[Distributed] Re-implement ad-hoc requirements into dynamic witness table lookup for SerializationRequirement conformance

include/swift/AST/Decl.h

@@ -7498,6 +7498,17 @@ class AbstractFunctionDecl : public GenericContext, public ValueDecl {
   /// 'DistributedTargetInvocationResultHandler' protocol.
   bool isDistributedTargetInvocationResultHandlerOnReturn() const;
 
+  /// Determines whether this declaration is a witness to a
+  /// protocol requirement with ad-hoc `SerializationRequirement`
+  /// conformance.
+  bool isDistributedWitnessWithAdHocSerializationRequirement() const {
+    return isDistributedActorSystemRemoteCall(/*isVoidResult=*/false) ||
+           isDistributedTargetInvocationEncoderRecordArgument() ||
+           isDistributedTargetInvocationEncoderRecordReturnType() ||
+           isDistributedTargetInvocationDecoderDecodeNextArgument() ||
+           isDistributedTargetInvocationResultHandlerOnReturn();
+  }
+
   /// For a method of a class, checks whether it will require a new entry in the
   /// vtable.
   bool needsNewVTableEntry() const;

include/swift/SIL/SILFunction.h

@@ -264,17 +264,6 @@ class SILFunction
   /// @_dynamicReplacement(for:) function.
   SILFunction *ReplacedFunction = nullptr;
 
-  /// This SILFunction REFerences an ad-hoc protocol requirement witness in
-  /// order to keep it alive, such that it main be obtained in IRGen. Without
-  /// this explicit reference, the witness would seem not-used, and not be
-  /// accessible for IRGen.
-  ///
-  /// Specifically, one such case is the DistributedTargetInvocationDecoder's
-  /// 'decodeNextArgument' which must be retained, as it is only used from IRGen
-  /// and such, appears as-if unused in SIL and would get optimized away.
-  // TODO: Consider making this a general "references adhoc functions" and make it an array?
-  SILFunction *RefAdHocRequirementFunction = nullptr;
-
   Identifier ObjCReplacementFor;
 
   /// The head of a single-linked list of currently alive BasicBlockBitfield.
@@ -596,27 +585,6 @@ class SILFunction
     ReplacedFunction = nullptr;
   }
 
-  SILFunction *getReferencedAdHocRequirementWitnessFunction() const {
-    return RefAdHocRequirementFunction;
-  }
-  // Marks that this `SILFunction` uses the passed in ad-hoc protocol
-  // requirement witness `f` and therefore must retain it explicitly,
-  // otherwise we might not be able to get a reference to it.
-  void setReferencedAdHocRequirementWitnessFunction(SILFunction *f) {
-    assert(RefAdHocRequirementFunction == nullptr && "already set");
-
-    if (f == nullptr)
-      return;
-    RefAdHocRequirementFunction = f;
-    RefAdHocRequirementFunction->incrementRefCount();
-  }
-  void dropReferencedAdHocRequirementWitnessFunction() {
-    if (!RefAdHocRequirementFunction)
-      return;
-    RefAdHocRequirementFunction->decrementRefCount();
-    RefAdHocRequirementFunction = nullptr;
-  }
-
   bool hasObjCReplacement() const {
     return !ObjCReplacementFor.empty();
   }

include/swift/Sema/ConstraintLocator.h

@@ -1293,6 +1293,37 @@ class ConstraintLocatorBuilder {
     return false;
   }
 
+  std::optional<std::pair</*witness=*/ValueDecl *, GenericTypeParamType *>>
+  isForWitnessGenericParameterRequirement() const {
+    SmallVector<LocatorPathElt, 2> path;
+    getLocatorParts(path);
+
+    // -> witness -> generic env -> requirement
+    if (path.size() < 3)
+      return std::nullopt;
+
+    GenericTypeParamType *GP = nullptr;
+    if (auto reqLoc =
+            path.back().getAs<LocatorPathElt::TypeParameterRequirement>()) {
+      path.pop_back();
+      if (auto openedGeneric =
+              path.back().getAs<LocatorPathElt::OpenedGeneric>()) {
+        auto signature = openedGeneric->getSignature();
+        auto requirement = signature.getRequirements()[reqLoc->getIndex()];
+        GP = requirement.getFirstType()->getAs<GenericTypeParamType>();
+      }
+    }
+
+    if (!GP)
+      return std::nullopt;
+
+    auto witness = path.front().getAs<LocatorPathElt::Witness>();
+    if (!witness)
+      return std::nullopt;
+
+    return std::make_pair(witness->getDecl(), GP);
+  }
+
   /// Checks whether this locator is describing an argument application for a
   /// non-ephemeral parameter.
   bool isNonEphemeralParameterApplication() const {

include/swift/Sema/ConstraintSystem.h

@@ -1627,6 +1627,11 @@ class Solution {
   llvm::DenseMap<ConstraintLocator *, UnresolvedDotExpr *>
       ImplicitCallAsFunctionRoots;
 
+  /// The set of conformances synthesized during solving (i.e. for
+  /// ad-hoc distributed `SerializationRequirement` conformances).
+  llvm::MapVector<ConstraintLocator *, ProtocolConformanceRef>
+      SynthesizedConformances;
+
   /// Record a new argument matching choice for given locator that maps a
   /// single argument to a single parameter.
   void recordSingleArgMatchingChoice(ConstraintLocator *locator);
@@ -1667,11 +1672,15 @@ class Solution {
   /// Compute the set of substitutions for a generic signature opened at the
   /// given locator.
   ///
+  /// \param decl The underlying declaration for which the substitutions are
+  /// computed.
+  ///
   /// \param sig The generic signature.
   ///
   /// \param locator The locator that describes where the substitutions came
   /// from.
-  SubstitutionMap computeSubstitutions(GenericSignature sig,
+  SubstitutionMap computeSubstitutions(NullablePtr<ValueDecl> decl,
+                                       GenericSignature sig,
                                        ConstraintLocator *locator) const;
 
   /// Resolves the contextual substitutions for a reference to a declaration
@@ -2411,6 +2420,11 @@ class ConstraintSystem {
   llvm::SmallMapVector<ConstraintLocator *, UnresolvedDotExpr *, 2>
       ImplicitCallAsFunctionRoots;
 
+  /// The set of conformances synthesized during solving (i.e. for
+  /// ad-hoc distributed `SerializationRequirement` conformances).
+  llvm::MapVector<ConstraintLocator *, ProtocolConformanceRef>
+      SynthesizedConformances;
+
 private:
   /// Describe the candidate expression for partial solving.
   /// This class used by shrink & solve methods which apply
@@ -2934,6 +2948,9 @@ class ConstraintSystem {
     /// The length of \c ImplicitCallAsFunctionRoots.
     unsigned numImplicitCallAsFunctionRoots;
 
+    /// The length of \c SynthesizedConformances.
+    unsigned numSynthesizedConformances;
+
     /// The previous score.
     Score PreviousScore;
 

lib/AST/DistributedDecl.cpp

@@ -387,6 +387,10 @@ bool swift::checkDistributedSerializationRequirementIsExactlyCodable(
 bool AbstractFunctionDecl::isDistributedActorSystemRemoteCall(bool isVoidReturn) const {
   auto &C = getASTContext();
   auto module = getParentModule();
+  auto *DC = getDeclContext();
+
+  if (!DC->isTypeContext() || !isGeneric())
+    return false;
 
   // === Check the name
   auto callId = isVoidReturn ? C.Id_remoteCallVoid : C.Id_remoteCall;
@@ -398,7 +402,7 @@ bool AbstractFunctionDecl::isDistributedActorSystemRemoteCall(bool isVoidReturn)
   ProtocolDecl *systemProto =
       C.getDistributedActorSystemDecl();
 
-  auto systemNominal = getDeclContext()->getSelfNominalTypeDecl();
+  auto systemNominal = DC->getSelfNominalTypeDecl();
   auto distSystemConformance = module->lookupConformance(
       systemNominal->getDeclaredInterfaceType(), systemProto);
 

lib/IRGen/GenDistributed.cpp

@@ -81,10 +81,6 @@ struct ArgumentDecoderInfo {
   /// The type of `decodeNextArgument` method.
   CanSILFunctionType MethodType;
 
-  /// Protocol requirements associated with the generic
-  /// parameter `Argument` of this decode method.
-  GenericSignature::RequiredProtocols ProtocolRequirements;
-
   // Witness metadata for conformance to DistributedTargetInvocationDecoder
   // protocol.
   WitnessMetadata Witness;
@@ -94,31 +90,19 @@ struct ArgumentDecoderInfo {
                       FunctionPointer decodeNextArgumentPtr,
                       CanSILFunctionType decodeNextArgumentTy)
       : Decoder(decoder), MethodPtr(decodeNextArgumentPtr),
-        MethodType(decodeNextArgumentTy),
-        ProtocolRequirements(findProtocolRequirements(decodeNextArgumentTy)) {
+        MethodType(decodeNextArgumentTy) {
     Witness.SelfMetadata = decoderType;
     Witness.SelfWitnessTable = decoderWitnessTable;
   }
 
   CanSILFunctionType getMethodType() const { return MethodType; }
 
-  ArrayRef<ProtocolDecl *> getProtocolRequirements() const {
-    return ProtocolRequirements;
+  WitnessMetadata *getWitnessMetadata() const {
+    return const_cast<WitnessMetadata *>(&Witness);
   }
 
   /// Form a callee to a decode method - `decodeNextArgument`.
   Callee getCallee() const;
-
-private:
-  static GenericSignature::RequiredProtocols
-  findProtocolRequirements(CanSILFunctionType decodeMethodTy) {
-    auto signature = decodeMethodTy->getInvocationGenericSignature();
-    auto genericParams = signature.getGenericParams();
-
-    // func decodeNextArgument<Arg : #SerializationRequirement#>() throws -> Arg
-    assert(genericParams.size() == 1);
-    return signature->getRequiredProtocols(genericParams.front());
-  }
 };
 
 class DistributedAccessor {
@@ -156,10 +140,6 @@ class DistributedAccessor {
                       llvm::Value *argumentType, const SILParameterInfo &param,
                       Explosion &arguments);
 
-  void lookupWitnessTables(llvm::Value *value,
-                           ArrayRef<ProtocolDecl *> protocols,
-                           Explosion &witnessTables);
-
   /// Load witness table addresses (if any) from the given buffer
   /// into the given argument explosion.
   ///
@@ -417,17 +397,13 @@ void DistributedAccessor::decodeArgument(unsigned argumentIdx,
   // substitution Argument -> <argument metadata>
   decodeArgs.add(argumentType);
 
-  // Lookup witness tables for the requirement on the argument type.
-  lookupWitnessTables(argumentType, decoder.getProtocolRequirements(),
-                      decodeArgs);
-
   Address calleeErrorSlot;
   llvm::Value *decodeError = nullptr;
 
   emission->begin();
   {
     emission->setArgs(decodeArgs, /*isOutlined=*/false,
-                      /*witnessMetadata=*/nullptr);
+                      /*witnessMetadata=*/decoder.getWitnessMetadata());
 
     Explosion result;
     emission->emitToExplosion(result, /*isOutlined=*/false);
@@ -528,37 +504,6 @@ void DistributedAccessor::decodeArgument(unsigned argumentIdx,
   }
 }
 
-void DistributedAccessor::lookupWitnessTables(
-    llvm::Value *value, ArrayRef<ProtocolDecl *> protocols,
-    Explosion &witnessTables) {
-  auto conformsToProtocol = IGM.getConformsToProtocolFunctionPointer();
-
-  for (auto *protocol : protocols) {
-    auto *protocolDescriptor = IGM.getAddrOfProtocolDescriptor(protocol);
-    auto *witnessTable =
-        IGF.Builder.CreateCall(conformsToProtocol, {value, protocolDescriptor});
-
-    auto failBB = IGF.createBasicBlock("missing-witness");
-    auto contBB = IGF.createBasicBlock("");
-
-    auto isNull = IGF.Builder.CreateICmpEQ(
-        witnessTable, llvm::ConstantPointerNull::get(IGM.WitnessTablePtrTy));
-    IGF.Builder.CreateCondBr(isNull, failBB, contBB);
-
-    // This operation shouldn't fail because runtime should have checked that
-    // a particular argument type conforms to `SerializationRequirement`
-    // of the distributed actor the decoder is used for. If it does fail
-    // then accessor should trap.
-    {
-      IGF.Builder.emitBlock(failBB);
-      IGF.emitTrap("missing witness table", /*EmitUnreachable=*/true);
-    }
-
-    IGF.Builder.emitBlock(contBB);
-    witnessTables.add(witnessTable);
-  }
-}
-
 void DistributedAccessor::emitLoadOfWitnessTables(llvm::Value *witnessTables,
                                                   llvm::Value *numTables,
                                                   unsigned expectedWitnessTables,
@@ -803,70 +748,22 @@ DistributedAccessor::getCalleeForDistributedTarget(llvm::Value *self) const {
 
 ArgumentDecoderInfo DistributedAccessor::findArgumentDecoder(
     llvm::Value *decoder, llvm::Value *decoderTy, llvm::Value *witnessTable) {
-  auto *actor = getDistributedActorOf(Target);
-  auto expansionContext = IGM.getMaximalTypeExpansionContext();
-
-  auto *decodeFn = IGM.Context.getDistributedActorArgumentDecodingMethod(actor);
-  assert(decodeFn && "no suitable decoder?");
-
-  auto methodTy = IGM.getSILTypes().getConstantFunctionType(
-      expansionContext, SILDeclRef(decodeFn));
-
-  auto fpKind = FunctionPointerKind::defaultAsync();
-  auto signature = IGM.getSignature(methodTy, fpKind);
-
-  // If the decoder class is `final`, let's emit a direct reference.
-  auto *decoderDecl = decodeFn->getDeclContext()->getSelfNominalTypeDecl();
-
-  // If decoder is a class, need to load it first because generic parameter
-  // is passed indirectly. This is good for structs and enums because
-  // `decodeNextArgument` is a mutating method, but not for classes because
-  // in that case heap object is mutated directly.
-  bool usesDispatchThunk = false;
+  auto &C = IGM.Context;
 
-  if (auto classDecl = dyn_cast<ClassDecl>(decoderDecl)) {
-    auto selfTy = methodTy->getSelfParameter().getSILStorageType(
-        IGM.getSILModule(), methodTy, expansionContext);
+  auto decoderProtocol = C.getDistributedTargetInvocationDecoderDecl();
+  SILDeclRef decodeNextArgumentRef(
+      decoderProtocol->getSingleRequirement(C.Id_decodeNextArgument));
 
-    auto &classTI = IGM.getTypeInfo(selfTy).as<ClassTypeInfo>();
-    auto &classLayout = classTI.getClassLayout(IGM, selfTy,
-                                               /*forBackwardDeployment=*/false);
+  llvm::Constant *fnPtr =
+      IGM.getAddrOfDispatchThunk(decodeNextArgumentRef, NotForDefinition);
 
-    llvm::Value *typedDecoderPtr = IGF.Builder.CreateBitCast(
-        decoder, classLayout.getType()->getPointerTo()->getPointerTo());
-
-    Explosion instance;
-
-    classTI.loadAsTake(IGF,
-                       {typedDecoderPtr, classTI.getStorageType(),
-                        classTI.getBestKnownAlignment()},
-                       instance);
-
-    decoder = instance.claimNext();
-
-    /// When using library evolution functions have another "dispatch thunk"
-    /// so we must use this instead of the decodeFn directly.
-    usesDispatchThunk =
-        getMethodDispatch(decodeFn) == swift::MethodDispatch::Class &&
-        classDecl->hasResilientMetadata();
-  }
-
-  FunctionPointer methodPtr;
-
-  if (usesDispatchThunk) {
-    auto fnPtr = IGM.getAddrOfDispatchThunk(SILDeclRef(decodeFn), NotForDefinition);
-    methodPtr = FunctionPointer::createUnsigned(
-        methodTy, fnPtr, signature, /*useSignature=*/true);
-  } else {
-    SILFunction *decodeSILFn = IGM.getSILModule().lookUpFunction(SILDeclRef(decodeFn));
-    auto fnPtr = IGM.getAddrOfSILFunction(decodeSILFn, NotForDefinition,
-        /*isDynamicallyReplaceable=*/false);
-    methodPtr = FunctionPointer::forDirect(
-        classifyFunctionPointerKind(decodeSILFn), fnPtr,
-        /*secondaryValue=*/nullptr, signature);
-  }
+  auto fnType = IGM.getSILTypes().getConstantFunctionType(
+      IGM.getMaximalTypeExpansionContext(), decodeNextArgumentRef);
 
-  return {decoder, decoderTy, witnessTable, methodPtr, methodTy};
+  auto sig = IGM.getSignature(fnType);
+  auto fn = FunctionPointer::forDirect(fnType, fnPtr,
+                                       /*secondaryValue=*/nullptr, sig, true);
+  return {decoder, decoderTy, witnessTable, fn, fnType};
 }
 
 SILType DistributedAccessor::getResultType() const {