Runtime support for identity key paths.

Make sure the implementation can handle a key path with zero components by removing inappropriate assumptions that the number of components is always non-empty. Identity key paths also need some special behavior:

- Appending an identity key path should be an identity operation for the other operand
- Identity key paths have a `MemoryLayout.offset(of:)` zero
- Identity key paths interop with KVC as key paths to `@"self"`

To be able to exercise and test this behavior, add a `Builtin.identityKeyPath()` function and `WritableKeyPath._identity` accessor in lieu of finalized syntax.

include/swift/AST/Builtins.def

@@ -390,6 +390,11 @@ BUILTIN_SIL_OPERATION(AllocWithTailElems, "allocWithTailElems", Special)
 /// Projects the first tail-allocated element of type E from a class C.
 BUILTIN_SIL_OPERATION(ProjectTailElems, "projectTailElems", Special)
 
+/// identityKeyPath : <T> () -> WritableKeyPath<T, T>
+///
+/// Creates an identity key path object. (TODO: replace with proper syntax)
+BUILTIN_SIL_OPERATION(IdentityKeyPath, "identityKeyPath", Special)
+
 #undef BUILTIN_SIL_OPERATION
 
 // BUILTIN_RUNTIME_CALL - A call into a runtime function.

include/swift/AST/DiagnosticsParse.def

@@ -583,8 +583,6 @@ ERROR(sil_keypath_computed_property_missing_part,none,
 ERROR(sil_keypath_external_missing_part,none,
       "keypath external component with indices needs an indices_equals and "
       "indices_hash function", ())
-ERROR(sil_keypath_no_components,none,
-      "keypath must have at least one component", ())
 ERROR(sil_keypath_no_root,none,
       "keypath must have a root component declared",())
 ERROR(sil_keypath_index_not_hashable,none,

lib/AST/Builtins.cpp

@@ -556,6 +556,20 @@ makeTuple(const Gs & ...elementGenerators) {
   };
 }
 
+template <class... Gs>
+static BuiltinGenericSignatureBuilder::LambdaGenerator
+makeBoundGenericType(NominalTypeDecl *decl,
+                     const Gs & ...argumentGenerators) {
+  return {
+    [=](BuiltinGenericSignatureBuilder &builder) -> Type {
+      Type args[] = {
+        argumentGenerators.build(builder)...
+      };
+      return BoundGenericType::get(decl, Type(), args);
+    }
+  };
+}
+
 template <class T>
 static BuiltinGenericSignatureBuilder::MetatypeGenerator<T>
 makeMetatype(const T &object, Optional<MetatypeRepresentation> repr = None) {
@@ -972,6 +986,15 @@ static ValueDecl *getTypeJoinMetaOperation(ASTContext &Context, Identifier Id) {
   return builder.build(Id);
 }
 
+static ValueDecl *getIdentityKeyPathOperation(ASTContext &Context,
+                                              Identifier Id) {
+  BuiltinGenericSignatureBuilder builder(Context, 1);
+  auto arg = makeGenericParam();
+  builder.setResult(makeBoundGenericType(Context.getWritableKeyPathDecl(),
+                                         arg, arg));
+  return builder.build(Id);
+}
+
 static ValueDecl *getCanBeObjCClassOperation(ASTContext &Context,
                                              Identifier Id) {
   // <T> T.Type -> Builtin.Int8
@@ -1865,6 +1888,9 @@ ValueDecl *swift::getBuiltinValueDecl(ASTContext &Context, Identifier Id) {
 
   case BuiltinValueKind::TypeJoinMeta:
     return getTypeJoinMetaOperation(Context, Id);
+
+  case BuiltinValueKind::IdentityKeyPath:
+    return getIdentityKeyPathOperation(Context, Id);
   }
 
   llvm_unreachable("bad builtin value!");

lib/ParseSIL/ParseSIL.cpp

@@ -2884,8 +2884,6 @@ bool SILParser::parseSILInstruction(SILBuilder &B) {
         return true;
     }
 
-    if (components.empty())
-      P.diagnose(InstLoc.getSourceLoc(), diag::sil_keypath_no_components);
     if (rootType.isNull())
       P.diagnose(InstLoc.getSourceLoc(), diag::sil_keypath_no_root);
 
@@ -2943,8 +2941,13 @@ bool SILParser::parseSILInstruction(SILBuilder &B) {
     if (patternEnv && patternEnv->getGenericSignature()) {
       canSig = patternEnv->getGenericSignature()->getCanonicalSignature();
     }
+    CanType leafType;
+    if (!components.empty())
+      leafType = components.back().getComponentType();
+    else
+      leafType = rootType;
     auto pattern = KeyPathPattern::get(B.getModule(), canSig,
-                     rootType, components.back().getComponentType(),
+                     rootType, leafType,
                      components, objcString);
 
     ResultVal = B.createKeyPath(InstLoc, pattern, subMap, operands, Ty);

lib/SILGen/SILGenBuiltin.cpp

@@ -21,6 +21,7 @@
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/Builtins.h"
 #include "swift/AST/DiagnosticsSIL.h"
+#include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/Module.h"
 #include "swift/AST/ReferenceCounting.h"
 #include "swift/SIL/SILArgument.h"
@@ -1055,6 +1056,58 @@ static ManagedValue emitBuiltinProjectTailElems(SILGenFunction &SGF,
   return ManagedValue::forUnmanaged(result);
 }
 
+static ManagedValue emitBuiltinIdentityKeyPath(SILGenFunction &SGF,
+                                               SILLocation loc,
+                                               SubstitutionMap subs,
+                                               ArrayRef<ManagedValue> args,
+                                               SGFContext C) {
+  assert(subs.getReplacementTypes().size() == 1 &&
+         "identityKeyPath should have one substitution");
+  assert(args.size() == 0 &&
+         "identityKeyPath should have no args");
+
+  auto identityTy = subs.getReplacementTypes()[0]->getCanonicalType();
+
+  // The `self` key can be used for identity in Cocoa KVC as well.
+  StringRef objcString = SGF.getASTContext().LangOpts.EnableObjCInterop
+    ? "self" : "";
+
+  // The key path pattern has to capture some generic context if the type is
+  // dependent on this generic context. We only need the specific type, though,
+  // not the entire generic environment.
+  bool isDependent = identityTy->hasArchetype();
+  CanType identityPatternTy = identityTy;
+  CanGenericSignature patternSig = nullptr;
+  SubstitutionMap patternSubs;
+  if (isDependent) {
+    auto param = GenericTypeParamType::get(0, 0, SGF.getASTContext());
+    identityPatternTy = param->getCanonicalType();
+    patternSig = GenericSignature::get(param, {})->getCanonicalSignature();
+    patternSubs = SubstitutionMap::get(patternSig,
+                                       llvm::makeArrayRef((Type)identityTy),
+                                       {});
+  }
+
+  auto identityPattern = KeyPathPattern::get(SGF.SGM.M,
+                                             patternSig,
+                                             identityPatternTy,
+                                             identityPatternTy,
+                                             {},
+                                             objcString);
+
+  auto kpTy = BoundGenericType::get(SGF.getASTContext().getWritableKeyPathDecl(),
+                                    Type(),
+                                    {identityTy, identityTy})
+    ->getCanonicalType();
+
+  auto keyPath = SGF.B.createKeyPath(loc, identityPattern,
+                                     patternSubs,
+                                     {},
+                                     SILType::getPrimitiveObjectType(kpTy));
+  return SGF.emitManagedRValueWithCleanup(keyPath);
+}
+
+
 /// Specialized emitter for type traits.
 template<TypeTraitResult (TypeBase::*Trait)(),
          BuiltinValueKind Kind>

stdlib/public/core/KeyPath.swift

@@ -57,6 +57,7 @@ public class AnyKeyPath: Hashable, _AppendKeyPath {
     ObjectIdentifier(type(of: self)).hash(into: &hasher)
     return withBuffer {
       var buffer = $0
+      if buffer.data.isEmpty { return }
       while true {
         let (component, type) = buffer.next()
         hasher.combine(component.value)
@@ -88,6 +89,11 @@ public class AnyKeyPath: Hashable, _AppendKeyPath {
           return false
         }
 
+        // Identity is equal to identity
+        if aBuffer.data.isEmpty {
+          return bBuffer.data.isEmpty
+        }
+
         while true {
           let (aComponent, aType) = aBuffer.next()
           let (bComponent, bType) = bBuffer.next()
@@ -153,6 +159,11 @@ public class AnyKeyPath: Hashable, _AppendKeyPath {
   internal var _storedInlineOffset: Int? {
     return withBuffer {
       var buffer = $0
+
+      // The identity key path is effectively a stored keypath of type Self
+      // at offset zero
+      if buffer.data.isEmpty { return 0 }
+
       var offset = 0
       while true {
         let (rawComponent, optNextType) = buffer.next()
@@ -220,6 +231,9 @@ public class KeyPath<Root, Value>: PartialKeyPath<Root> {
     var curBase: Any = root
     return withBuffer {
       var buffer = $0
+      if buffer.data.isEmpty {
+        return unsafeBitCast(root, to: Value.self)
+      }
       while true {
         let (rawComponent, optNextType) = buffer.next()
         let valueType = optNextType ?? Value.self
@@ -279,6 +293,13 @@ public class WritableKeyPath<Root, Value>: KeyPath<Root, Value> {
       _sanityCheck(!buffer.hasReferencePrefix,
                    "WritableKeyPath should not have a reference prefix")
 
+      if buffer.data.isEmpty {
+        return (
+          UnsafeMutablePointer<Value>(
+            mutating: p.assumingMemoryBound(to: Value.self)),
+          nil)
+      }
+
       while true {
         let (rawComponent, optNextType) = buffer.next()
         let nextType = optNextType ?? Value.self
@@ -306,7 +327,16 @@ public class WritableKeyPath<Root, Value>: KeyPath<Root, Value> {
               owner: keepAlive)
     }
   }
+}
 
+extension WritableKeyPath where Root == Value {
+  // FIXME: Replace with proper surface syntax
+
+  /// Returns an identity key path that references the entire input value.
+  @inlinable
+  public static var _identity: WritableKeyPath<Root, Root> {
+    return Builtin.identityKeyPath()
+  }
 }
 
 /// A key path that supports reading from and writing to the resulting value
@@ -1941,6 +1971,16 @@ internal func _appendingKeyPaths<
     var rootBuffer = $0
     return leaf.withBuffer {
       var leafBuffer = $0
+
+      // If either operand is the identity key path, then we should return
+      // the other operand back untouched.
+      if leafBuffer.data.isEmpty {
+        return unsafeDowncast(root, to: Result.self)
+      }
+      if rootBuffer.data.isEmpty {
+        return unsafeDowncast(leaf, to: Result.self)
+      }
+
       // Reserve room for the appended KVC string, if both key paths are
       // KVC-compatible.
       let appendedKVCLength: Int, rootKVCLength: Int, leafKVCLength: Int
@@ -2275,7 +2315,8 @@ internal func _getKeyPathClassAndInstanceSizeFromPattern(
   var buffer = KeyPathBuffer(base: bufferPtr)
   var size = buffer.data.count + MemoryLayout<Int>.size
 
-  while true {
+  if !buffer.data.isEmpty {
+   while true {
     let header = buffer.pop(RawKeyPathComponent.Header.self)
 
     // Ensure that we pop an amount of data consistent with what
@@ -2552,6 +2593,7 @@ internal func _getKeyPathClassAndInstanceSizeFromPattern(
     // Pop the type accessor reference.
     _ = buffer.popRaw(size: MemoryLayout<Int>.size,
                       alignment: Int.self)
+   }
   }
 
   _sanityCheck(buffer.data.isEmpty, "didn't read entire pattern")
@@ -2628,7 +2670,8 @@ internal func _instantiateKeyPathBuffer(
   var base: Any.Type = rootType
   // Some pattern forms are pessimistically larger than what we need in the
   // instantiated key path. Keep track of this.
-  while true {
+  if !patternBuffer.data.isEmpty {
+   while true {
     let componentAddr = destData.baseAddress.unsafelyUnwrapped
     let header = patternBuffer.pop(RawKeyPathComponent.Header.self)
 
@@ -2996,6 +3039,7 @@ internal func _instantiateKeyPathBuffer(
     base = unsafeBitCast(componentTyAccessor(arguments), to: Any.Type.self)
     pushDest(base)
     previousComponentAddr = componentAddr
+   }
   }
 
   // We should have traversed both buffers.

test/IRGen/keypaths.sil

@@ -448,6 +448,14 @@ entry(%0 : @trivial $*A, %1 : @trivial $*B, %2 : @trivial $*A, %3 : @trivial $*B
   return undef : $()
 }
 
+sil @identity : $@convention(thin) <T> () -> () {
+entry:
+  %v = keypath $WritableKeyPath<T, T>, <A> (root $A; objc "self") <T>
+  %w = keypath $WritableKeyPath<Int, Int>, (root $Int; objc "self")
+
+  return undef : $()
+}
+
 sil @s_get : $@convention(thin) <A: Hashable, B: Hashable> (@in_guaranteed A, UnsafeRawPointer) -> @out B
 sil @s_set : $@convention(thin) <A: Hashable, B: Hashable> (@in_guaranteed B, @in_guaranteed A, UnsafeRawPointer) -> ()
 sil @s_equals : $@convention(thin) <A: Hashable, B: Hashable> (UnsafeRawPointer, UnsafeRawPointer) -> Bool

test/SILGen/keypaths.swift

@@ -1,4 +1,6 @@
-// RUN: %target-swift-emit-silgen -module-name keypaths %s | %FileCheck %s
+// RUN: %target-swift-emit-silgen -parse-stdlib -module-name keypaths %s | %FileCheck %s
+
+import Swift
 
 struct S<T> {
   var x: T
@@ -383,3 +385,14 @@ func subclass_generics<T: C<Int>, U: C<V>, V/*: PoC*/>(_: T, _: U, _: V) {
   _ = \PoC.extension
  */
 }
+
+// CHECK-LABEL: sil hidden @{{.*}}identity
+func identity<T>(_: T) {
+  // CHECK: keypath $WritableKeyPath<T, T>, <τ_0_0> ({{.*}}root $τ_0_0) <T>
+  let _: WritableKeyPath<T, T> = Builtin.identityKeyPath()
+  // CHECK: keypath $WritableKeyPath<Array<T>, Array<T>>, <τ_0_0> ({{.*}}root $τ_0_0) <Array<T>>
+  let _: WritableKeyPath<[T], [T]> = Builtin.identityKeyPath()
+  // CHECK: keypath $WritableKeyPath<String, String>, ({{.*}}root $String)
+  let _: WritableKeyPath<String, String> = Builtin.identityKeyPath()
+}
+

test/stdlib/KeyPath.swift

@@ -684,6 +684,10 @@ struct NonOffsetableProperties {
   var z: Int { return 0 }
 }
 
+func getIdentityKeyPathOfType<T>(_: T.Type) -> KeyPath<T, T> {
+  return WritableKeyPath<T, T>._identity
+}
+
 keyPath.test("offsets") {
   let SLayout = MemoryLayout<S<Int>>.self
   expectNotNil(SLayout.offset(of: \S<Int>.x))
@@ -699,6 +703,43 @@ keyPath.test("offsets") {
   expectNil(NOPLayout.offset(of: \NonOffsetableProperties.x))
   expectNil(NOPLayout.offset(of: \NonOffsetableProperties.y))
   expectNil(NOPLayout.offset(of: \NonOffsetableProperties.z))
+
+  expectEqual(SLayout.offset(of: WritableKeyPath<S<Int>, S<Int>>._identity),
+              0)
+  expectEqual(SLayout.offset(of: getIdentityKeyPathOfType(S<Int>.self)), 0)
+}
+
+keyPath.test("identity key path") {
+  var x = LifetimeTracked(1738)
+
+  let id = WritableKeyPath<LifetimeTracked, LifetimeTracked>._identity
+  expectTrue(x === x[keyPath: id])
+
+  let newX = LifetimeTracked(679)
+  x[keyPath: id] = newX
+  expectTrue(x === newX)
+
+  let id2 = getIdentityKeyPathOfType(LifetimeTracked.self)
+  expectEqual(id, id2)
+  expectEqual(id.hashValue, id2.hashValue)
+  expectNotNil(id2 as? WritableKeyPath)
+
+  let id3 = id.appending(path: id2)
+  expectEqual(id, id3)
+  expectEqual(id.hashValue, id3.hashValue)
+  expectNotNil(id3 as? WritableKeyPath)
+
+  let valueKey = \LifetimeTracked.value
+  let valueKey2 = id.appending(path: valueKey)
+  let valueKey3 = (valueKey as KeyPath).appending(path: WritableKeyPath<Int, Int>._identity)
+
+  expectEqual(valueKey, valueKey2)
+  expectEqual(valueKey.hashValue, valueKey2.hashValue)
+  expectEqual(valueKey, valueKey3)
+  expectEqual(valueKey.hashValue, valueKey3.hashValue)
+
+  expectEqual(x[keyPath: valueKey2], 679)
+  expectEqual(x[keyPath: valueKey3], 679)
 }
 
 keyPath.test("let-ness") {