[AutoDiff upstream] Add @derivative(of:) attribute.

The `@derivative(of:)` attribute registers a function as a derivative of another
function. This patch adds the `@derivative(of:)` attribute definition, syntax,
parsing, and printing.

Resolves TF-826.

Todos:
- Type-checking (TF-829).
- Serialization (TF-837).

Author: dan-zheng

include/swift/AST/Attr.def

@@ -529,6 +529,11 @@ DECL_ATTR(_originallyDefinedIn, OriginallyDefinedIn,
   ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
   96)
 
+DECL_ATTR(derivative, Derivative,
+  OnFunc | LongAttribute | AllowMultipleAttributes |
+  ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
+  97)
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef CONTEXTUAL_DECL_ATTR_ALIAS

include/swift/AST/Attr.h

@@ -1747,6 +1747,84 @@ class DifferentiableAttr final
   }
 };
 
+/// Attribute that registers a function as a derivative of another function.
+///
+/// Examples:
+///   @derivative(of: sin(_:))
+///   @derivative(of: +, wrt: (lhs, rhs))
+class DerivativeAttr final
+    : public DeclAttribute,
+      private llvm::TrailingObjects<DerivativeAttr, ParsedAutoDiffParameter> {
+  friend TrailingObjects;
+
+  /// The original function name.
+  DeclNameWithLoc OriginalFunctionName;
+  /// The original function declaration, resolved by the type checker.
+  AbstractFunctionDecl *OriginalFunction = nullptr;
+  /// The number of parsed parameters specified in 'wrt:'.
+  unsigned NumParsedParameters = 0;
+  /// The differentiation parameters' indices, resolved by the type checker.
+  IndexSubset *ParameterIndices = nullptr;
+  /// The derivative function kind (JVP or VJP), resolved by the type checker.
+  Optional<AutoDiffDerivativeFunctionKind> Kind = None;
+
+  explicit DerivativeAttr(bool implicit, SourceLoc atLoc, SourceRange baseRange,
+                          DeclNameWithLoc original,
+                          ArrayRef<ParsedAutoDiffParameter> params);
+
+  explicit DerivativeAttr(bool implicit, SourceLoc atLoc, SourceRange baseRange,
+                          DeclNameWithLoc original, IndexSubset *indices);
+
+public:
+  static DerivativeAttr *create(ASTContext &context, bool implicit,
+                                SourceLoc atLoc, SourceRange baseRange,
+                                DeclNameWithLoc original,
+                                ArrayRef<ParsedAutoDiffParameter> params);
+
+  static DerivativeAttr *create(ASTContext &context, bool implicit,
+                                SourceLoc atLoc, SourceRange baseRange,
+                                DeclNameWithLoc original, IndexSubset *indices);
+
+  DeclNameWithLoc getOriginalFunctionName() const {
+    return OriginalFunctionName;
+  }
+  AbstractFunctionDecl *getOriginalFunction() const {
+    return OriginalFunction;
+  }
+  void setOriginalFunction(AbstractFunctionDecl *decl) {
+    OriginalFunction = decl;
+  }
+
+  AutoDiffDerivativeFunctionKind getDerivativeKind() const {
+    assert(Kind && "Derivative function kind has not yet been resolved");
+    return *Kind;
+  }
+  void setDerivativeKind(AutoDiffDerivativeFunctionKind kind) { Kind = kind; }
+
+  /// The parsed differentiation parameters, i.e. the list of parameters
+  /// specified in 'wrt:'.
+  ArrayRef<ParsedAutoDiffParameter> getParsedParameters() const {
+    return {getTrailingObjects<ParsedAutoDiffParameter>(), NumParsedParameters};
+  }
+  MutableArrayRef<ParsedAutoDiffParameter> getParsedParameters() {
+    return {getTrailingObjects<ParsedAutoDiffParameter>(), NumParsedParameters};
+  }
+  size_t numTrailingObjects(OverloadToken<ParsedAutoDiffParameter>) const {
+    return NumParsedParameters;
+  }
+
+  IndexSubset *getParameterIndices() const {
+    return ParameterIndices;
+  }
+  void setParameterIndices(IndexSubset *parameterIndices) {
+    ParameterIndices = parameterIndices;
+  }
+
+  static bool classof(const DeclAttribute *DA) {
+    return DA->getKind() == DAK_Derivative;
+  }
+};
+
 /// Attributes that may be applied to declarations.
 class DeclAttributes {
   /// Linked list of declaration attributes.

include/swift/AST/AutoDiff.h

@@ -26,6 +26,47 @@
 
 namespace swift {
 
+/// A function type differentiability kind.
+enum class DifferentiabilityKind : uint8_t {
+  NonDifferentiable = 0,
+  Normal = 1,
+  Linear = 2
+};
+
+/// The kind of an linear map.
+struct AutoDiffLinearMapKind {
+  enum innerty : uint8_t {
+    // The differential function.
+    Differential = 0,
+    // The pullback function.
+    Pullback = 1
+  } rawValue;
+
+  AutoDiffLinearMapKind() = default;
+  AutoDiffLinearMapKind(innerty rawValue) : rawValue(rawValue) {}
+  operator innerty() const { return rawValue; }
+};
+
+/// The kind of a derivative function.
+struct AutoDiffDerivativeFunctionKind {
+  enum innerty : uint8_t {
+    // The Jacobian-vector products function.
+    JVP = 0,
+    // The vector-Jacobian products function.
+    VJP = 1
+  } rawValue;
+
+  AutoDiffDerivativeFunctionKind() = default;
+  AutoDiffDerivativeFunctionKind(innerty rawValue) : rawValue(rawValue) {}
+  AutoDiffDerivativeFunctionKind(AutoDiffLinearMapKind linMapKind)
+      : rawValue(static_cast<innerty>(linMapKind.rawValue)) {}
+  explicit AutoDiffDerivativeFunctionKind(StringRef string);
+  operator innerty() const { return rawValue; }
+  AutoDiffLinearMapKind getLinearMapKind() {
+    return (AutoDiffLinearMapKind::innerty)rawValue;
+  }
+};
+
 class ParsedAutoDiffParameter {
 public:
   enum class Kind { Named, Ordered, Self };
@@ -89,12 +130,6 @@ class ParsedAutoDiffParameter {
   }
 };
 
-enum class DifferentiabilityKind : uint8_t {
-  NonDifferentiable = 0,
-  Normal = 1,
-  Linear = 2
-};
-
 } // end namespace swift
 
 #endif // SWIFT_AST_AUTODIFF_H

include/swift/AST/DiagnosticsParse.def

@@ -1329,6 +1329,11 @@ ERROR(attr_expected_comma,none,
 ERROR(attr_expected_string_literal,none,
 "expected string literal in '%0' attribute", (StringRef))
 
+ERROR(attr_missing_label,PointsToFirstBadToken,
+      "missing label '%0:' in '@%1' attribute", (StringRef, StringRef))
+ERROR(attr_expected_label,none,
+      "expected label '%0:' in '@%1' attribute", (StringRef, StringRef))
+
 ERROR(alignment_must_be_positive_integer,none,
       "alignment value must be a positive integer literal", ())
 
@@ -1550,6 +1555,10 @@ ERROR(diff_params_clause_expected_parameter,PointsToFirstBadToken,
       "expected a parameter, which can be a function parameter name, "
       "parameter index, or 'self'", ())
 
+// derivative
+ERROR(attr_derivative_expected_original_name,PointsToFirstBadToken,
+      "expected an original function name", ())
+
 //------------------------------------------------------------------------------
 // MARK: Generics parsing diagnostics
 //------------------------------------------------------------------------------

include/swift/Parse/Parser.h

@@ -997,6 +997,10 @@ class Parser {
   bool parseDifferentiationParametersClause(
       SmallVectorImpl<ParsedAutoDiffParameter> &params, StringRef attrName);
 
+  /// Parse the @derivative attribute.
+  ParserResult<DerivativeAttr> parseDerivativeAttribute(SourceLoc AtLoc,
+                                                        SourceLoc Loc);
+
   /// Parse a specific attribute.
   ParserStatus parseDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc);
 

lib/AST/Attr.cpp

@@ -1027,10 +1027,12 @@ StringRef DeclAttribute::getAttrName() const {
     return "<<custom>>";
   case DAK_ProjectedValueProperty:
     return "_projectedValueProperty";
-  case DAK_Differentiable:
-    return "differentiable";
   case DAK_OriginallyDefinedIn:
     return "_originallyDefinedIn";
+  case DAK_Differentiable:
+    return "differentiable";
+  case DAK_Derivative:
+    return "derivative";
   }
   llvm_unreachable("bad DeclAttrKind");
 }
@@ -1450,6 +1452,44 @@ void DifferentiableAttr::print(llvm::raw_ostream &OS, const Decl *D,
                                    omitAssociatedFunctions);
 }
 
+DerivativeAttr::DerivativeAttr(bool implicit, SourceLoc atLoc,
+                               SourceRange baseRange,
+                               DeclNameWithLoc originalName,
+                               ArrayRef<ParsedAutoDiffParameter> params)
+    : DeclAttribute(DAK_Derivative, atLoc, baseRange, implicit),
+      OriginalFunctionName(std::move(originalName)),
+      NumParsedParameters(params.size()) {
+  std::copy(params.begin(), params.end(),
+            getTrailingObjects<ParsedAutoDiffParameter>());
+}
+
+DerivativeAttr::DerivativeAttr(bool implicit, SourceLoc atLoc,
+                               SourceRange baseRange,
+                               DeclNameWithLoc originalName,
+                               IndexSubset *indices)
+    : DeclAttribute(DAK_Derivative, atLoc, baseRange, implicit),
+      OriginalFunctionName(std::move(originalName)), ParameterIndices(indices) {
+}
+
+DerivativeAttr *
+DerivativeAttr::create(ASTContext &context, bool implicit, SourceLoc atLoc,
+                       SourceRange baseRange, DeclNameWithLoc originalName,
+                       ArrayRef<ParsedAutoDiffParameter> params) {
+  unsigned size = totalSizeToAlloc<ParsedAutoDiffParameter>(params.size());
+  void *mem = context.Allocate(size, alignof(DerivativeAttr));
+  return new (mem) DerivativeAttr(implicit, atLoc, baseRange,
+                                  std::move(originalName), params);
+}
+
+DerivativeAttr *DerivativeAttr::create(ASTContext &context, bool implicit,
+                                       SourceLoc atLoc, SourceRange baseRange,
+                                       DeclNameWithLoc originalName,
+                                       IndexSubset *indices) {
+  void *mem = context.Allocate(sizeof(DerivativeAttr), alignof(DerivativeAttr));
+  return new (mem) DerivativeAttr(implicit, atLoc, baseRange,
+                                  std::move(originalName), indices);
+}
+
 ImplementsAttr::ImplementsAttr(SourceLoc atLoc, SourceRange range,
                                TypeLoc ProtocolType,
                                DeclName MemberName,