[AutoDiff upstream] Add @differentiable function IRGen.

Lower `@differentiable` and `@differentiable(linear)` functions as structs of
function pointers.

Author: dan-zheng

include/swift/AST/AutoDiff.h

@@ -75,6 +75,41 @@ struct AutoDiffDerivativeFunctionKind {
   }
 };
 
+/// A component of a SIL `@differentiable` function-typed value.
+struct NormalDifferentiableFunctionTypeComponent {
+  enum innerty : unsigned { Original = 0, JVP = 1, VJP = 2 } rawValue;
+
+  NormalDifferentiableFunctionTypeComponent() = default;
+  NormalDifferentiableFunctionTypeComponent(innerty rawValue)
+      : rawValue(rawValue) {}
+  NormalDifferentiableFunctionTypeComponent(
+      AutoDiffDerivativeFunctionKind kind);
+  explicit NormalDifferentiableFunctionTypeComponent(unsigned rawValue)
+      : NormalDifferentiableFunctionTypeComponent((innerty)rawValue) {}
+  explicit NormalDifferentiableFunctionTypeComponent(StringRef name);
+  operator innerty() const { return rawValue; }
+
+  /// Returns the derivative function kind, if the component is a derivative
+  /// function.
+  Optional<AutoDiffDerivativeFunctionKind> getAsDerivativeFunctionKind() const;
+};
+
+/// A component of a SIL `@differentiable(linear)` function-typed value.
+struct LinearDifferentiableFunctionTypeComponent {
+  enum innerty : unsigned {
+    Original = 0,
+    Transpose = 1,
+  } rawValue;
+
+  LinearDifferentiableFunctionTypeComponent() = default;
+  LinearDifferentiableFunctionTypeComponent(innerty rawValue)
+      : rawValue(rawValue) {}
+  explicit LinearDifferentiableFunctionTypeComponent(unsigned rawValue)
+      : LinearDifferentiableFunctionTypeComponent((innerty)rawValue) {}
+  explicit LinearDifferentiableFunctionTypeComponent(StringRef name);
+  operator innerty() const { return rawValue; }
+};
+
 /// A derivative function configuration, uniqued in `ASTContext`.
 /// Identifies a specific derivative function given an original function.
 class AutoDiffDerivativeFunctionIdentifier : public llvm::FoldingSetNode {

lib/AST/AutoDiff.cpp

@@ -28,6 +28,50 @@ AutoDiffDerivativeFunctionKind::AutoDiffDerivativeFunctionKind(
   rawValue = *result;
 }
 
+NormalDifferentiableFunctionTypeComponent::
+    NormalDifferentiableFunctionTypeComponent(
+        AutoDiffDerivativeFunctionKind kind) {
+  switch (kind) {
+  case AutoDiffDerivativeFunctionKind::JVP:
+    rawValue = JVP;
+    return;
+  case AutoDiffDerivativeFunctionKind::VJP:
+    rawValue = VJP;
+    return;
+  }
+}
+
+NormalDifferentiableFunctionTypeComponent::
+    NormalDifferentiableFunctionTypeComponent(StringRef string) {
+  Optional<innerty> result = llvm::StringSwitch<Optional<innerty>>(string)
+                                 .Case("original", Original)
+                                 .Case("jvp", JVP)
+                                 .Case("vjp", VJP);
+  assert(result && "Invalid string");
+  rawValue = *result;
+}
+
+Optional<AutoDiffDerivativeFunctionKind>
+NormalDifferentiableFunctionTypeComponent::getAsDerivativeFunctionKind() const {
+  switch (rawValue) {
+  case Original:
+    return None;
+  case JVP:
+    return {AutoDiffDerivativeFunctionKind::JVP};
+  case VJP:
+    return {AutoDiffDerivativeFunctionKind::VJP};
+  }
+}
+
+LinearDifferentiableFunctionTypeComponent::
+    LinearDifferentiableFunctionTypeComponent(StringRef string) {
+  Optional<innerty> result = llvm::StringSwitch<Optional<innerty>>(string)
+                                 .Case("original", Original)
+                                 .Case("transpose", Transpose);
+  assert(result && "Invalid string");
+  rawValue = *result;
+}
+
 DifferentiabilityWitnessFunctionKind::DifferentiabilityWitnessFunctionKind(
     StringRef string) {
   Optional<innerty> result = llvm::StringSwitch<Optional<innerty>>(string)

lib/IRGen/CMakeLists.txt

@@ -16,6 +16,7 @@ add_swift_host_library(swiftIRGen STATIC
   GenControl.cpp
   GenCoverage.cpp
   GenDecl.cpp
+  GenDiffFunc.cpp
   GenDiffWitness.cpp
   GenEnum.cpp
   GenExistential.cpp