7 +1019,7 @@ namespace decls_block {
     BCFixed<1>,             // implicit?
     BCFixed<1>,             // explicitly objc?
     BCFixed<1>,             // inherits convenience initializers from its superclass?
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     TypeIDField,            // superclass
     AccessLevelField,       // access level
     BCVBR<4>,               // number of conformances
@@ -1066,7 +1061,7 @@ namespace decls_block {
     BCFixed<1>,  // stub implementation?
     BCFixed<1>,  // throws?
     CtorInitializerKindField,  // initializer kind
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     DeclIDField, // overridden decl
     AccessLevelField, // access level
     BCFixed<1>,   // requires a new vtable slot
@@ -1134,7 +1129,7 @@ namespace decls_block {
     SelfAccessKindField,   // self access kind
     BCFixed<1>,   // has forced static dispatch?
     BCFixed<1>,   // throws?
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     TypeIDField,  // result interface type
     BCFixed<1>,   // IUO result?
     DeclIDField,  // operator decl
@@ -1160,7 +1155,7 @@ namespace decls_block {
     DeclIDField, // naming decl
     GenericSignatureIDField, // interface generic signature
     TypeIDField, // interface type for opaque type
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     SubstitutionMapIDField // optional substitution map for underlying type
     // trailed by generic parameters
   >;
@@ -1176,7 +1171,7 @@ namespace decls_block {
     SelfAccessKindField,   // self access kind
     BCFixed<1>,   // has forced static dispatch?
     BCFixed<1>,   // throws?
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     TypeIDField,  // result interface type
     BCFixed<1>,   // IUO result?
     DeclIDField,  // overridden function
@@ -1263,7 +1258,7 @@ namespace decls_block {
     WriteImplKindField,   // write implementation
     ReadWriteImplKindField,   // read-write implementation
     AccessorCountField, // number of accessors
-    GenericEnvironmentIDField, // generic environment
+    GenericSignatureIDField, // generic environment
     TypeIDField, // element interface type
     BCFixed<1>,  // IUO element?
     DeclIDField, // overridden decl
@@ -1286,7 +1281,7 @@ namespace decls_block {
     TypeIDField, // base type
     DeclContextIDField, // context decl
     BCFixed<1>,  // implicit flag
-    GenericEnvironmentIDField,  // generic environment
+    GenericSignatureIDField,  // generic environment
     BCVBR<4>,    // # of protocol conformances
     BCVBR<4>,    // number of inherited types
     BCArray<TypeIDField> // inherited types, followed by TypeID dependencies
@@ -1299,7 +1294,7 @@ namespace decls_block {
     DeclContextIDField, // context decl
     BCFixed<1>,  // implicit?
     BCFixed<1>,  // objc?
-    GenericEnvironmentIDField // generic environment
+    GenericSignatureIDField // generic environment
     // This record is trailed by its inlinable body text
   >;
 
@@ -1379,8 +1374,8 @@ namespace decls_block {
     // Conformances trail the record.
   >;
 
-  using SILGenericEnvironmentLayout = BCRecordLayout<
-    SIL_GENERIC_ENVIRONMENT,
+  using SILGenericSignatureLayout = BCRecordLayout<
+    SIL_GENERIC_SIGNATURE,
     BCArray<TypeIDField>         // (generic parameter name, sugared interface
                                  //  type) pairs
   >;
@@ -1780,7 +1775,7 @@ namespace index_block {
     LOCAL_DECL_CONTEXT_OFFSETS,
     LOCAL_TYPE_DECLS,
     OPAQUE_RETURN_TYPE_DECLS,
-    GENERIC_ENVIRONMENT_OFFSETS,
+    GENERIC_SIGNATURE_OFFSETS,
     NORMAL_CONFORMANCE_OFFSETS,
     SIL_LAYOUT_OFFSETS,
 
@@ -1790,7 +1785,6 @@ namespace index_block {
 
     ORDERED_TOP_LEVEL_DECLS,
 
-    GENERIC_SIGNATURE_OFFSETS,
     SUBSTITUTION_MAP_OFFSETS,
     LastRecordKind = SUBSTITUTION_MAP_OFFSETS,
   };