EnumPayload.cpp

//===--- EnumPayload.cpp - Payload management for 'enum' Types ------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

#include "EnumPayload.h"
#include "Explosion.h"
#include "GenEnum.h"
#include "IRGenModule.h"
#include <map>

using namespace swift;
using namespace irgen;

// FIXME: Everything here brazenly assumes little-endian-ness.

static llvm::Value *forcePayloadValue(EnumPayload::LazyValue &value) {
  if (auto v = value.dyn_cast<llvm::Value *>())
    return v;
  
  auto null = llvm::Constant::getNullValue(value.dyn_cast<llvm::Type*>());
  value = null;
  return null;
}

static llvm::Type *getPayloadType(EnumPayload::LazyValue value) {
  if (auto t = value.dyn_cast<llvm::Type *>())
    return t;
  
  return value.dyn_cast<llvm::Value *>()->getType();
}

EnumPayload EnumPayload::zero(IRGenModule &IGM, EnumPayloadSchema schema) {
  // We don't need to create any values yet; they can be filled in when
  // real values are inserted.
  EnumPayload result;
  schema.forEachType(IGM, [&](llvm::Type *type) {
    result.PayloadValues.push_back(type);
  });
  return result;
}

EnumPayload EnumPayload::fromBitPattern(IRGenModule &IGM,
                                        APInt bitPattern,
                                        EnumPayloadSchema schema) {
  EnumPayload result;
  
  schema.forEachType(IGM, [&](llvm::Type *type) {
    unsigned bitSize = IGM.DataLayout.getTypeSizeInBits(type);

    llvm::IntegerType *intTy
      = llvm::IntegerType::get(IGM.getLLVMContext(), bitSize);
    
    // Take some bits off of the bottom of the pattern.
    auto bits = bitPattern.zextOrTrunc(bitSize);
    auto val = llvm::ConstantInt::get(intTy, bits);
    if (val->getType() != type) {
      if (type->isPointerTy())
        val = llvm::ConstantExpr::getIntToPtr(val, type);
      else
        val = llvm::ConstantExpr::getBitCast(val, type);
    }
    
    result.PayloadValues.push_back(val);
    
    // Shift the remaining bits down.
    bitPattern = bitPattern.lshr(bitSize);
  });
    
  return result;
}

// Fn: void(LazyValue &payloadValue, unsigned payloadBitWidth,
//          unsigned payloadValueOffset, unsigned valueBitWidth,
//          unsigned valueOffset)
template<typename Fn>
static void withValueInPayload(IRGenFunction &IGF,
                               const EnumPayload &payload,
                               llvm::Type *valueType,
                               unsigned payloadOffset,
                               Fn &&f) {
  auto &DataLayout = IGF.IGM.DataLayout;
  int valueBitWidth = DataLayout.getTypeSizeInBits(valueType);
  
  // Find the elements we need to touch.
  // TODO: Linear search through the payload elements is lame.
  MutableArrayRef<EnumPayload::LazyValue> payloads = payload.PayloadValues;
  llvm::Type *payloadType;
  int payloadBitWidth;
  int valueOffset = 0, payloadValueOffset = payloadOffset;
  for (;;) {
    payloadType = getPayloadType(payloads.front());
    payloadBitWidth = IGF.IGM.DataLayout.getTypeSizeInBits(payloadType);
    
    // Does this element overlap the area we need to touch?
    if (payloadValueOffset < payloadBitWidth) {
      // See how much of the value we can fit here.
      int valueChunkWidth = payloadBitWidth - payloadValueOffset;
      valueChunkWidth = std::min(valueChunkWidth, valueBitWidth - valueOffset);
    
      f(payloads.front(),
        payloadBitWidth, payloadValueOffset,
        valueBitWidth, valueOffset);
      
      // If we used the entire value, we're done.
      valueOffset += valueChunkWidth;
      if (valueOffset >= valueBitWidth)
        return;
    }
    
    payloadValueOffset = std::max(payloadValueOffset - payloadBitWidth, 0);
    payloads = payloads.slice(1);
  }
}

void EnumPayload::insertValue(IRGenFunction &IGF, llvm::Value *value,
                              unsigned payloadOffset) {
  withValueInPayload(IGF, *this, value->getType(), payloadOffset,
    [&](LazyValue &payloadValue,
        unsigned payloadBitWidth,
        unsigned payloadValueOffset,
        unsigned valueBitWidth,
        unsigned valueOffset) {
      auto payloadType = getPayloadType(payloadValue);
      // See if the value matches the payload type exactly. In this case we
      // don't need to do any work to use the value.
      if (payloadValueOffset == 0 && valueOffset == 0) {
        if (value->getType() == payloadType) {
          payloadValue = value;
          return;
        }
        // If only the width matches exactly, we can still do a bitcast.
        if (payloadBitWidth == valueBitWidth) {
          auto bitcast = IGF.Builder.CreateBitOrPointerCast(value, payloadType);
          payloadValue = bitcast;
          return;
        }
      }
      
      // Select out the chunk of the value to merge with the existing payload.
      llvm::Value *subvalue = value;
      
      auto valueIntTy =
        llvm::IntegerType::get(IGF.IGM.getLLVMContext(), valueBitWidth);
      auto payloadIntTy =
        llvm::IntegerType::get(IGF.IGM.getLLVMContext(), payloadBitWidth);
      auto payloadTy = getPayloadType(payloadValue);
      subvalue = IGF.Builder.CreateBitOrPointerCast(subvalue, valueIntTy);
      if (valueOffset > 0)
        subvalue = IGF.Builder.CreateLShr(subvalue,
                               llvm::ConstantInt::get(valueIntTy, valueOffset));
      subvalue = IGF.Builder.CreateZExtOrTrunc(subvalue, payloadIntTy);
      if (payloadValueOffset > 0)
        subvalue = IGF.Builder.CreateShl(subvalue,
                      llvm::ConstantInt::get(payloadIntTy, payloadValueOffset));
      
      // If there hasn't yet been a value stored here, we can use the adjusted
      // value directly.
      if (payloadValue.is<llvm::Type *>()) {
        payloadValue = IGF.Builder.CreateBitOrPointerCast(subvalue, payloadTy);
      }
      // Otherwise, bitwise-or it in, brazenly assuming there are zeroes
      // underneath.
      else {
        // TODO: This creates a bunch of bitcasting noise for non-integer
        // payload fields.
        auto lastValue = payloadValue.get<llvm::Value *>();
        lastValue = IGF.Builder.CreateBitOrPointerCast(lastValue, payloadIntTy);
        lastValue = IGF.Builder.CreateOr(lastValue, subvalue);
        payloadValue = IGF.Builder.CreateBitOrPointerCast(lastValue, payloadTy);
      }
    });
}

llvm::Value *EnumPayload::extractValue(IRGenFunction &IGF, llvm::Type *type,
                                       unsigned payloadOffset) const {
  llvm::Value *result = nullptr;
  withValueInPayload(IGF, *this, type, payloadOffset,
    [&](LazyValue &payloadValue,
        unsigned payloadBitWidth,
        unsigned payloadValueOffset,
        unsigned valueBitWidth,
        unsigned valueOffset) {
      auto payloadType = getPayloadType(payloadValue);
      // If the desired type matches the payload slot exactly, we don't need
      // to do anything.
      if (payloadValueOffset == 0 && valueOffset == 0) {
        if (type == payloadType) {
          result = forcePayloadValue(payloadValue);
          return;
        }
        // If only the width matches exactly, do a bitcast.
        if (payloadBitWidth == valueBitWidth) {
          result =
            IGF.Builder.CreateBitOrPointerCast(forcePayloadValue(payloadValue),
                                               type);
          return;
        }
      }
      
      // Integrate the chunk of payload into the result value.
      auto value = forcePayloadValue(payloadValue);
      auto valueIntTy =
        llvm::IntegerType::get(IGF.IGM.getLLVMContext(), valueBitWidth);
      auto payloadIntTy =
        llvm::IntegerType::get(IGF.IGM.getLLVMContext(), payloadBitWidth);

      value = IGF.Builder.CreateBitOrPointerCast(value, payloadIntTy);
      if (payloadValueOffset > 0)
        value = IGF.Builder.CreateLShr(value,
                  llvm::ConstantInt::get(value->getType(), payloadValueOffset));
      if (valueBitWidth > payloadBitWidth)
        value = IGF.Builder.CreateZExt(value, valueIntTy);
      if (valueOffset > 0)
        value = IGF.Builder.CreateShl(value,
                         llvm::ConstantInt::get(value->getType(), valueOffset));
      if (valueBitWidth < payloadBitWidth)
        value = IGF.Builder.CreateTrunc(value, valueIntTy);
      
      if (!result)
        result = value;
      else
        result = IGF.Builder.CreateOr(result, value);
    });
  return IGF.Builder.CreateBitOrPointerCast(result, type);
}

EnumPayload EnumPayload::fromExplosion(IRGenModule &IGM,
                                       Explosion &in, EnumPayloadSchema schema){
  EnumPayload result;
  
  schema.forEachType(IGM, [&](llvm::Type *type) {
    auto next = in.claimNext();
    assert(next->getType() == type && "explosion doesn't match payload schema");
    result.PayloadValues.push_back(next);
  });
  
  return result;
}

void EnumPayload::explode(IRGenModule &IGM, Explosion &out) const {
  for (LazyValue &value : PayloadValues) {
    out.add(forcePayloadValue(value));
  }
}

void EnumPayload::packIntoEnumPayload(IRGenFunction &IGF,
                                      EnumPayload &outerPayload,
                                      unsigned bitOffset) const {
  auto &DL = IGF.IGM.DataLayout;
  for (auto &value : PayloadValues) {
    auto v = forcePayloadValue(value);
    outerPayload.insertValue(IGF, v, bitOffset);
    bitOffset += DL.getTypeSizeInBits(v->getType());
  }
}

EnumPayload EnumPayload::unpackFromEnumPayload(IRGenFunction &IGF,
                                        const EnumPayload &outerPayload,
                                        unsigned bitOffset,
                                        EnumPayloadSchema schema) {
  EnumPayload result;
  auto &DL = IGF.IGM.DataLayout;
  schema.forEachType(IGF.IGM, [&](llvm::Type *type) {
    auto v = outerPayload.extractValue(IGF, type, bitOffset);
    result.PayloadValues.push_back(v);
    bitOffset += DL.getTypeSizeInBits(type);
  });
  return result;
}

static llvm::Type *getPayloadStorageType(IRGenModule &IGM,
                                         const EnumPayload &payload) {
  if (payload.StorageType)
    return payload.StorageType;

  if (payload.PayloadValues.size() == 1) {
    payload.StorageType = getPayloadType(payload.PayloadValues.front());
    return payload.StorageType;
  }
  
  SmallVector<llvm::Type *, 2> elementTypes;
  for (auto value : payload.PayloadValues) {
    elementTypes.push_back(getPayloadType(value));
  }
  
  payload.StorageType = llvm::StructType::get(IGM.getLLVMContext(),
                                              elementTypes);
  return payload.StorageType;
}

EnumPayload EnumPayload::load(IRGenFunction &IGF, Address address,
                              EnumPayloadSchema schema) {
  EnumPayload result = EnumPayload::zero(IGF.IGM, schema);
  if (result.PayloadValues.empty())
    return result;
  
  auto storageTy = getPayloadStorageType(IGF.IGM, result);
  address = IGF.Builder.CreateBitCast(address, storageTy->getPointerTo());
  
  if (result.PayloadValues.size() == 1) {
    result.PayloadValues.front() = IGF.Builder.CreateLoad(address);
  } else {
    Size offset(0);
    for (unsigned i : indices(result.PayloadValues)) {
      auto &value = result.PayloadValues[i];
      auto member = IGF.Builder.CreateStructGEP(address, i, offset);
      auto loadedValue = IGF.Builder.CreateLoad(member);
      value = loadedValue;
      offset += Size(IGF.IGM.DataLayout.getTypeAllocSize(loadedValue->getType()));
    }
  }
  
  return result;
}

void EnumPayload::store(IRGenFunction &IGF, Address address) const {
  if (PayloadValues.empty())
    return;

  auto storageTy = getPayloadStorageType(IGF.IGM, *this);
  address = IGF.Builder.CreateBitCast(address, storageTy->getPointerTo());
  
  if (PayloadValues.size() == 1) {
    IGF.Builder.CreateStore(forcePayloadValue(PayloadValues.front()), address);
    return;
  } else {
    Size offset(0);
    for (unsigned i : indices(PayloadValues)) {
      auto &value = PayloadValues[i];
      auto member = IGF.Builder.CreateStructGEP(address, i, offset);
      auto valueToStore = forcePayloadValue(value);
      IGF.Builder.CreateStore(valueToStore, member);
      offset += Size(IGF.IGM.DataLayout
                       .getTypeAllocSize(valueToStore->getType()));
    }
  }
}

namespace {
struct ult {
  bool operator()(const APInt &a, const APInt &b) const {
    return a.ult(b);
  }
};
}

static void emitSubSwitch(IRGenFunction &IGF,
                    MutableArrayRef<EnumPayload::LazyValue> values,
                    APInt mask,
                    MutableArrayRef<std::pair<APInt, llvm::BasicBlock *>> cases,
                    SwitchDefaultDest dflt) {
recur:
  assert(!values.empty() && "didn't exit out when exhausting all values?!");
  
  assert(!cases.empty() && "switching with no cases?!");
  
  auto &DL = IGF.IGM.DataLayout;
  auto &pv = values.front();
  values = values.slice(1);
  auto payloadTy = getPayloadType(pv);
  unsigned size = DL.getTypeSizeInBits(payloadTy);
  
  // Grab a chunk of the mask.
  auto maskPiece = mask.zextOrTrunc(size);
  mask = mask.lshr(size);
  
  // If the piece is zero, this doesn't affect the switch. We can just move
  // forward and recur.
  if (maskPiece == 0) {
    for (auto &casePair : cases)
      casePair.first = casePair.first.lshr(size);
    goto recur;
  }
  
  // Force the value we will test.
  auto v = forcePayloadValue(pv);
  auto payloadIntTy = llvm::IntegerType::get(IGF.IGM.getLLVMContext(), size);
  
  // Need to coerce to integer for 'icmp eq' if it's not already an integer
  // or pointer. (Switching or masking will also require a cast to integer.)
  if (!isa<llvm::IntegerType>(v->getType())
      && !isa<llvm::PointerType>(v->getType()))
    v = IGF.Builder.CreateBitOrPointerCast(v, payloadIntTy);
  
  // Apply the mask if it's interesting.
  if (!maskPiece.isAllOnesValue()) {
    v = IGF.Builder.CreateBitOrPointerCast(v, payloadIntTy);
    auto maskConstant = llvm::ConstantInt::get(payloadIntTy, maskPiece);
    v = IGF.Builder.CreateAnd(v, maskConstant);
  }
  
  // Gather the values we will switch over for this payload chunk.
  // FIXME: std::map is lame. Should hash APInts.
  std::map<APInt, SmallVector<std::pair<APInt, llvm::BasicBlock*>, 2>, ult>
    subCases;
  
  for (auto casePair : cases) {
    // Grab a chunk of the value.
    auto valuePiece = casePair.first.zextOrTrunc(size);
    // Index the case according to this chunk.
    subCases[valuePiece].push_back({std::move(casePair.first).lshr(size),
                                    casePair.second});
  }
  
  bool needsAdditionalCases = !values.empty() && mask != 0;
  SmallVector<std::pair<llvm::BasicBlock *, decltype(cases)>, 2> recursiveCases;
  
  auto blockForCases
    = [&](MutableArrayRef<std::pair<APInt, llvm::BasicBlock*>> cases)
        -> llvm::BasicBlock *
    {
      // If we need to recur, emit a new block.
      if (needsAdditionalCases) {
        auto newBB = IGF.createBasicBlock("");
        recursiveCases.push_back({newBB, cases});
        return newBB;
      }
      // Otherwise, we can jump directly to the ultimate destination.
      assert(cases.size() == 1 && "more than one case for final destination?!");
      return cases.front().second;
    };
  
  // If there's only one case, do a cond_br.
  if (subCases.size() == 1) {
    auto &subCase = *subCases.begin();
    llvm::BasicBlock *block = blockForCases(subCase.second);
    // If the default case is unreachable, we don't need to conditionally
    // branch.
    if (dflt.getInt()) {
      IGF.Builder.CreateBr(block);
      goto next;
    }
  
    auto &valuePiece = subCase.first;
    llvm::Value *valueConstant = llvm::ConstantInt::get(payloadIntTy,
                                                        valuePiece);
    valueConstant = IGF.Builder.CreateBitOrPointerCast(valueConstant,
                                                       v->getType());
    auto cmp = IGF.Builder.CreateICmpEQ(v, valueConstant);
    IGF.Builder.CreateCondBr(cmp, block, dflt.getPointer());
    goto next;
  }
  
  // Otherwise, do a switch.
  {
    v = IGF.Builder.CreateBitOrPointerCast(v, payloadIntTy);
    auto swi = IGF.Builder.CreateSwitch(v, dflt.getPointer(), subCases.size());
    
    for (auto &subCase : subCases) {
      auto &valuePiece = subCase.first;
      auto valueConstant = llvm::ConstantInt::get(IGF.IGM.getLLVMContext(),
                                                  valuePiece);

      swi->addCase(valueConstant, blockForCases(subCase.second));
    }
  }
  
next:
  // Emit the recursive cases.
  for (auto &recursive : recursiveCases) {
    IGF.Builder.emitBlock(recursive.first);
    emitSubSwitch(IGF, values, mask, recursive.second, dflt);
  }
}

void EnumPayload::emitSwitch(IRGenFunction &IGF,
                           APInt mask,
                           ArrayRef<std::pair<APInt, llvm::BasicBlock *>> cases,
                           SwitchDefaultDest dflt) const {
  // If there's only one case to test, do a simple compare and branch.
  if (cases.size() == 1) {
    // If the default case is unreachable, don't bother branching at all.
    if (dflt.getInt()) {
      IGF.Builder.CreateBr(cases[0].second);
      return;
    }
  
    auto *cmp = emitCompare(IGF, mask, cases[0].first);
    IGF.Builder.CreateCondBr(cmp, cases[0].second, dflt.getPointer());
    return;
  }

  // Otherwise, break down the decision tree.
  SmallVector<std::pair<APInt, llvm::BasicBlock*>, 4> mutableCases
    (cases.begin(), cases.end());
  
  emitSubSwitch(IGF, PayloadValues, mask, mutableCases, dflt);
  
  assert(IGF.Builder.hasPostTerminatorIP());
}

llvm::Value *
EnumPayload::emitCompare(IRGenFunction &IGF, APInt mask, APInt value) const {
  // Succeed trivially for an empty payload, or if the payload is masked
  // out completely.
  if (PayloadValues.empty() || mask == 0)
    return llvm::ConstantInt::get(IGF.IGM.Int1Ty, 1U);

  assert((~mask & value) == 0
         && "value has masked out bits set?!");
  
  auto &DL = IGF.IGM.DataLayout;
  llvm::Value *condition = nullptr;
  for (auto &pv : PayloadValues) {
    auto v = forcePayloadValue(pv);
    unsigned size = DL.getTypeSizeInBits(v->getType());

    // Break off a piece of the mask and value.
    auto maskPiece = mask.zextOrTrunc(size);
    auto valuePiece = value.zextOrTrunc(size);
    
    mask = mask.lshr(size);
    value = value.lshr(size);
    
    // If this piece is zero, it doesn't affect the comparison.
    if (maskPiece == 0)
      continue;
    
    // Apply the mask and test.
    bool isMasked = !maskPiece.isAllOnesValue();
    auto intTy = llvm::IntegerType::get(IGF.IGM.getLLVMContext(), size);
    // Need to bitcast to an integer in order to use 'icmp eq' if the piece
    // isn't already an int or pointer, or in order to apply a mask.
    if (isMasked
        || (!isa<llvm::IntegerType>(v->getType())
            && !isa<llvm::PointerType>(v->getType())))
      v = IGF.Builder.CreateBitOrPointerCast(v, intTy);
    
    if (isMasked) {
      auto maskConstant = llvm::ConstantInt::get(intTy, maskPiece);
      v = IGF.Builder.CreateAnd(v, maskConstant);
    }
    
    llvm::Value *valueConstant = llvm::ConstantInt::get(intTy, valuePiece);
    valueConstant = IGF.Builder.CreateBitOrPointerCast(valueConstant,
                                                       v->getType());
    auto cmp = IGF.Builder.CreateICmpEQ(v, valueConstant);
    if (!condition)
      condition = cmp;
    else
      condition = IGF.Builder.CreateAnd(condition, cmp);
  }
  
  // We should have handled the cases where there are no significant conditions
  // in the early exit.
  assert(condition && "no significant condition?!");
  return condition;
}

void
EnumPayload::emitApplyAndMask(IRGenFunction &IGF, APInt mask) {
  // Early exit if the mask has no effect.
  if (mask.isAllOnesValue())
    return;

  auto &DL = IGF.IGM.DataLayout;
  for (auto &pv : PayloadValues) {
    auto payloadTy = getPayloadType(pv);
    unsigned size = DL.getTypeSizeInBits(payloadTy);

    // Break off a chunk of the mask.
    auto maskPiece = mask.zextOrTrunc(size);
    mask = mask.lshr(size);
    
    // If this piece is all ones, it has no effect.
    if (maskPiece.isAllOnesValue())
      continue;

    // If the payload value is vacant, the mask can't change it.
    if (pv.is<llvm::Type *>())
      continue;

    // If this piece is zero, it wipes out the chunk entirely, and we can
    // drop it.
    if (maskPiece == 0) {
      pv = payloadTy;
      continue;
    }
    
    // Otherwise, apply the mask to the existing value.
    auto v = pv.get<llvm::Value*>();
    auto payloadIntTy = llvm::IntegerType::get(IGF.IGM.getLLVMContext(), size);
    auto maskConstant = llvm::ConstantInt::get(payloadIntTy, maskPiece);
    v = IGF.Builder.CreateBitOrPointerCast(v, payloadIntTy);
    v = IGF.Builder.CreateAnd(v, maskConstant);
    v = IGF.Builder.CreateBitOrPointerCast(v, payloadTy);
    pv = v;
  }
}

void
EnumPayload::emitApplyOrMask(IRGenFunction &IGF, APInt mask) {
  // Early exit if the mask has no effect.
  if (mask == 0)
    return;

  auto &DL = IGF.IGM.DataLayout;
  for (auto &pv : PayloadValues) {
    auto payloadTy = getPayloadType(pv);
    unsigned size = DL.getTypeSizeInBits(payloadTy);

    // Break off a chunk of the mask.
    auto maskPiece = mask.zextOrTrunc(size);
    mask = mask.lshr(size);

    // If this piece is zero, it has no effect.
    if (maskPiece == 0)
      continue;
    
    auto payloadIntTy = llvm::IntegerType::get(IGF.IGM.getLLVMContext(), size);
    auto maskConstant = llvm::ConstantInt::get(payloadIntTy, maskPiece);
    
    // If the payload value is vacant, or the mask is all ones,
    // we can adopt the mask value directly.
    if (pv.is<llvm::Type *>() || maskPiece.isAllOnesValue()) {
      pv = IGF.Builder.CreateBitOrPointerCast(maskConstant, payloadTy);
      continue;
    }
    
    // Otherwise, apply the mask to the existing value.
    auto v = pv.get<llvm::Value*>();
    v = IGF.Builder.CreateBitOrPointerCast(v, payloadIntTy);
    v = IGF.Builder.CreateOr(v, maskConstant);
    v = IGF.Builder.CreateBitOrPointerCast(v, payloadTy);
    pv = v;
  }
}

void
EnumPayload::emitApplyOrMask(IRGenFunction &IGF,
                             EnumPayload mask) {
  unsigned count = PayloadValues.size();
  assert(count == mask.PayloadValues.size());

  auto &DL = IGF.IGM.DataLayout;
  for (unsigned i = 0; i < count; i++ ) {
    auto payloadTy = getPayloadType(PayloadValues[i]);
    unsigned size = DL.getTypeSizeInBits(payloadTy);

    auto payloadIntTy = llvm::IntegerType::get(IGF.IGM.getLLVMContext(), size);

    if (mask.PayloadValues[i].is<llvm::Type *>()) {
      // We're ORing with zero, do nothing
    } else if (PayloadValues[i].is<llvm::Type *>()) {
      PayloadValues[i] = mask.PayloadValues[i];
    } else {
      auto v1 = IGF.Builder.CreateBitOrPointerCast(
          PayloadValues[i].get<llvm::Value *>(),
          payloadIntTy);

      auto v2 = IGF.Builder.CreateBitOrPointerCast(
          mask.PayloadValues[i].get<llvm::Value *>(),
          payloadIntTy);

      PayloadValues[i] = IGF.Builder.CreateBitOrPointerCast(
          IGF.Builder.CreateOr(v1, v2),
          payloadTy);
    }
  }
}

llvm::Value *
EnumPayload::emitGatherSpareBits(IRGenFunction &IGF,
                                 const SpareBitVector &spareBits,
                                 unsigned firstBitOffset,
                                 unsigned bitWidth) const {
  auto &DL = IGF.IGM.DataLayout;
  unsigned payloadOffset = 0;
  llvm::Value *spareBitValue = nullptr;
  auto destTy = llvm::IntegerType::get(IGF.IGM.getLLVMContext(), bitWidth);
  for (auto &pv : PayloadValues) {
    // If this value is zero, it has nothing to add to the spare bits.
    auto v = pv.dyn_cast<llvm::Value*>();
    if (!v) {
      payloadOffset += DL.getTypeSizeInBits(pv.get<llvm::Type*>());
      continue;
    }
    
    unsigned size = DL.getTypeSizeInBits(v->getType());
    // Slice the spare bit vector.
    // FIXME: this is inefficient.
    auto spareBitsPart = SpareBitVector::getConstant(size, false);
    unsigned numBitsInPart = 0;
    for (unsigned i = 0; i < size; ++i)
      if (spareBits[payloadOffset + i]) {
        spareBitsPart.setBit(i);
        ++numBitsInPart;
      }
    
    payloadOffset += size;
    
    // If there were no spare bits in this part, it has nothing to add.
    if (numBitsInPart == 0)
      continue;
    
    if (firstBitOffset >= bitWidth)
      break;

    // Get the spare bits from this part.
    auto bits = irgen::emitGatherSpareBits(IGF, spareBitsPart,
                                           v, firstBitOffset, bitWidth);
    firstBitOffset += numBitsInPart;
    
    // Accumulate it into the full set.
    if (!spareBitValue)
      spareBitValue = bits;
    else
      spareBitValue = IGF.Builder.CreateOr(spareBitValue, bits);
  }
  if (!spareBitValue)
    return llvm::ConstantInt::get(destTy, 0);
  return spareBitValue;
}