swift-mirror/lib/SILPasses/ConstantPropagation.cpp

//===--- ConstantPropagation.cpp - Constant fold and diagnose overflows ---===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 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
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "constant-propagation"
#include "swift/Subsystems.h"
#include "swift/AST/Diagnostics.h"
#include "swift/SIL/SILBuilder.h"
#include "swift/SILPasses/Utils/Local.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/Debug.h"
using namespace swift;

STATISTIC(NumInstFolded, "Number of constant folded instructions");

template<typename...T, typename...U>
static void diagnose(ASTContext &Context, SourceLoc loc, Diag<T...> diag,
                     U &&...args) {
  Context.Diags.diagnose(loc,
                         diag, std::forward<U>(args)...);
}

/// \brief Construct (int, overflow) result tuple.
static SILInstruction *constructResultWithOverflowTuple(ApplyInst *AI,
                                                        APInt Res,
                                                        bool Overflow) {
  // Get the SIL subtypes of the returned tuple type.
  SILType FuncResType = AI->getSubstCalleeType()->getResult().getSILType();
  assert(FuncResType.castTo<TupleType>()->getNumElements() == 2);
  SILType ResTy1 = FuncResType.getTupleElementType(0);
  SILType ResTy2 = FuncResType.getTupleElementType(1);

  // Construct the folded instruction - a tuple of two literals, the
  // result and overflow.
  SILBuilder B(AI);
  SILLocation Loc = AI->getLoc();
  SILValue Result[] = {
    B.createIntegerLiteral(Loc, ResTy1, Res),
    B.createIntegerLiteral(Loc, ResTy2, Overflow)
  };
  return B.createTuple(Loc, FuncResType, Result);
}

/// \brief Fold arithmetic intrinsics with overflow.
static SILInstruction *constantFoldBinaryWithOverflow(ApplyInst *AI,
                                                      llvm::Intrinsic::ID ID,
                                                      bool ReportOverflow,
                                                      bool &ResultsInError) {
  OperandValueArrayRef Args = AI->getArguments();
  assert(Args.size() >= 2);

  // Check if both arguments are literals.
  IntegerLiteralInst *Op1 = dyn_cast<IntegerLiteralInst>(Args[0]);
  IntegerLiteralInst *Op2 = dyn_cast<IntegerLiteralInst>(Args[1]);

  // We cannot fold a builtin if one of the arguments is not a constant.
  if (!Op1 || !Op2)
    return nullptr;

  // Calculate the result.
  APInt LHSInt = Op1->getValue();
  APInt RHSInt = Op2->getValue();
  APInt Res;
  bool Overflow;
  bool Signed = false;
  StringRef Operator = "+";

  switch (ID) {
  default: llvm_unreachable("Invalid case");
  case llvm::Intrinsic::sadd_with_overflow:
    Res = LHSInt.sadd_ov(RHSInt, Overflow);
    Signed = true;
    break;
  case llvm::Intrinsic::uadd_with_overflow:
    Res = LHSInt.uadd_ov(RHSInt, Overflow);
    break;
  case llvm::Intrinsic::ssub_with_overflow:
    Res = LHSInt.ssub_ov(RHSInt, Overflow);
    Operator = "-";
    Signed = true;
    break;
  case llvm::Intrinsic::usub_with_overflow:
    Res = LHSInt.usub_ov(RHSInt, Overflow);
    Operator = "-";
    break;
  case llvm::Intrinsic::smul_with_overflow:
    Res = LHSInt.smul_ov(RHSInt, Overflow);
    Operator = "*";
    Signed = true;
    break;
  case llvm::Intrinsic::umul_with_overflow:
    Res = LHSInt.umul_ov(RHSInt, Overflow);
    Operator = "*";
    break;
  }

  // If we can statically determine that the operation overflows,
  // warn about it.
  if (Overflow && ReportOverflow) {
    // Try to infer the type of the constant expression that the user operates
    // on. If the intrinsic was lowered from a call to a function that takes
    // two arguments of the same type, use the type of the LHS argument.
    // This would detect '+'/'+=' and such.
    Type OpType;
    SILLocation Loc = AI->getLoc();
    const ApplyExpr *CE = Loc.getAsASTNode<ApplyExpr>();
    if (CE) {
      const TupleExpr *Args = dyn_cast_or_null<TupleExpr>(CE->getArg());
      if (Args && Args->getNumElements() == 2) {
        CanType LHSTy = Args->getElement(0)->getType()->getCanonicalType();
        CanType RHSTy = Args->getElement(0)->getType()->getCanonicalType();
        if (LHSTy == RHSTy)
          OpType = Args->getElement(1)->getType();
      }
    }

    if (!OpType.isNull()) {
      diagnose(AI->getModule().getASTContext(),
               Loc.getSourceLoc(),
               diag::arithmetic_operation_overflow,
               LHSInt.toString(/*Radix*/ 10, Signed),
               Operator,
               RHSInt.toString(/*Radix*/ 10, Signed),
               OpType);
      ResultsInError = true;
    } else {
      // If we cannot get the type info in an expected way, describe the type.
      diagnose(AI->getModule().getASTContext(),
               Loc.getSourceLoc(),
               diag::arithmetic_operation_overflow_generic_type,
               LHSInt.toString(/*Radix*/ 10, Signed),
               Operator,
               RHSInt.toString(/*Radix*/ 10, Signed),
               Signed,
               LHSInt.getBitWidth());

      ResultsInError = true;
    }
  }

  return constructResultWithOverflowTuple(AI, Res, Overflow);
}

static SILInstruction *constantFoldOverflowBuiltin(ApplyInst *AI,
                                                   BuiltinValueKind ID,
                                                   bool &ResultsInError) {
  OperandValueArrayRef Args = AI->getArguments();
  IntegerLiteralInst *ShouldReportFlag = dyn_cast<IntegerLiteralInst>(Args[2]);
  return constantFoldBinaryWithOverflow(AI,
           getLLVMIntrinsicIDForBuiltinWithOverflow(ID),
           ShouldReportFlag && (ShouldReportFlag->getValue() == 1),
           ResultsInError);
}

static SILInstruction *constantFoldIntrinsic(ApplyInst *AI,
                                             llvm::Intrinsic::ID ID,
                                             bool &ResultsInError) {
  switch (ID) {
  default: break;
  case llvm::Intrinsic::sadd_with_overflow:
  case llvm::Intrinsic::uadd_with_overflow:
  case llvm::Intrinsic::ssub_with_overflow:
  case llvm::Intrinsic::usub_with_overflow:
  case llvm::Intrinsic::smul_with_overflow:
  case llvm::Intrinsic::umul_with_overflow:
    return constantFoldBinaryWithOverflow(AI, ID,
                                          /* ReportOverflow */ false,
                                          ResultsInError);
  }
  return nullptr;
}

static SILInstruction *constantFoldCompareBuiltin(ApplyInst *AI,
                                                  BuiltinValueKind ID) {
  OperandValueArrayRef Args = AI->getArguments();

  // Fold for integer constant arguments.
  IntegerLiteralInst *LHS = dyn_cast<IntegerLiteralInst>(Args[0]);
  IntegerLiteralInst *RHS = dyn_cast<IntegerLiteralInst>(Args[1]);
  if (LHS && RHS) {
    APInt V1 = LHS->getValue();
    APInt V2 = RHS->getValue();
    APInt Res;
    switch (ID) {
    default: llvm_unreachable("Invalid integer compare kind");
    case BuiltinValueKind::ICMP_EQ:  Res = V1 == V2; break;
    case BuiltinValueKind::ICMP_NE:  Res = V1 != V2; break;
    case BuiltinValueKind::ICMP_SLT: Res = V1.slt(V2); break;
    case BuiltinValueKind::ICMP_SGT: Res = V1.sgt(V2); break;
    case BuiltinValueKind::ICMP_SLE: Res = V1.sle(V2); break;
    case BuiltinValueKind::ICMP_SGE: Res = V1.sge(V2); break;
    case BuiltinValueKind::ICMP_ULT: Res = V1.ult(V2); break;
    case BuiltinValueKind::ICMP_UGT: Res = V1.ugt(V2); break;
    case BuiltinValueKind::ICMP_ULE: Res = V1.ule(V2); break;
    case BuiltinValueKind::ICMP_UGE: Res = V1.uge(V2); break;
    }
    SILBuilder B(AI);
    return B.createIntegerLiteral(AI->getLoc(), AI->getType(), Res);
  }

  return nullptr;
}

static std::pair<bool, bool> getTypeSigndness(const BuiltinInfo &Builtin) {
  bool SrcTySigned =
  (Builtin.ID == BuiltinValueKind::SToSCheckedTrunc ||
   Builtin.ID == BuiltinValueKind::SToUCheckedTrunc ||
   Builtin.ID == BuiltinValueKind::SUCheckedConversion);

  bool DstTySigned =
  (Builtin.ID == BuiltinValueKind::SToSCheckedTrunc ||
   Builtin.ID == BuiltinValueKind::UToSCheckedTrunc ||
   Builtin.ID == BuiltinValueKind::USCheckedConversion);

  return std::pair<bool, bool>(SrcTySigned, DstTySigned);
}

static SILInstruction *
constantFoldAndCheckIntegerConversions(ApplyInst *AI,
                                       const BuiltinInfo &Builtin,
                                       bool &ResultsInError) {
  assert(Builtin.ID == BuiltinValueKind::SToSCheckedTrunc ||
         Builtin.ID == BuiltinValueKind::UToUCheckedTrunc ||
         Builtin.ID == BuiltinValueKind::SToUCheckedTrunc ||
         Builtin.ID == BuiltinValueKind::UToSCheckedTrunc ||
         Builtin.ID == BuiltinValueKind::SUCheckedConversion ||
         Builtin.ID == BuiltinValueKind::USCheckedConversion);

  // Check if we are converting a constant integer.
  OperandValueArrayRef Args = AI->getArguments();
  IntegerLiteralInst *V = dyn_cast<IntegerLiteralInst>(Args[0]);
  if (!V)
    return nullptr;
  APInt SrcVal = V->getValue();

  // Get source type and bit width.
  Type SrcTy = Builtin.Types[0];
  uint32_t SrcBitWidth =
    Builtin.Types[0]->castTo<BuiltinIntegerType>()->getGreatestWidth();

  // Compute the destination (for SrcBitWidth < DestBitWidth) and enough info
  // to check for overflow.
  APInt Result;
  bool OverflowError;
  Type DstTy;

  // Process conversions signed <-> unsigned for same size integers.
  if (Builtin.ID == BuiltinValueKind::SUCheckedConversion ||
      Builtin.ID == BuiltinValueKind::USCheckedConversion) {
    DstTy = SrcTy;
    Result = SrcVal;
    // Report an error if the sign bit is set.
    OverflowError = SrcVal.isNegative();

  // Process truncation from unsigned to signed.
  } else if (Builtin.ID != BuiltinValueKind::UToSCheckedTrunc) {
    assert(Builtin.Types.size() == 2);
    DstTy = Builtin.Types[1];
    uint32_t DstBitWidth =
      DstTy->castTo<BuiltinIntegerType>()->getGreatestWidth();
    //     Result = trunc_IntFrom_IntTo(Val)
    //   For signed destination:
    //     sext_IntFrom(Result) == Val ? Result : overflow_error
    //   For signed destination:
    //     zext_IntFrom(Result) == Val ? Result : overflow_error
    Result = SrcVal.trunc(DstBitWidth);
    // Get the signedness of the destination.
    bool Signed = (Builtin.ID == BuiltinValueKind::SToSCheckedTrunc);
    APInt Ext = Signed ? Result.sext(SrcBitWidth) : Result.zext(SrcBitWidth);
    OverflowError = (SrcVal != Ext);

  // Process the rest of truncations.
  } else {
    assert(Builtin.Types.size() == 2);
    DstTy = Builtin.Types[1];
    uint32_t DstBitWidth =
      Builtin.Types[1]->castTo<BuiltinIntegerType>()->getGreatestWidth();
    // Compute the destination (for SrcBitWidth < DestBitWidth):
    //   Result = trunc_IntTo(Val)
    //   Trunc  = trunc_'IntTo-1bit'(Val)
    //   zext_IntFrom(Trunc) == Val ? Result : overflow_error
    Result = SrcVal.trunc(DstBitWidth);
    APInt TruncVal = SrcVal.trunc(DstBitWidth - 1);
    OverflowError = (SrcVal != TruncVal.zext(SrcBitWidth));
  }

  // Check for overflow.
  if (OverflowError) {
    SILLocation Loc = AI->getLoc();
    SILModule &M = AI->getModule();
    const ApplyExpr *CE = Loc.getAsASTNode<ApplyExpr>();
    Type UserSrcTy;
    Type UserDstTy;
    // Primitive heuristics to get the user-written type.
    // Eventually we might be able to use SILLocatuion (when it contains info
    // about inlined call chains).
    if (CE)
      if (const TupleType *RTy = CE->getArg()->getType()->getAs<TupleType>()) {
        if (RTy->getNumElements() == 1) {
          UserSrcTy = RTy->getElementType(0);
          UserDstTy = CE->getType();
        }
      }

    // Assume that we are converting from a literal if the Source size is
    // 2048. Is there a better way to identify conversions from literals?
    bool Literal = (SrcBitWidth == 2048);

    // FIXME: This will prevent hard error in cases the error is comming
    // from ObjC interoperability code. Currently, we treat NSUInteger as
    // Int.
    if (Loc.getSourceLoc().isInvalid()) {
      if (Literal)
        diagnose(M.getASTContext(), Loc.getSourceLoc(),
                 diag::integer_literal_overflow_warn,
                 UserDstTy.isNull() ? DstTy : UserDstTy);
      else
        diagnose(M.getASTContext(), Loc.getSourceLoc(),
                 diag::integer_conversion_overflow_warn,
                 UserSrcTy.isNull() ? SrcTy : UserSrcTy,
                 UserDstTy.isNull() ? DstTy : UserDstTy);

      ResultsInError = true;
      return nullptr;
    }

    // Report the overflow error.
    if (Literal) {
      // Try to print user-visible types if they are available.
      if (!UserDstTy.isNull()) {
        diagnose(M.getASTContext(), Loc.getSourceLoc(),
                 diag::integer_literal_overflow, UserDstTy);
      // Otherwise, print the Builtin Types.
      } else {
        bool SrcTySigned, DstTySigned;
        llvm::tie(SrcTySigned, DstTySigned) = getTypeSigndness(Builtin);
        diagnose(M.getASTContext(), Loc.getSourceLoc(),
                 diag::integer_literal_overflow_builtin_types,
                 DstTySigned, DstTy);
      }
    } else
      if (Builtin.ID == BuiltinValueKind::SUCheckedConversion) {
        diagnose(M.getASTContext(), Loc.getSourceLoc(),
                 diag::integer_conversion_sign_error,
                 UserDstTy.isNull() ? DstTy : UserDstTy);
      } else {
        // Try to print user-visible types if they are available.
        if (!UserSrcTy.isNull()) {
          diagnose(M.getASTContext(), Loc.getSourceLoc(),
                   diag::integer_conversion_overflow,
                   UserSrcTy, UserDstTy);

        // Otherwise, print the Builtin Types.
        } else {
          // Since builtin types are sign-agnostic, print the signdness
          // separately.
          bool SrcTySigned, DstTySigned;
          llvm::tie(SrcTySigned, DstTySigned) = getTypeSigndness(Builtin);
          diagnose(M.getASTContext(), Loc.getSourceLoc(),
                   diag::integer_conversion_overflow_builtin_types,
                   SrcTySigned, SrcTy, DstTySigned, DstTy);
        }
      }
    ResultsInError = true;
    return nullptr;
  }

  // The call to the builtin should be replaced with the constant value.
  return constructResultWithOverflowTuple(AI, Result, false);

}

static SILInstruction *constantFoldBuiltin(ApplyInst *AI,
                                           BuiltinFunctionRefInst *FR,
                                           bool &ResultsInError) {
  const IntrinsicInfo &Intrinsic = FR->getIntrinsicInfo();
  SILModule &M = AI->getModule();

  // If it's an llvm intrinsic, fold the intrinsic.
  if (Intrinsic.ID != llvm::Intrinsic::not_intrinsic)
    return constantFoldIntrinsic(AI, Intrinsic.ID, ResultsInError);

  // Otherwise, it should be one of the builin functions.
  OperandValueArrayRef Args = AI->getArguments();
  const BuiltinInfo &Builtin = M.getBuiltinInfo(FR->getReferencedFunction());

  switch (Builtin.ID) {
  default: break;

// Check and fold binary arithmetic with overflow.
#define BUILTIN(id, name, Attrs)
#define BUILTIN_BINARY_OPERATION_WITH_OVERFLOW(id, name, attrs, overload) \
  case BuiltinValueKind::id:
#include "swift/AST/Builtins.def"
    return constantFoldOverflowBuiltin(AI, Builtin.ID, ResultsInError);

// Fold comparison predicates.
#define BUILTIN(id, name, Attrs)
#define BUILTIN_BINARY_PREDICATE(id, name, attrs, overload) \
case BuiltinValueKind::id:
#include "swift/AST/Builtins.def"
      return constantFoldCompareBuiltin(AI, Builtin.ID);

  case BuiltinValueKind::Trunc:
  case BuiltinValueKind::ZExt:
  case BuiltinValueKind::SExt:
  case BuiltinValueKind::TruncOrBitCast:
  case BuiltinValueKind::ZExtOrBitCast:
  case BuiltinValueKind::SExtOrBitCast: {

    // We can fold if the value being cast is a constant.
    IntegerLiteralInst *V = dyn_cast<IntegerLiteralInst>(Args[0]);
    if (!V)
      return nullptr;

    // Get the cast result.
    Type SrcTy = Builtin.Types[0];
    Type DestTy = Builtin.Types.size() == 2 ? Builtin.Types[1] : Type();
    uint32_t SrcBitWidth = 
      SrcTy->castTo<BuiltinIntegerType>()->getGreatestWidth();
    uint32_t DestBitWidth =
      DestTy->castTo<BuiltinIntegerType>()->getGreatestWidth();

    APInt CastResV;
    if (SrcBitWidth == DestBitWidth) {
      CastResV = V->getValue();
    } else switch (Builtin.ID) {
      default : llvm_unreachable("Invalid case.");
      case BuiltinValueKind::Trunc:
      case BuiltinValueKind::TruncOrBitCast:
        CastResV = V->getValue().trunc(DestBitWidth);
        break;
      case BuiltinValueKind::ZExt:
      case BuiltinValueKind::ZExtOrBitCast:
        CastResV = V->getValue().zext(DestBitWidth);
        break;
      case BuiltinValueKind::SExt:
      case BuiltinValueKind::SExtOrBitCast:
        CastResV = V->getValue().sext(DestBitWidth);
        break;
    }

    // Add the literal instruction to represent the result of the cast.
    SILBuilder B(AI);
    return B.createIntegerLiteral(AI->getLoc(), AI->getType(), CastResV);
  }

  // Fold constant division operations and report div by zero.
  case BuiltinValueKind::SDiv:
  case BuiltinValueKind::ExactSDiv:
  case BuiltinValueKind::SRem:
  case BuiltinValueKind::UDiv:
  case BuiltinValueKind::ExactUDiv:
  case BuiltinValueKind::URem: {
    // Get the denominator.
    IntegerLiteralInst *Denom = dyn_cast<IntegerLiteralInst>(Args[1]);
    if (!Denom)
      return nullptr;
    APInt DenomVal = Denom->getValue();

    // Reoprt an error if the denominator is zero.
    if (DenomVal == 0) {
      diagnose(M.getASTContext(),
               AI->getLoc().getSourceLoc(),
               diag::division_by_zero);
      ResultsInError = true;
      return nullptr;
    }

    // Get the numerator.
    IntegerLiteralInst *Num = dyn_cast<IntegerLiteralInst>(Args[0]);
    if (!Num)
      return nullptr;
    APInt NumVal = Num->getValue();

    APInt ResVal;
    bool Overflowed = false;
    switch (Builtin.ID) {
    // We do not cover all the cases below - only the ones that are easily
    // computable for APInt.
    default : return nullptr;
    case BuiltinValueKind::SDiv:
      ResVal = NumVal.sdiv_ov(DenomVal, Overflowed);
      break;
    case BuiltinValueKind::SRem:
      ResVal = NumVal.srem(DenomVal);
      break;
    case BuiltinValueKind::UDiv:
      ResVal = NumVal.udiv(DenomVal);
      break;
    case BuiltinValueKind::URem:
      ResVal = NumVal.urem(DenomVal);
      break;
    }

    if (Overflowed) {
      diagnose(M.getASTContext(),
               AI->getLoc().getSourceLoc(),
               diag::division_overflow,
               NumVal.toString(/*Radix*/ 10, /*Signed*/true),
               "/",
               DenomVal.toString(/*Radix*/ 10, /*Signed*/true));
      ResultsInError = true;
      return nullptr;
    }

    // Add the literal instruction to represnet the result of the division.
    SILBuilder B(AI);
    return B.createIntegerLiteral(AI->getLoc(), AI->getType(), ResVal);
  }

  // Process special builtins that are designed to check for overflows in
  // integer conversions.
  case BuiltinValueKind::SToSCheckedTrunc:
  case BuiltinValueKind::UToUCheckedTrunc:
  case BuiltinValueKind::SToUCheckedTrunc:
  case BuiltinValueKind::UToSCheckedTrunc:
  case BuiltinValueKind::SUCheckedConversion:
  case BuiltinValueKind::USCheckedConversion: {
    return constantFoldAndCheckIntegerConversions(AI, Builtin, ResultsInError);
  }

  case BuiltinValueKind::IntToFPWithOverflow: {
    // Get the value. It should be a constant in most cases.
    // Note, this will not always be a constant, for example, when analyzing
    // _convertFromBuiltinIntegerLiteral function itself.
    IntegerLiteralInst *V = dyn_cast<IntegerLiteralInst>(Args[0]);
    if (!V)
      return nullptr;
    APInt SrcVal = V->getValue();
    Type DestTy = Builtin.Types[1];

    APFloat TruncVal(
        DestTy->castTo<BuiltinFloatType>()->getAPFloatSemantics());
    APFloat::opStatus ConversionStatus = TruncVal.convertFromAPInt(
        SrcVal, /*isSigned=*/true, APFloat::rmNearestTiesToEven);

    SILLocation Loc = AI->getLoc();
    const ApplyExpr *CE = Loc.getAsASTNode<ApplyExpr>();

    // Check for overflow.
    if (ConversionStatus & APFloat::opOverflow) {
      diagnose(M.getASTContext(), Loc.getSourceLoc(),
               diag::integer_literal_overflow,
               CE ? CE->getType() : DestTy);
      ResultsInError = true;
      return nullptr;
    }

    // The call to the builtin should be replaced with the constant value.
    SILBuilder B(AI);
    return B.createFloatLiteral(Loc, AI->getType(), TruncVal);
  }
  }
  return nullptr;
}

static SILValue constantFoldInstruction(SILInstruction &I,
                                        bool &ResultsInError) {
  // Constant fold function calls.
  if (ApplyInst *AI = dyn_cast<ApplyInst>(&I)) {
    // Constant fold calls to builtins.
    if (auto *FR = dyn_cast<BuiltinFunctionRefInst>(AI->getCallee())) {
      return constantFoldBuiltin(AI, FR, ResultsInError);
    }
    return SILValue();
  }

  // Constant fold extraction of a constant element.
  if (TupleExtractInst *TEI = dyn_cast<TupleExtractInst>(&I)) {
    if (TupleInst *TheTuple = dyn_cast<TupleInst>(TEI->getOperand()))
      return TheTuple->getElements()[TEI->getFieldNo()];
  }

  // Constant fold extraction of a constant struct element.
  if (StructExtractInst *SEI = dyn_cast<StructExtractInst>(&I)) {
    if (StructInst *Struct = dyn_cast<StructInst>(SEI->getOperand()))
      return Struct->getOperandForField(SEI->getField())->get();
  }

  return SILValue();
}

static bool isFoldable(SILInstruction *I) {
  return isa<IntegerLiteralInst>(I);
}

static bool CCPFunctionBody(SILFunction &F) {
  DEBUG(llvm::errs() << "*** ConstPropagation processing: " << F.getName()
        << "\n");

  // The list of instructions whose evaluation resulted in errror or warning.
  // This is used to avoid duplicate error reporting in case we reach the same
  // instruction from different entry points in the WorkList.
  llvm::DenseSet<SILInstruction*> ErrorSet;

  // The worklist of the constants that could be folded into their users.
  llvm::SetVector<ValueBase*> WorkList;
  // Initialize the worklist to all of the constant instructions.
  for (auto &BB : F) {
    for (auto &I : BB) {
      if (isFoldable(&I) && !I.use_empty())
        WorkList.insert(&I);
    }
  }

  llvm::DenseSet<SILInstruction*> FoldedUsers;

  while (!WorkList.empty()) {
    ValueBase *I = *WorkList.begin();
    WorkList.remove(I);

    // Go through all users of the constant and try to fold them.
    FoldedUsers.clear();
    for (auto Use : I->getUses()) {
      SILInstruction *User = Use->getUser();

      // It is possible that we had processed this user already. Do not try
      // to fold it again if we had previously produced an error while folding
      // it.  It is not always possible to fold an instruction in case of error.
      if (ErrorSet.count(User))
        continue;

      // Some constant users may indirectly cause folding of their users.
      if (isa<StructInst>(User) || isa<TupleInst>(User)) {
        WorkList.insert(User);
        continue;
      }

      // Try to fold the user.
      bool ResultsInError = false;
      SILValue C = constantFoldInstruction(*User, ResultsInError);
      if (ResultsInError)
        ErrorSet.insert(User);

      if (!C) continue;

      FoldedUsers.insert(User);
      ++NumInstFolded;

      // If the constant produced a tuple, be smarter than RAUW: explicitly nuke
      // any tuple_extract instructions using the apply.  This is a common case
      // for functions returning multiple values.
      if (auto *TI = dyn_cast<TupleInst>(C)) {
        for (auto UI = User->use_begin(), E = User->use_end(); UI != E;) {
          Operand *O = *UI++;

          // If the user is a tuple_extract, just substitute the right value in.
          if (auto *TEI = dyn_cast<TupleExtractInst>(O->getUser())) {
            SILValue NewVal = TI->getOperand(TEI->getFieldNo());
            SILValue(TEI, 0).replaceAllUsesWith(NewVal);
            TEI->dropAllReferences();
            FoldedUsers.insert(TEI);
            WorkList.insert(NewVal.getDef());
          }
        }
        
        if (User->use_empty())
          FoldedUsers.insert(TI);
      }
      
      
      // We were able to fold, so all users should use the new folded value.
      assert(User->getTypes().size() == 1 &&
             "Currently, we only support single result instructions");
      SILValue(User).replaceAllUsesWith(C);
      
      // The new constant could be further folded now, add it to the worklist.
      WorkList.insert(C.getDef());
    }

    // Eagerly DCE. We do this after visiting all users to ensure we don't
    // invalidate the uses iterator.
    for (auto U : FoldedUsers)
      recursivelyDeleteTriviallyDeadInstructions(U);

  }

  return false;
}

//===----------------------------------------------------------------------===//
//                          Top Level Driver
//===----------------------------------------------------------------------===//
void swift::performSILConstantPropagation(SILModule *M) {
  for (auto &Fn : *M)
    CCPFunctionBody(Fn);
}