Esempio n. 1
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static void maybeEmitDebugInfoForLocalTypeData(IRGenFunction &IGF,
                                               LocalTypeDataKey key,
                                               llvm::Value *data) {
  // Only if debug info is enabled.
  if (!IGF.IGM.DebugInfo) return;

  // Only for type metadata.
  if (key.Kind != LocalTypeDataKind::forTypeMetadata()) return;

  // Only for archetypes, and not for opened archetypes.
  auto type = dyn_cast<ArchetypeType>(key.Type);
  if (!type) return;
  if (type->getOpenedExistentialType()) return;

  // At -O0, create an alloca to keep the type alive.
  auto name = type->getFullName();
  if (!IGF.IGM.IRGen.Opts.Optimize) {
    auto temp = IGF.createAlloca(data->getType(), IGF.IGM.getPointerAlignment(),
                                 name);
    IGF.Builder.CreateStore(data, temp);
    data = temp.getAddress();
  }

  // Emit debug info for the metadata.
  IGF.IGM.DebugInfo->emitTypeMetadata(IGF, data, name);
}
Esempio n. 2
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static void maybeEmitDebugInfoForLocalTypeData(IRGenFunction &IGF,
                                               LocalTypeDataKey key,
                                               MetadataResponse value) {
  // FIXME: This check doesn't entirely behave correctly for non-transparent
  // functions that were inlined into transparent functions. Correct would be to
  // check which instruction requests the type metadata and see whether its
  // inlined function is transparent.
  auto * DS = IGF.getDebugScope();
  if (DS && DS->getInlinedFunction() &&
      DS->getInlinedFunction()->isTransparent())
    return;
  
  // Only for formal type metadata.
  if (key.Kind != LocalTypeDataKind::forFormalTypeMetadata())
    return;

  // Only for archetypes, and not for opened archetypes.
  auto type = dyn_cast<ArchetypeType>(key.Type);
  if (!type)
    return;
  if (type->getOpenedExistentialType())
    return;

  llvm::Value *data = value.getMetadata();

  // At -O0, create an alloca to keep the type alive.
  auto name = type->getFullName();
  if (!IGF.IGM.IRGen.Opts.shouldOptimize()) {
    auto alloca =
        IGF.createAlloca(data->getType(), IGF.IGM.getPointerAlignment(), name);
    IGF.Builder.CreateStore(data, alloca);
    data = alloca.getAddress();
  }

  // Only if debug info is enabled.
  if (!IGF.IGM.DebugInfo)
    return;

  // Emit debug info for the metadata.
  llvm::SmallString<8> AssocType;
  auto *oocTy = type->mapTypeOutOfContext().getPointer();
  {
    llvm::raw_svector_ostream OS(AssocType);
    while (auto *dependentMemberType = dyn_cast<DependentMemberType>(oocTy)) {
      OS << '.' << dependentMemberType->getName();
      oocTy = dependentMemberType->getBase().getPointer();
    }
  }
  auto *typeParam = cast<GenericTypeParamType>(oocTy);
  IGF.IGM.DebugInfo->emitTypeMetadata(IGF, data, typeParam->getDepth(),
                                      typeParam->getIndex(), AssocType);
}
Esempio n. 3
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ContainedAddress FixedTypeInfo::allocateStack(IRGenFunction &IGF, SILType T,
                                              const Twine &name) const {
  // If the type is known to be empty, don't actually allocate anything.
  if (isKnownEmpty(ResilienceExpansion::Maximal)) {
    auto addr = getUndefAddress();
    return { addr, addr };
  }

  Address alloca =
    IGF.createAlloca(getStorageType(), getFixedAlignment(), name);
  IGF.Builder.CreateLifetimeStart(alloca, getFixedSize());
  
  return { alloca, alloca };
}
Esempio n. 4
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ContainedAddress FixedTypeInfo::allocateStack(IRGenFunction &IGF, SILType T,
                                              const Twine &name) const {
  // If the type is known to be empty, don't actually allocate anything.
  if (isKnownEmpty()) {
    auto addr = getUndefAddress();
    return { addr, addr };
  }

  Address alloca =
    IGF.createAlloca(getStorageType(), getFixedAlignment(), name);
  // TODO: lifetime intrinsics?

  return { alloca, alloca };
}
Esempio n. 5
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static void setMetadataRef(IRGenFunction &IGF,
                           ArchetypeType *archetype,
                           llvm::Value *metadata) {
  assert(metadata->getType() == IGF.IGM.TypeMetadataPtrTy);
  IGF.setUnscopedLocalTypeData(CanType(archetype),
                               LocalTypeData::forMetatype(),
                               metadata);

  // Create a shadow copy of the metadata in an alloca for the debug info.
  StringRef Name = metadata->getName();
  if (!IGF.IGM.Opts.Optimize) {
    auto Alloca = IGF.createAlloca(metadata->getType(),
                                   IGF.IGM.getPointerAlignment(), Name);
    IGF.Builder.CreateAlignedStore(metadata, Alloca.getAddress(),
                                   IGF.IGM.getPointerAlignment().getValue());
    metadata = Alloca.getAddress();
  }

  // Emit debug info for the metadata.
  if (IGF.IGM.DebugInfo)
    IGF.IGM.DebugInfo->emitTypeMetadata(IGF, metadata, Name);
}
Esempio n. 6
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void
irgen::emitTypeLayoutVerifier(IRGenFunction &IGF,
                              ArrayRef<CanType> formalTypes) {
  llvm::Type *verifierArgTys[] = {
    IGF.IGM.TypeMetadataPtrTy,
    IGF.IGM.Int8PtrTy,
    IGF.IGM.Int8PtrTy,
    IGF.IGM.SizeTy,
    IGF.IGM.Int8PtrTy,
  };
  auto verifierFnTy = llvm::FunctionType::get(IGF.IGM.VoidTy,
                                              verifierArgTys,
                                              /*var arg*/ false);
  auto verifierFn = IGF.IGM.Module.getOrInsertFunction(
                                       "_swift_debug_verifyTypeLayoutAttribute",
                                       verifierFnTy);
  struct VerifierArgumentBuffers {
    Address runtimeBuf, staticBuf;
  };
  llvm::DenseMap<llvm::Type *, VerifierArgumentBuffers>
    verifierArgBufs;
  
  auto getSizeConstant = [&](Size sz) -> llvm::Constant * {
    return llvm::ConstantInt::get(IGF.IGM.SizeTy, sz.getValue());
  };
  auto getAlignmentMaskConstant = [&](Alignment a) -> llvm::Constant * {
    return llvm::ConstantInt::get(IGF.IGM.SizeTy, a.getValue() - 1);
  };
  auto getBoolConstant = [&](bool b) -> llvm::Constant * {
    return llvm::ConstantInt::get(IGF.IGM.Int1Ty, b);
  };

  SmallString<20> numberBuf;

  for (auto formalType : formalTypes) {
    // Runtime type metadata always represents the maximal abstraction level of
    // the type.
    auto anyTy = ProtocolCompositionType::get(IGF.IGM.Context, {});
    auto openedAnyTy = ArchetypeType::getOpened(anyTy);
    auto maxAbstraction = AbstractionPattern(openedAnyTy);
    auto &ti = IGF.getTypeInfoForUnlowered(maxAbstraction, formalType);
    
    // If there's no fixed type info, we rely on the runtime anyway, so there's
    // nothing to verify.
    // TODO: There are some traits of partially-fixed layouts we could check too.
    auto *fixedTI = dyn_cast<FixedTypeInfo>(&ti);
    if (!fixedTI)
      return;
    
    auto metadata = IGF.emitTypeMetadataRef(formalType);
    
    auto verify = [&](llvm::Value *runtimeVal,
                      llvm::Value *staticVal,
                      const llvm::Twine &description) {
      assert(runtimeVal->getType() == staticVal->getType());
      // Get or create buffers for the arguments.
      VerifierArgumentBuffers bufs;
      auto foundBufs = verifierArgBufs.find(runtimeVal->getType());
      if (foundBufs != verifierArgBufs.end()) {
        bufs = foundBufs->second;
      } else {
        Address runtimeBuf = IGF.createAlloca(runtimeVal->getType(),
                                              IGF.IGM.getPointerAlignment(),
                                              "runtime");
        Address staticBuf = IGF.createAlloca(staticVal->getType(),
                                             IGF.IGM.getPointerAlignment(),
                                             "static");
        bufs = {runtimeBuf, staticBuf};
        verifierArgBufs[runtimeVal->getType()] = bufs;
      }
      
      IGF.Builder.CreateStore(runtimeVal, bufs.runtimeBuf);
      IGF.Builder.CreateStore(staticVal, bufs.staticBuf);
      
      auto runtimePtr = IGF.Builder.CreateBitCast(bufs.runtimeBuf.getAddress(),
                                                  IGF.IGM.Int8PtrTy);
      auto staticPtr = IGF.Builder.CreateBitCast(bufs.staticBuf.getAddress(),
                                                 IGF.IGM.Int8PtrTy);
      auto count = llvm::ConstantInt::get(IGF.IGM.SizeTy,
                    IGF.IGM.DataLayout.getTypeStoreSize(runtimeVal->getType()));
      auto msg
        = IGF.IGM.getAddrOfGlobalString(description.str());
      
      IGF.Builder.CreateCall(
          verifierFn, {metadata, runtimePtr, staticPtr, count, msg});
    };

    // Check that the fixed layout matches the runtime layout.
    SILType layoutType = SILType::getPrimitiveObjectType(formalType);
    verify(emitLoadOfSize(IGF, layoutType),
           getSizeConstant(fixedTI->getFixedSize()),
           "size");
    verify(emitLoadOfAlignmentMask(IGF, layoutType),
           getAlignmentMaskConstant(fixedTI->getFixedAlignment()),
           "alignment mask");
    verify(emitLoadOfStride(IGF, layoutType),
           getSizeConstant(fixedTI->getFixedStride()),
           "stride");
    verify(emitLoadOfIsInline(IGF, layoutType),
           getBoolConstant(fixedTI->getFixedPacking(IGF.IGM)
                             == FixedPacking::OffsetZero),
           "is-inline bit");
    verify(emitLoadOfIsPOD(IGF, layoutType),
           getBoolConstant(fixedTI->isPOD(ResilienceScope::Component)),
           "is-POD bit");
    verify(emitLoadOfIsBitwiseTakable(IGF, layoutType),
           getBoolConstant(fixedTI->isBitwiseTakable(ResilienceScope::Component)),
           "is-bitwise-takable bit");
    unsigned xiCount = fixedTI->getFixedExtraInhabitantCount(IGF.IGM);
    verify(emitLoadOfHasExtraInhabitants(IGF, layoutType),
           getBoolConstant(xiCount != 0),
           "has-extra-inhabitants bit");

    // Check extra inhabitants.
    if (xiCount > 0) {
      verify(emitLoadOfExtraInhabitantCount(IGF, layoutType),
             getSizeConstant(Size(xiCount)),
             "extra inhabitant count");
      
      // Verify that the extra inhabitant representations are consistent.
      
      /* TODO: Update for EnumPayload implementation changes.
      
      auto xiBuf = IGF.createAlloca(fixedTI->getStorageType(),
                                    fixedTI->getFixedAlignment(),
                                    "extra-inhabitant");
      auto xiOpaque = IGF.Builder.CreateBitCast(xiBuf, IGF.IGM.OpaquePtrTy);
      
      // TODO: Randomize the set of extra inhabitants we check.
      unsigned bits = fixedTI->getFixedSize().getValueInBits();
      for (unsigned i = 0, e = std::min(xiCount, 1024u);
           i < e; ++i) {
        // Initialize the buffer with junk, to help ensure we're insensitive to
        // insignificant bits.
        // TODO: Randomize the filler.
        IGF.Builder.CreateMemSet(xiBuf.getAddress(),
                                 llvm::ConstantInt::get(IGF.IGM.Int8Ty, 0x5A),
                                 fixedTI->getFixedSize().getValue(),
                                 fixedTI->getFixedAlignment().getValue());
        
        // Ask the runtime to store an extra inhabitant.
        auto index = llvm::ConstantInt::get(IGF.IGM.Int32Ty, i);
        emitStoreExtraInhabitantCall(IGF, layoutType, index,
                                     xiOpaque.getAddress());
        
        // Compare the stored extra inhabitant against the fixed extra
        // inhabitant pattern.
        auto fixedXI = fixedTI->getFixedExtraInhabitantValue(IGF.IGM, bits, i);
        auto xiBuf2 = IGF.Builder.CreateBitCast(xiBuf,
                                            fixedXI->getType()->getPointerTo());
        llvm::Value *runtimeXI = IGF.Builder.CreateLoad(xiBuf2);
        runtimeXI = fixedTI->maskFixedExtraInhabitant(IGF, runtimeXI);
        
        numberBuf.clear();
        {
          llvm::raw_svector_ostream os(numberBuf);
          os << i;
          os.flush();
        }
        
        verify(runtimeXI, fixedXI,
               llvm::Twine("stored extra inhabitant ") + numberBuf.str());
        
        // Now store the fixed extra inhabitant and ask the runtime to identify
        // it.
        // Mask in junk to make sure the runtime correctly ignores it.
        auto xiMask = fixedTI->getFixedExtraInhabitantMask(IGF.IGM).asAPInt();
        auto maskVal = llvm::ConstantInt::get(IGF.IGM.getLLVMContext(), xiMask);
        auto notMaskVal
          = llvm::ConstantInt::get(IGF.IGM.getLLVMContext(), ~xiMask);
        // TODO: Randomize the filler.
        auto xiFill = llvm::ConstantInt::getAllOnesValue(fixedXI->getType());
        llvm::Value *xiFillMask = IGF.Builder.CreateAnd(notMaskVal, xiFill);
        llvm::Value *xiValMask = IGF.Builder.CreateAnd(maskVal, fixedXI);
        llvm::Value *filledXI = IGF.Builder.CreateOr(xiFillMask, xiValMask);
        
        IGF.Builder.CreateStore(filledXI, xiBuf2);
        
        auto runtimeIndex = emitGetExtraInhabitantIndexCall(IGF, layoutType,
                                                        xiOpaque.getAddress());
        verify(runtimeIndex, index,
               llvm::Twine("extra inhabitant index calculation ")
                 + numberBuf.str());
      }
       */
    }

    // TODO: Verify interesting layout properties specific to the kind of type,
    // such as struct or class field offsets, enum case tags, vtable entries,
    // etc.
  }
}
Esempio n. 7
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/// Emit a checked cast to a protocol or protocol composition.
void irgen::emitScalarExistentialDowncast(IRGenFunction &IGF,
                                  llvm::Value *value,
                                  SILType srcType,
                                  SILType destType,
                                  CheckedCastMode mode,
                                  Optional<MetatypeRepresentation> metatypeKind,
                                  Explosion &ex) {
  SmallVector<ProtocolDecl*, 4> allProtos;
  destType.getSwiftRValueType().getAnyExistentialTypeProtocols(allProtos);

  // Look up witness tables for the protocols that need them and get
  // references to the ObjC Protocol* values for the objc protocols.
  SmallVector<llvm::Value*, 4> objcProtos;
  SmallVector<llvm::Value*, 4> witnessTableProtos;

  bool hasClassConstraint = false;
  bool hasClassConstraintByProtocol = false;

  for (auto proto : allProtos) {
    // If the protocol introduces a class constraint, track whether we need
    // to check for it independent of protocol witnesses.
    if (proto->requiresClass()) {
      hasClassConstraint = true;
      if (proto->getKnownProtocolKind()
          && *proto->getKnownProtocolKind() == KnownProtocolKind::AnyObject) {
        // AnyObject only requires that the type be a class.
        continue;
      }
      
      // If this protocol is class-constrained but not AnyObject, checking its
      // conformance will check the class constraint too.
      hasClassConstraintByProtocol = true;
    }

    if (Lowering::TypeConverter::protocolRequiresWitnessTable(proto)) {
      auto descriptor = emitProtocolDescriptorRef(IGF, proto);
      witnessTableProtos.push_back(descriptor);
    }

    if (!proto->isObjC())
      continue;

    objcProtos.push_back(emitReferenceToObjCProtocol(IGF, proto));
  }
  
  llvm::Type *resultType;
  if (metatypeKind) {
    switch (*metatypeKind) {
    case MetatypeRepresentation::Thin:
      llvm_unreachable("can't cast to thin metatype");
    case MetatypeRepresentation::Thick:
      resultType = IGF.IGM.TypeMetadataPtrTy;
      break;
    case MetatypeRepresentation::ObjC:
      resultType = IGF.IGM.ObjCClassPtrTy;
      break;
    }
  } else {
    auto schema = IGF.getTypeInfo(destType).getSchema();
    resultType = schema[0].getScalarType();
  }
  // We only need to check the class constraint for metatype casts where
  // no protocol conformance indirectly requires the constraint for us.
  bool checkClassConstraint =
    (bool)metatypeKind && hasClassConstraint && !hasClassConstraintByProtocol;

  llvm::Value *resultValue = value;

  // If we don't have anything we really need to check, then trivially succeed.
  if (objcProtos.empty() && witnessTableProtos.empty() &&
      !checkClassConstraint) {
    resultValue = IGF.Builder.CreateBitCast(value, resultType);
    ex.add(resultValue);
    return;
  }

  // Check the ObjC protocol conformances if there were any.
  llvm::Value *objcCast = nullptr;
  if (!objcProtos.empty()) {
    // Get the ObjC instance or class object to check for these conformances.
    llvm::Value *objcObject;
    if (metatypeKind) {
      switch (*metatypeKind) {
      case MetatypeRepresentation::Thin:
        llvm_unreachable("can't cast to thin metatype");
      case MetatypeRepresentation::Thick: {
        // The metadata might be for a non-class type, which wouldn't have
        // an ObjC class object.
        objcObject = nullptr;
        break;
      }
      case MetatypeRepresentation::ObjC:
        // Metatype is already an ObjC object.
        objcObject = value;
        break;
      }
    } else {
      // Class instance is already an ObjC object.
      objcObject = value;
    }
    if (objcObject)
      objcObject = IGF.Builder.CreateBitCast(objcObject,
                                             IGF.IGM.UnknownRefCountedPtrTy);
    
    // Pick the cast function based on the cast mode and on whether we're
    // casting a Swift metatype or ObjC object.
    llvm::Value *castFn;
    switch (mode) {
    case CheckedCastMode::Unconditional:
      castFn = objcObject
        ? IGF.IGM.getDynamicCastObjCProtocolUnconditionalFn()
        : IGF.IGM.getDynamicCastTypeToObjCProtocolUnconditionalFn();
      break;
    case CheckedCastMode::Conditional:
      castFn = objcObject
        ? IGF.IGM.getDynamicCastObjCProtocolConditionalFn()
        : IGF.IGM.getDynamicCastTypeToObjCProtocolConditionalFn();
      break;
    }
    llvm::Value *objcCastObject = objcObject ? objcObject : value;
    
    Address protoRefsBuf = IGF.createAlloca(
                                        llvm::ArrayType::get(IGF.IGM.Int8PtrTy,
                                                             objcProtos.size()),
                                        IGF.IGM.getPointerAlignment(),
                                        "objc_protocols");
    protoRefsBuf = IGF.Builder.CreateBitCast(protoRefsBuf,
                                             IGF.IGM.Int8PtrPtrTy);

    for (unsigned index : indices(objcProtos)) {
      Address protoRefSlot = IGF.Builder.CreateConstArrayGEP(
                                                     protoRefsBuf, index,
                                                     IGF.IGM.getPointerSize());
      IGF.Builder.CreateStore(objcProtos[index], protoRefSlot);
      ++index;
    }

    
    objcCast = IGF.Builder.CreateCall(
        castFn,
        {objcCastObject, IGF.IGM.getSize(Size(objcProtos.size())),
         protoRefsBuf.getAddress()});
    resultValue = IGF.Builder.CreateBitCast(objcCast, resultType);
  }

  // If we don't need to look up any witness tables, we're done.
  if (witnessTableProtos.empty() && !checkClassConstraint) {
    ex.add(resultValue);
    return;
  }

  // If we're doing a conditional cast, and the ObjC protocol checks failed,
  // then the cast is done.
  Optional<ConditionalDominanceScope> condition;
  llvm::BasicBlock *origBB = nullptr, *successBB = nullptr, *contBB = nullptr;
  if (!objcProtos.empty()) {
    switch (mode) {
    case CheckedCastMode::Unconditional:
      break;
    case CheckedCastMode::Conditional: {
      origBB = IGF.Builder.GetInsertBlock();
      successBB = IGF.createBasicBlock("success");
      contBB = IGF.createBasicBlock("cont");
      auto isNull = IGF.Builder.CreateICmpEQ(objcCast,
                               llvm::ConstantPointerNull::get(
                                 cast<llvm::PointerType>(objcCast->getType())));
      IGF.Builder.CreateCondBr(isNull, contBB, successBB);
      IGF.Builder.emitBlock(successBB);
      condition.emplace(IGF);
    }
    }
  }

  // Get the Swift type metadata for the type.
  llvm::Value *metadataValue;
  if (metatypeKind) {
    switch (*metatypeKind) {
    case MetatypeRepresentation::Thin:
      llvm_unreachable("can't cast to thin metatype");
    case MetatypeRepresentation::Thick:
      // The value is already a native metatype.
      metadataValue = value;
      break;
    case MetatypeRepresentation::ObjC:
      // Get the type metadata from the ObjC class, which may be a wrapper.
      metadataValue = emitObjCMetadataRefForMetadata(IGF, value);
    }
  } else {
    // Get the type metadata for the instance.
    metadataValue = emitDynamicTypeOfHeapObject(IGF, value, srcType);
  }

  // Look up witness tables for the protocols that need them.
  auto fn = emitExistentialScalarCastFn(IGF.IGM, witnessTableProtos.size(),
                                        mode, checkClassConstraint);

  llvm::SmallVector<llvm::Value *, 4> args;

  if (resultValue->getType() != IGF.IGM.Int8PtrTy)
    resultValue = IGF.Builder.CreateBitCast(resultValue, IGF.IGM.Int8PtrTy);
  args.push_back(resultValue);

  args.push_back(metadataValue);
  for (auto proto : witnessTableProtos)
    args.push_back(proto);

  auto valueAndWitnessTables = IGF.Builder.CreateCall(fn, args);

  resultValue = IGF.Builder.CreateExtractValue(valueAndWitnessTables, 0);
  if (resultValue->getType() != resultType)
    resultValue = IGF.Builder.CreateBitCast(resultValue, resultType);
  ex.add(resultValue);

  for (unsigned i = 0, e = witnessTableProtos.size(); i < e; ++i) {
    auto wt = IGF.Builder.CreateExtractValue(valueAndWitnessTables, i + 1);
    ex.add(wt);
  }

  // If we had conditional ObjC checks, join the failure paths.
  if (contBB) {
    condition.reset();
    IGF.Builder.CreateBr(contBB);
    IGF.Builder.emitBlock(contBB);
    
    // Return null on the failure path.
    Explosion successEx = std::move(ex);
    ex.reset();
    
    while (!successEx.empty()) {
      auto successVal = successEx.claimNext();
      auto failureVal = llvm::Constant::getNullValue(successVal->getType());
      auto phi = IGF.Builder.CreatePHI(successVal->getType(), 2);
      phi->addIncoming(successVal, successBB);
      phi->addIncoming(failureVal, origBB);
      ex.add(phi);
    }
  }
}
Esempio n. 8
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/// Emit a checked cast to a protocol or protocol composition.
void irgen::emitScalarExistentialDowncast(IRGenFunction &IGF,
                                  llvm::Value *value,
                                  SILType srcType,
                                  SILType destType,
                                  CheckedCastMode mode,
                                  Optional<MetatypeRepresentation> metatypeKind,
                                  Explosion &ex) {
  auto srcInstanceType = srcType.getSwiftRValueType();
  auto destInstanceType = destType.getSwiftRValueType();
  while (auto metatypeType = dyn_cast<ExistentialMetatypeType>(
           destInstanceType)) {
    destInstanceType = metatypeType.getInstanceType();
    srcInstanceType = cast<AnyMetatypeType>(srcInstanceType).getInstanceType();
  }

  auto layout = destInstanceType.getExistentialLayout();

  // Look up witness tables for the protocols that need them and get
  // references to the ObjC Protocol* values for the objc protocols.
  SmallVector<llvm::Value*, 4> objcProtos;
  SmallVector<llvm::Value*, 4> witnessTableProtos;

  bool hasClassConstraint = layout.requiresClass();
  bool hasClassConstraintByProtocol = false;

  bool hasSuperclassConstraint = bool(layout.superclass);

  for (auto protoTy : layout.getProtocols()) {
    auto *protoDecl = protoTy->getDecl();

    // If the protocol introduces a class constraint, track whether we need
    // to check for it independent of protocol witnesses.
    if (protoDecl->requiresClass()) {
      assert(hasClassConstraint);
      hasClassConstraintByProtocol = true;
    }

    if (Lowering::TypeConverter::protocolRequiresWitnessTable(protoDecl)) {
      auto descriptor = emitProtocolDescriptorRef(IGF, protoDecl);
      witnessTableProtos.push_back(descriptor);
    }

    if (protoDecl->isObjC())
      objcProtos.push_back(emitReferenceToObjCProtocol(IGF, protoDecl));
  }
  
  llvm::Type *resultType;
  if (metatypeKind) {
    switch (*metatypeKind) {
    case MetatypeRepresentation::Thin:
      llvm_unreachable("can't cast to thin metatype");
    case MetatypeRepresentation::Thick:
      resultType = IGF.IGM.TypeMetadataPtrTy;
      break;
    case MetatypeRepresentation::ObjC:
      resultType = IGF.IGM.ObjCClassPtrTy;
      break;
    }
  } else {
    auto schema = IGF.getTypeInfo(destType).getSchema();
    resultType = schema[0].getScalarType();
  }

  // The source of a scalar cast is statically known to be a class or a
  // metatype, so we only have to check the class constraint in two cases:
  //
  // 1) The destination type has an explicit superclass constraint that is
  //    more derived than what the source type is known to be.
  //
  // 2) We are casting between metatypes, in which case the source might
  //    be a non-class metatype.
  bool checkClassConstraint = false;
  if ((bool)metatypeKind &&
      hasClassConstraint &&
      !hasClassConstraintByProtocol &&
      !srcInstanceType->mayHaveSuperclass())
    checkClassConstraint = true;

  // If the source has an equal or more derived superclass constraint than
  // the destination, we can elide the superclass check.
  //
  // Note that destInstanceType is always an existential type, so calling
  // getSuperclass() returns the superclass constraint of the existential,
  // not the superclass of some concrete class.
  bool checkSuperclassConstraint =
    hasSuperclassConstraint &&
    !destInstanceType->getSuperclass()->isExactSuperclassOf(srcInstanceType);

  if (checkSuperclassConstraint)
    checkClassConstraint = true;

  llvm::Value *resultValue = value;

  // If we don't have anything we really need to check, then trivially succeed.
  if (objcProtos.empty() && witnessTableProtos.empty() &&
      !checkClassConstraint) {
    resultValue = IGF.Builder.CreateBitCast(value, resultType);
    ex.add(resultValue);
    return;
  }

  // Check the ObjC protocol conformances if there were any.
  llvm::Value *objcCast = nullptr;
  if (!objcProtos.empty()) {
    // Get the ObjC instance or class object to check for these conformances.
    llvm::Value *objcObject;
    if (metatypeKind) {
      switch (*metatypeKind) {
      case MetatypeRepresentation::Thin:
        llvm_unreachable("can't cast to thin metatype");
      case MetatypeRepresentation::Thick: {
        // The metadata might be for a non-class type, which wouldn't have
        // an ObjC class object.
        objcObject = nullptr;
        break;
      }
      case MetatypeRepresentation::ObjC:
        // Metatype is already an ObjC object.
        objcObject = value;
        break;
      }
    } else {
      // Class instance is already an ObjC object.
      objcObject = value;
    }
    if (objcObject)
      objcObject = IGF.Builder.CreateBitCast(objcObject,
                                             IGF.IGM.UnknownRefCountedPtrTy);
    
    // Pick the cast function based on the cast mode and on whether we're
    // casting a Swift metatype or ObjC object.
    llvm::Constant *castFn;
    switch (mode) {
    case CheckedCastMode::Unconditional:
      castFn = objcObject
        ? IGF.IGM.getDynamicCastObjCProtocolUnconditionalFn()
        : IGF.IGM.getDynamicCastTypeToObjCProtocolUnconditionalFn();
      break;
    case CheckedCastMode::Conditional:
      castFn = objcObject
        ? IGF.IGM.getDynamicCastObjCProtocolConditionalFn()
        : IGF.IGM.getDynamicCastTypeToObjCProtocolConditionalFn();
      break;
    }
    llvm::Value *objcCastObject = objcObject ? objcObject : value;
    
    Address protoRefsBuf = IGF.createAlloca(
                                        llvm::ArrayType::get(IGF.IGM.Int8PtrTy,
                                                             objcProtos.size()),
                                        IGF.IGM.getPointerAlignment(),
                                        "objc_protocols");
    protoRefsBuf = IGF.Builder.CreateBitCast(protoRefsBuf,
                                             IGF.IGM.Int8PtrPtrTy);

    for (unsigned index : indices(objcProtos)) {
      Address protoRefSlot = IGF.Builder.CreateConstArrayGEP(
                                                     protoRefsBuf, index,
                                                     IGF.IGM.getPointerSize());
      IGF.Builder.CreateStore(objcProtos[index], protoRefSlot);
      ++index;
    }

    
    auto cc = IGF.IGM.DefaultCC;
    if (auto fun = dyn_cast<llvm::Function>(castFn))
      cc = fun->getCallingConv();


    auto call = IGF.Builder.CreateCall(
        castFn,
        {objcCastObject, IGF.IGM.getSize(Size(objcProtos.size())),
         protoRefsBuf.getAddress()});
    call->setCallingConv(cc);
    objcCast = call;
    resultValue = IGF.Builder.CreateBitCast(objcCast, resultType);
  }

  // If we don't need to look up any witness tables, we're done.
  if (witnessTableProtos.empty() && !checkClassConstraint) {
    ex.add(resultValue);
    return;
  }

  // If we're doing a conditional cast, and the ObjC protocol checks failed,
  // then the cast is done.
  Optional<ConditionalDominanceScope> condition;
  llvm::BasicBlock *origBB = nullptr, *successBB = nullptr, *contBB = nullptr;
  if (!objcProtos.empty()) {
    switch (mode) {
    case CheckedCastMode::Unconditional:
      break;
    case CheckedCastMode::Conditional: {
      origBB = IGF.Builder.GetInsertBlock();
      successBB = IGF.createBasicBlock("success");
      contBB = IGF.createBasicBlock("cont");
      auto isNull = IGF.Builder.CreateICmpEQ(objcCast,
                               llvm::ConstantPointerNull::get(
                                 cast<llvm::PointerType>(objcCast->getType())));
      IGF.Builder.CreateCondBr(isNull, contBB, successBB);
      IGF.Builder.emitBlock(successBB);
      condition.emplace(IGF);
    }
    }
  }

  // Get the Swift type metadata for the type.
  llvm::Value *metadataValue;
  if (metatypeKind) {
    switch (*metatypeKind) {
    case MetatypeRepresentation::Thin:
      llvm_unreachable("can't cast to thin metatype");
    case MetatypeRepresentation::Thick:
      // The value is already a native metatype.
      metadataValue = value;
      break;
    case MetatypeRepresentation::ObjC:
      // Get the type metadata from the ObjC class, which may be a wrapper.
      metadataValue = emitObjCMetadataRefForMetadata(IGF, value);
    }
  } else {
    // Get the type metadata for the instance.
    metadataValue = emitDynamicTypeOfHeapObject(IGF, value, srcType);
  }

  // Look up witness tables for the protocols that need them.
  auto fn = emitExistentialScalarCastFn(IGF.IGM,
                                        witnessTableProtos.size(),
                                        mode,
                                        checkClassConstraint,
                                        checkSuperclassConstraint);

  llvm::SmallVector<llvm::Value *, 4> args;

  if (resultValue->getType() != IGF.IGM.Int8PtrTy)
    resultValue = IGF.Builder.CreateBitCast(resultValue, IGF.IGM.Int8PtrTy);
  args.push_back(resultValue);

  args.push_back(metadataValue);

  if (checkSuperclassConstraint)
    args.push_back(IGF.emitTypeMetadataRef(CanType(layout.superclass)));

  for (auto proto : witnessTableProtos)
    args.push_back(proto);

  auto valueAndWitnessTables = IGF.Builder.CreateCall(fn, args);

  resultValue = IGF.Builder.CreateExtractValue(valueAndWitnessTables, 0);
  if (resultValue->getType() != resultType)
    resultValue = IGF.Builder.CreateBitCast(resultValue, resultType);
  ex.add(resultValue);

  for (unsigned i = 0, e = witnessTableProtos.size(); i < e; ++i) {
    auto wt = IGF.Builder.CreateExtractValue(valueAndWitnessTables, i + 1);
    ex.add(wt);
  }

  // If we had conditional ObjC checks, join the failure paths.
  if (contBB) {
    condition.reset();
    IGF.Builder.CreateBr(contBB);
    IGF.Builder.emitBlock(contBB);
    
    // Return null on the failure path.
    Explosion successEx = std::move(ex);
    ex.reset();
    
    while (!successEx.empty()) {
      auto successVal = successEx.claimNext();
      auto failureVal = llvm::Constant::getNullValue(successVal->getType());
      auto phi = IGF.Builder.CreatePHI(successVal->getType(), 2);
      phi->addIncoming(successVal, successBB);
      phi->addIncoming(failureVal, origBB);
      ex.add(phi);
    }
  }
}