AST: Cache ProtocolDecl::getInheritedProtocols()

Fixes <rdar://problem/47371754>.

Author: slavapestov

include/swift/AST/Decl.h

@@ -493,7 +493,7 @@ class alignas(1 << DeclAlignInBits) Decl {
     HasLazyConformances : 1
   );
 
-  SWIFT_INLINE_BITFIELD_FULL(ProtocolDecl, NominalTypeDecl, 1+1+1+1+1+1+1+2+8+16,
+  SWIFT_INLINE_BITFIELD_FULL(ProtocolDecl, NominalTypeDecl, 1+1+1+1+1+1+1+2+1+8+16,
     /// Whether the \c RequiresClass bit is valid.
     RequiresClassValid : 1,
 
@@ -519,6 +519,9 @@ class alignas(1 << DeclAlignInBits) Decl {
     /// The stage of the circularity check for this protocol.
     Circularity : 2,
 
+    /// Whether we've computed the inherited protocols list yet.
+    InheritedProtocolsValid : 1,
+
     : NumPadBits,
 
     /// If this is a compiler-known protocol, this will be a KnownProtocolKind
@@ -3893,15 +3896,7 @@ struct SelfReferenceKind {
 class ProtocolDecl final : public NominalTypeDecl {
   SourceLoc ProtocolLoc;
 
-  /// The generic signature representing exactly the new requirements introduced
-  /// by this protocol.
-  const Requirement *RequirementSignature = nullptr;
-
-  bool requiresClassSlow();
-
-  bool existentialConformsToSelfSlow();
-
-  bool existentialTypeSupportedSlow(LazyResolver *resolver);
+  ArrayRef<ProtocolDecl *> InheritedProtocols;
 
   struct {
     /// The superclass decl and a bit to indicate whether the
@@ -3913,6 +3908,18 @@ class ProtocolDecl final : public NominalTypeDecl {
     llvm::PointerIntPair<Type, 1, bool> SuperclassType;
   } LazySemanticInfo;
 
+  /// The generic signature representing exactly the new requirements introduced
+  /// by this protocol.
+  const Requirement *RequirementSignature = nullptr;
+
+  bool requiresClassSlow();
+
+  bool existentialConformsToSelfSlow();
+
+  bool existentialTypeSupportedSlow(LazyResolver *resolver);
+
+  ArrayRef<ProtocolDecl *> getInheritedProtocolsSlow();
+
   friend class SuperclassDeclRequest;
   friend class SuperclassTypeRequest;
   friend class TypeChecker;
@@ -3925,7 +3932,12 @@ class ProtocolDecl final : public NominalTypeDecl {
   using Decl::getASTContext;
 
   /// Retrieve the set of protocols inherited from this protocol.
-  llvm::TinyPtrVector<ProtocolDecl *> getInheritedProtocols() const;
+  ArrayRef<ProtocolDecl *> getInheritedProtocols() const {
+    if (Bits.ProtocolDecl.InheritedProtocolsValid)
+      return InheritedProtocols;
+
+    return const_cast<ProtocolDecl *>(this)->getInheritedProtocolsSlow();
+  }
 
   /// Determine whether this protocol has a superclass.
   bool hasSuperclass() const { return (bool)getSuperclassDecl(); }

lib/AST/Decl.cpp

@@ -3909,16 +3909,19 @@ ProtocolDecl::ProtocolDecl(DeclContext *DC, SourceLoc ProtocolLoc,
   Bits.ProtocolDecl.ExistentialConformsToSelf = false;
   Bits.ProtocolDecl.Circularity
     = static_cast<unsigned>(CircularityCheck::Unchecked);
+  Bits.ProtocolDecl.InheritedProtocolsValid = 0;
   Bits.ProtocolDecl.NumRequirementsInSignature = 0;
   Bits.ProtocolDecl.HasMissingRequirements = false;
   Bits.ProtocolDecl.KnownProtocol = 0;
-  setTrailingWhereClause(TrailingWhere);
+    setTrailingWhereClause(TrailingWhere);
 }
 
-llvm::TinyPtrVector<ProtocolDecl *>
-ProtocolDecl::getInheritedProtocols() const {
-  llvm::TinyPtrVector<ProtocolDecl *> result;
-  SmallPtrSet<const ProtocolDecl *, 4> known;
+ArrayRef<ProtocolDecl *>
+ProtocolDecl::getInheritedProtocolsSlow() {
+  Bits.ProtocolDecl.InheritedProtocolsValid = true;
+
+  llvm::SmallVector<ProtocolDecl *, 2> result;
+  SmallPtrSet<const ProtocolDecl *, 2> known;
   known.insert(this);
   bool anyObject = false;
   for (const auto found :
@@ -3930,7 +3933,9 @@ ProtocolDecl::getInheritedProtocols() const {
     }
   }
 
-  return result;
+  auto &ctx = getASTContext();
+  InheritedProtocols = ctx.AllocateCopy(result);
+  return InheritedProtocols;
 }
 
 llvm::TinyPtrVector<AssociatedTypeDecl *>