[Serialization] Implement serialization for @differentiable attribute. (#17155)

Implement (de)serialization for all components of `@differentiable` attribute
except the trailing where clause (which needs to be type-checked).

This is a necessary step for the `#adjoint` expression to look up
`@differentiable` attributes declared on functions in other modules correctly.

Addresses SR-7977.

Author: dan-zheng

include/swift/AST/Attr.def

@@ -381,10 +381,8 @@ SIMPLE_DECL_ATTR(_forbidSerializingReference, ForbidSerializingReference,
   77)
 
 // SWIFT_ENABLE_TENSORFLOW
-// FIXME: Make it serialized
 DECL_ATTR(differentiable, Differentiable,
-          OnFunc | LongAttribute | NotSerialized,
-          /* Not serialized */ 78)
+          OnFunc | LongAttribute, 78)
 
 SIMPLE_DECL_ATTR(compilerEvaluable, CompilerEvaluable,
                  OnFunc | OnConstructor, /* Not serialized */ 79)

include/swift/Serialization/ModuleFormat.h

@@ -1447,8 +1447,6 @@ namespace decls_block {
     = BCRecordLayout<RestatedObjCConformance_DECL_ATTR>;
   using ClangImporterSynthesizedTypeDeclAttrLayout
     = BCRecordLayout<ClangImporterSynthesizedType_DECL_ATTR>;
-  // SWIFT_ENABLE_TENSORFLOW
-  using DifferentiableDeclAttrLayout = BCRecordLayout<Differentiable_DECL_ATTR>;
 
   using InlineDeclAttrLayout = BCRecordLayout<
     Inline_DECL_ATTR,
@@ -1505,6 +1503,17 @@ namespace decls_block {
     BCFixed<1> // specialization kind
   >;
 
+  // SWIFT_ENABLE_TENSORFLOW
+  using DifferentiableDeclAttrLayout = BCRecordLayout<
+    Differentiable_DECL_ATTR,
+    BCFixed<1>, // Differentiation mode ('forward' or 'reverse').
+    IdentifierIDField, // Primal name.
+    DeclIDField, // Primal function declaration.
+    IdentifierIDField, // Adjoint name.
+    DeclIDField, // Adjoint function declaration.
+    BCArray<BCFixed<32>> // Differentiation parameters.
+  >;
+
 #define SIMPLE_DECL_ATTR(X, CLASS, ...) \
   using CLASS##DeclAttrLayout = BCRecordLayout< \
     CLASS##_DECL_ATTR, \

lib/Serialization/Deserialization.cpp

@@ -2506,6 +2506,52 @@ ModuleFile::getDeclCheckedImpl(DeclID DID, Optional<DeclContext *> ForcedContext
         break;
       }
 
+      // SWIFT_ENABLE_TENSORFLOW
+      case decls_block::Differentiable_DECL_ATTR: {
+        AutoDiffMode autodiffMode = AutoDiffMode::Reverse;
+        unsigned autodiffModeValue;
+        uint64_t primalNameId;
+        DeclID primalDeclId;
+        uint64_t adjointNameId;
+        DeclID adjointDeclId;
+        ArrayRef<uint64_t> paramValues;
+
+        serialization::decls_block::DifferentiableDeclAttrLayout::readRecord(
+          scratch, autodiffModeValue, primalNameId, primalDeclId, adjointNameId,
+          adjointDeclId, paramValues);
+        autodiffMode = autodiffModeValue
+          ? AutoDiffMode::Reverse
+          : AutoDiffMode::Forward;
+
+        using FuncSpecifier = DifferentiableAttr::FunctionSpecifier;
+        Optional<FuncSpecifier> primal;
+        FuncDecl *primalDecl = nullptr;
+        if (primalNameId != 0 && primalDeclId != 0) {
+          primal = { getIdentifier(primalNameId), DeclNameLoc() };
+          primalDecl = cast<FuncDecl>(getDecl(primalDeclId));
+        }
+        FuncSpecifier adjoint = { getIdentifier(adjointNameId), DeclNameLoc() };
+        FuncDecl *adjointDecl = cast<FuncDecl>(getDecl(adjointDeclId));
+
+        SmallVector<AutoDiffParameter, 4> parameters;
+        SourceLoc loc;
+        for (auto paramValue : paramValues) {
+          auto parameter = paramValue & 0x01
+            ? AutoDiffParameter::getSelfParameter(loc)
+            : AutoDiffParameter::getIndexParameter(loc, paramValue >> 1);
+          parameters.push_back(parameter);
+        }
+        // TODO: Deserialize trailing where clause.
+        auto diffAttr =
+          DifferentiableAttr::create(ctx, loc, SourceRange(), autodiffMode,
+                                     loc, parameters, primal, adjoint,
+                                     /*TrailingWhereClause*/ nullptr);
+        diffAttr->setPrimalFunction(primalDecl);
+        diffAttr->setAdjointFunction(adjointDecl);
+        Attr = diffAttr;
+        break;
+      }
+
 #define SIMPLE_DECL_ATTR(NAME, CLASS, ...) \
       case decls_block::CLASS##_DECL_ATTR: { \
         bool isImplicit; \

lib/Serialization/Serialization.cpp

@@ -2174,8 +2174,6 @@ void Serializer::writeDeclAttribute(const DeclAttribute *DA) {
   case DAK_ObjCRuntimeName:
   case DAK_RestatedObjCConformance:
   case DAK_ClangImporterSynthesizedType:
-  // SWIFT_ENABLE_TENSORFLOW
-  case DAK_Differentiable:
     llvm_unreachable("cannot serialize attribute");
 
   case DAK_Count:
@@ -2333,6 +2331,43 @@ void Serializer::writeDeclAttribute(const DeclAttribute *DA) {
     writeGenericRequirements(SA->getRequirements(), DeclTypeAbbrCodes);
     return;
   }
+
+  // SWIFT_ENABLE_TENSORFLOW
+  case DAK_Differentiable: {
+    auto abbrCode = DeclTypeAbbrCodes[DifferentiableDeclAttrLayout::Code];
+    auto attr = cast<DifferentiableAttr>(DA);
+
+    IdentifierID primalName = 0;
+    DeclID primalRef = 0;
+    if (auto primal = attr->getPrimal()) {
+      primalName = addDeclBaseNameRef(primal->Name.getBaseName());
+      primalRef = addDeclRef(attr->getPrimalFunction());
+    }
+    auto adjointName = addDeclBaseNameRef(attr->getAdjoint().Name.getBaseName());
+    auto adjointRef = addDeclRef(attr->getAdjointFunction());
+
+    SmallVector<uint32_t, 4> parameters;
+    for (auto param : attr->getParameters()) {
+      switch (param.getKind()) {
+      // The self parameter is uniquely identified by 0x01.
+      case AutoDiffParameter::Kind::Self:
+        parameters.push_back(1);
+        break;
+      // Index parameters are left-shifted by 1.
+      case AutoDiffParameter::Kind::Index:
+        parameters.push_back(param.getIndex() << 1);
+        break;
+      }
+    }
+
+    DifferentiableDeclAttrLayout::emitRecord(
+      Out, ScratchRecord, abbrCode, (unsigned) attr->getMode(), primalName,
+      primalRef, adjointName, adjointRef, parameters);
+    // TODO: Serialize trailing where clause.
+    // Type-checking where clause should be done first (mimicking the
+    // @_specialize attribute).
+    return;
+  }
   }
 }
 

test/Serialization/differentiable_attr.swift

@@ -0,0 +1,68 @@
+// SWIFT_ENABLE_TENSORFLOW
+// TODO: Handle trailing where clause in @differentiable attribute.
+
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -emit-module -parse-as-library -o %t
+// RUN: %target-sil-opt -disable-sil-linking -enable-sil-verify-all %t/differentiable_attr.swiftmodule -o - | %FileCheck %s
+
+struct CheckpointsFoo {}
+func pfoo(_ x: Float) -> (checkpoints: CheckpointsFoo, originalValue: Float) {
+  return (CheckpointsFoo(), x * x)
+}
+func dfoo_checkpointed(_ x: Float, checkpoints: CheckpointsFoo, originalValue: Float, seed: Float) -> Float {
+  return 2 * x
+}
+// CHECK-DAG: @differentiable(reverse, primal: pfoo, adjoint: dfoo_checkpointed)
+// CHECK-DAG: func foo_checkpointed(_ x: Float) -> Float
+@differentiable(reverse, primal: pfoo(_:), adjoint: dfoo_checkpointed(_:checkpoints:originalValue:seed:))
+func foo_checkpointed(_ x: Float) -> Float {
+  return x * x
+}
+
+struct S<T> {
+  struct Checkpoints {
+    let s: S
+  }
+  func primal(x: Float) -> (Checkpoints, Float) {
+    return (Checkpoints(s: self), x)
+  }
+  func adjoint_checkpointed(x: Float, _: Checkpoints, _: Float, _: Float) -> S {
+    return self
+  }
+
+  // CHECK-DAG: @differentiable(reverse, (self), primal: primal, adjoint: adjoint_checkpointed)
+  // CHECK-DAG: func original(x: Float) -> Float
+  @differentiable(reverse, withRespectTo: (self), primal: primal, adjoint: adjoint_checkpointed)
+  func original(x: Float) -> Float {
+    return x
+  }
+}
+
+func pbaz1<T>(_ x: T, _ y: T) -> ((T, T), T) {
+  return ((y, y), x)
+}
+func dbaz1_checkpointed<T>(_ x: T, _ y: T, primal: (T, T), originalValue: T, seed: T) -> (T, T) {
+  return (y, x)
+}
+// CHECK-DAG: @differentiable(reverse, primal: pbaz1, adjoint: dbaz1_checkpointed)
+// CHECK-DAG: func baz1_checkpointed<T>(_ x: T, _ y: T) -> T
+@differentiable(reverse, primal: pbaz1(_:_:), adjoint: dbaz1_checkpointed(_:_:primal:originalValue:seed:))
+func baz1_checkpointed<T>(_ x: T, _ y: T) -> T {
+  return x
+}
+
+struct CheckpointsFP<T : FloatingPoint> {
+  let meow: T
+}
+func pbaz2<T : FloatingPoint>(_ x: T, _ y: T) -> (CheckpointsFP<T>, T) {
+  return (CheckpointsFP(meow: 1), x + y)
+}
+func dbaz2_checkpointed<T : FloatingPoint>(_ x: T, _ y: T, primal: CheckpointsFP<T>, originalValue: T, seed: T) -> (T, T) {
+  return (1, 1)
+}
+// CHECK-DAG: @differentiable(reverse, primal: pbaz2, adjoint: dbaz2_checkpointed)
+// CHECK-DAG: func baz2_checkpointed<T>(_ x: T, _ y: T) -> T where T : FloatingPoint
+@differentiable(reverse, primal: pbaz2(_:_:), adjoint: dbaz2_checkpointed(_:_:primal:originalValue:seed:))
+func baz2_checkpointed<T : FloatingPoint>(_ x: T, _ y: T) -> T {
+  return x
+}