[AutoDiff] Improve getConstrainedDerivativeGenericSignature helper. (#29621)

Move `getConstrainedDerivativeGenericSignature` under `autodiff` namespace.
Improve naming and documentation.

Author: dan-zheng

include/swift/AST/AutoDiff.h

@@ -31,7 +31,7 @@ namespace swift {
 class AnyFunctionType;
 class TupleType;
 struct SILAutoDiffIndices;
-
+class SILFunctionType;
 
 /// A function type differentiability kind.
 enum class DifferentiabilityKind : uint8_t {
@@ -240,6 +240,18 @@ void getSubsetParameterTypes(IndexSubset *indices, AnyFunctionType *type,
                              SmallVectorImpl<Type> &results,
                              bool reverseCurryLevels = false);
 
+/// "Constrained" derivative generic signatures require all differentiability
+/// parameters to conform to the `Differentiable` protocol.
+///
+/// Returns the "constrained" derivative generic signature given:
+/// - An original SIL function type.
+/// - Differentiability parameter indices.
+/// - A possibly "unconstrained" derivative generic signature.
+GenericSignature
+getConstrainedDerivativeGenericSignature(SILFunctionType *originalFnTy,
+                                         IndexSubset *diffParamIndices,
+                                         GenericSignature derivativeGenSig);
+
 } // end namespace autodiff
 
 } // end namespace swift
@@ -342,7 +354,6 @@ namespace swift {
 
 class ASTContext;
 class AnyFunctionType;
-class SILFunctionType;
 typedef CanTypeWrapper<SILFunctionType> CanSILFunctionType;
 enum class SILLinkage : uint8_t;
 

lib/AST/AutoDiff.cpp

@@ -10,7 +10,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "swift/AST/ASTContext.h"
 #include "swift/AST/AutoDiff.h"
+#include "swift/AST/TypeCheckRequests.h"
 #include "swift/AST/Types.h"
 
 using namespace swift;
@@ -67,6 +69,31 @@ void autodiff::getSubsetParameterTypes(IndexSubset *subset,
   }
 }
 
+GenericSignature autodiff::getConstrainedDerivativeGenericSignature(
+    SILFunctionType *originalFnTy, IndexSubset *diffParamIndices,
+    GenericSignature derivativeGenSig) {
+  if (!derivativeGenSig)
+    derivativeGenSig = originalFnTy->getSubstGenericSignature();
+  if (!derivativeGenSig)
+    return nullptr;
+  // Constrain all differentiability parameters to `Differentiable`.
+  auto &ctx = originalFnTy->getASTContext();
+  auto *diffableProto = ctx.getProtocol(KnownProtocolKind::Differentiable);
+  SmallVector<Requirement, 4> requirements;
+  for (unsigned paramIdx : diffParamIndices->getIndices()) {
+    auto paramType = originalFnTy->getParameters()[paramIdx].getInterfaceType();
+    Requirement req(RequirementKind::Conformance, paramType,
+                    diffableProto->getDeclaredType());
+    requirements.push_back(req);
+  }
+  return evaluateOrDefault(
+      ctx.evaluator,
+      AbstractGenericSignatureRequest{derivativeGenSig.getPointer(),
+                                      /*addedGenericParams*/ {},
+                                      std::move(requirements)},
+      nullptr);
+}
+
 // SWIFT_ENABLE_TENSORFLOW
 // Not-yet-upstreamed `tensorflow` branch additions are below.
 

lib/SIL/SILFunctionType.cpp

@@ -226,35 +226,6 @@ CanSILFunctionType SILFunctionType::getWithoutDifferentiability() {
                               isGenericSignatureImplied(), getASTContext());
 }
 
-// Returns the canonical generic signature for an autodiff derivative function
-// given an existing derivative function generic signature. All differentiation
-// parameters are constrained to conform to `Differentiable`.
-static CanGenericSignature getAutoDiffDerivativeFunctionGenericSignature(
-    CanGenericSignature derivativeFnGenSig,
-    ArrayRef<SILParameterInfo> originalParameters,
-    IndexSubset *parameterIndices, ModuleDecl *module) {
-  if (!derivativeFnGenSig)
-    return nullptr;
-  auto &ctx = module->getASTContext();
-  GenericSignatureBuilder builder(ctx);
-
-  // Add derivative function generic signature.
-  builder.addGenericSignature(derivativeFnGenSig);
-  // Constrain all wrt parameters to conform to `Differentiable`.
-  auto source =
-      GenericSignatureBuilder::FloatingRequirementSource::forAbstract();
-  auto *diffableProto = ctx.getProtocol(KnownProtocolKind::Differentiable);
-  for (unsigned paramIdx : parameterIndices->getIndices()) {
-    auto paramType = originalParameters[paramIdx].getInterfaceType();
-    Requirement req(RequirementKind::Conformance, paramType,
-                    diffableProto->getDeclaredType());
-    builder.addRequirement(req, source, module);
-  }
-  return std::move(builder)
-      .computeGenericSignature(SourceLoc(), /*allowConcreteGenericParams*/ true)
-      ->getCanonicalSignature();
-}
-
 CanSILFunctionType SILFunctionType::getAutoDiffDerivativeFunctionType(
     IndexSubset *parameterIndices, unsigned resultIndex,
     AutoDiffDerivativeFunctionKind kind, TypeConverter &TC,
@@ -293,8 +264,9 @@ CanSILFunctionType SILFunctionType::getAutoDiffDerivativeFunctionType(
   // Get the canonical derivative function generic signature.
   if (!derivativeFnGenSig)
     derivativeFnGenSig = getSubstGenericSignature();
-  derivativeFnGenSig = getAutoDiffDerivativeFunctionGenericSignature(
-      derivativeFnGenSig, getParameters(), parameterIndices, &TC.M);
+  derivativeFnGenSig = autodiff::getConstrainedDerivativeGenericSignature(
+                           this, parameterIndices, derivativeFnGenSig)
+                           .getCanonicalSignature();
 
   // Given a type, returns its formal SIL parameter info.
   auto getTangentParameterInfoForOriginalResult =
@@ -456,8 +428,9 @@ CanSILFunctionType SILFunctionType::getAutoDiffTransposeFunctionType(
   // Get the canonical transpose function generic signature.
   if (!genSig)
     genSig = getSubstGenericSignature();
-  genSig = getAutoDiffDerivativeFunctionGenericSignature(
-      genSig, getParameters(), parameterIndices, &TC.M);
+  genSig = autodiff::getConstrainedDerivativeGenericSignature(
+               this, parameterIndices, genSig)
+               .getCanonicalSignature();
 
   // Given a type, returns its formal SIL parameter info.
   auto getParameterInfoForOriginalResult =

lib/SILOptimizer/Mandatory/Differentiation.cpp

@@ -84,39 +84,6 @@ template <typename T> static inline void debugDump(T &v) {
                           << v << "\n==== END DEBUG DUMP ====\n");
 }
 
-/// Returns the "constrained" derivative generic signature given:
-/// - An original SIL function type.
-/// - A wrt parameter index subset.
-/// - A possibly uncanonical derivative generic signature (optional).
-/// - Additional derivative requirements (optional).
-/// The constrained derivative generic signature constrains all wrt parameters
-/// to conform to `Differentiable`.
-static GenericSignature
-getConstrainedDerivativeGenericSignature(CanSILFunctionType originalFnTy,
-                                         IndexSubset *paramIndexSet,
-                                         GenericSignature derivativeGenSig) {
-  if (!derivativeGenSig)
-    derivativeGenSig = originalFnTy->getSubstGenericSignature();
-  if (!derivativeGenSig)
-    return nullptr;
-  // Constrain all wrt parameters to `Differentiable`.
-  auto &ctx = derivativeGenSig->getASTContext();
-  auto *diffableProto = ctx.getProtocol(KnownProtocolKind::Differentiable);
-  SmallVector<Requirement, 4> requirements;
-  for (unsigned paramIdx : paramIndexSet->getIndices()) {
-    auto paramType = originalFnTy->getParameters()[paramIdx].getInterfaceType();
-    Requirement req(RequirementKind::Conformance, paramType,
-                    diffableProto->getDeclaredType());
-    requirements.push_back(req);
-  }
-  return evaluateOrDefault(
-      ctx.evaluator,
-      AbstractGenericSignatureRequest{derivativeGenSig.getPointer(),
-                                      /*addedGenericParams*/ {},
-                                      std::move(requirements)},
-      nullptr);
-}
-
 namespace {
 
 class DifferentiationTransformer {
@@ -597,7 +564,7 @@ emitDerivativeFunctionReference(
             invoker.getIndirectDifferentiation()
                 .second->getDerivativeGenericSignature();
       auto derivativeConstrainedGenSig =
-          getConstrainedDerivativeGenericSignature(
+          autodiff::getConstrainedDerivativeGenericSignature(
               originalFn->getLoweredFunctionType(), desiredParameterIndices,
               contextualDerivativeGenSig);
       minimalWitness = SILDifferentiabilityWitness::createDefinition(