ConstantFolding.cpp

//===-- ConstantFolding.cpp - Fold instructions into constants ------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines routines for folding instructions into constants.
//
// Also, to supplement the basic VMCore ConstantExpr simplifications,
// this file defines some additional folding routines that can make use of
// TargetData information. These functions cannot go in VMCore due to library
// dependency issues.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/System/FEnv.h"
#include <cerrno>
#include <cmath>
using namespace llvm;

//===----------------------------------------------------------------------===//
// Constant Folding internal helper functions
//===----------------------------------------------------------------------===//

/// FoldBitCast - Constant fold bitcast, symbolically evaluating it with 
/// TargetData.  This always returns a non-null constant, but it may be a
/// ConstantExpr if unfoldable.
static Constant *FoldBitCast(Constant *C, const Type *DestTy,
                             const TargetData &TD) {
  
  // This only handles casts to vectors currently.
  const VectorType *DestVTy = dyn_cast<VectorType>(DestTy);
  if (DestVTy == 0)
    return ConstantExpr::getBitCast(C, DestTy);
  
  // If this is a scalar -> vector cast, convert the input into a <1 x scalar>
  // vector so the code below can handle it uniformly.
  if (isa<ConstantFP>(C) || isa<ConstantInt>(C)) {
    Constant *Ops = C; // don't take the address of C!
    return FoldBitCast(ConstantVector::get(&Ops, 1), DestTy, TD);
  }
  
  // If this is a bitcast from constant vector -> vector, fold it.
  ConstantVector *CV = dyn_cast<ConstantVector>(C);
  if (CV == 0)
    return ConstantExpr::getBitCast(C, DestTy);
  
  // If the element types match, VMCore can fold it.
  unsigned NumDstElt = DestVTy->getNumElements();
  unsigned NumSrcElt = CV->getNumOperands();
  if (NumDstElt == NumSrcElt)
    return ConstantExpr::getBitCast(C, DestTy);
  
  const Type *SrcEltTy = CV->getType()->getElementType();
  const Type *DstEltTy = DestVTy->getElementType();
  
  // Otherwise, we're changing the number of elements in a vector, which 
  // requires endianness information to do the right thing.  For example,
  //    bitcast (<2 x i64> <i64 0, i64 1> to <4 x i32>)
  // folds to (little endian):
  //    <4 x i32> <i32 0, i32 0, i32 1, i32 0>
  // and to (big endian):
  //    <4 x i32> <i32 0, i32 0, i32 0, i32 1>
  
  // First thing is first.  We only want to think about integer here, so if
  // we have something in FP form, recast it as integer.
  if (DstEltTy->isFloatingPointTy()) {
    // Fold to an vector of integers with same size as our FP type.
    unsigned FPWidth = DstEltTy->getPrimitiveSizeInBits();
    const Type *DestIVTy =
      VectorType::get(IntegerType::get(C->getContext(), FPWidth), NumDstElt);
    // Recursively handle this integer conversion, if possible.
    C = FoldBitCast(C, DestIVTy, TD);
    if (!C) return ConstantExpr::getBitCast(C, DestTy);
    
    // Finally, VMCore can handle this now that #elts line up.
    return ConstantExpr::getBitCast(C, DestTy);
  }
  
  // Okay, we know the destination is integer, if the input is FP, convert
  // it to integer first.
  if (SrcEltTy->isFloatingPointTy()) {
    unsigned FPWidth = SrcEltTy->getPrimitiveSizeInBits();
    const Type *SrcIVTy =
      VectorType::get(IntegerType::get(C->getContext(), FPWidth), NumSrcElt);
    // Ask VMCore to do the conversion now that #elts line up.
    C = ConstantExpr::getBitCast(C, SrcIVTy);
    CV = dyn_cast<ConstantVector>(C);
    if (!CV)  // If VMCore wasn't able to fold it, bail out.
      return C;
  }
  
  // Now we know that the input and output vectors are both integer vectors
  // of the same size, and that their #elements is not the same.  Do the
  // conversion here, which depends on whether the input or output has
  // more elements.
  bool isLittleEndian = TD.isLittleEndian();
  
  SmallVector<Constant*, 32> Result;
  if (NumDstElt < NumSrcElt) {
    // Handle: bitcast (<4 x i32> <i32 0, i32 1, i32 2, i32 3> to <2 x i64>)
    Constant *Zero = Constant::getNullValue(DstEltTy);
    unsigned Ratio = NumSrcElt/NumDstElt;
    unsigned SrcBitSize = SrcEltTy->getPrimitiveSizeInBits();
    unsigned SrcElt = 0;
    for (unsigned i = 0; i != NumDstElt; ++i) {
      // Build each element of the result.
      Constant *Elt = Zero;
      unsigned ShiftAmt = isLittleEndian ? 0 : SrcBitSize*(Ratio-1);
      for (unsigned j = 0; j != Ratio; ++j) {
        Constant *Src = dyn_cast<ConstantInt>(CV->getOperand(SrcElt++));
        if (!Src)  // Reject constantexpr elements.
          return ConstantExpr::getBitCast(C, DestTy);
        
        // Zero extend the element to the right size.
        Src = ConstantExpr::getZExt(Src, Elt->getType());
        
        // Shift it to the right place, depending on endianness.
        Src = ConstantExpr::getShl(Src, 
                                   ConstantInt::get(Src->getType(), ShiftAmt));
        ShiftAmt += isLittleEndian ? SrcBitSize : -SrcBitSize;
        
        // Mix it in.
        Elt = ConstantExpr::getOr(Elt, Src);
      }
      Result.push_back(Elt);
    }
  } else {
    // Handle: bitcast (<2 x i64> <i64 0, i64 1> to <4 x i32>)
    unsigned Ratio = NumDstElt/NumSrcElt;
    unsigned DstBitSize = DstEltTy->getPrimitiveSizeInBits();
    
    // Loop over each source value, expanding into multiple results.
    for (unsigned i = 0; i != NumSrcElt; ++i) {
      Constant *Src = dyn_cast<ConstantInt>(CV->getOperand(i));
      if (!Src)  // Reject constantexpr elements.
        return ConstantExpr::getBitCast(C, DestTy);
      
      unsigned ShiftAmt = isLittleEndian ? 0 : DstBitSize*(Ratio-1);
      for (unsigned j = 0; j != Ratio; ++j) {
        // Shift the piece of the value into the right place, depending on
        // endianness.
        Constant *Elt = ConstantExpr::getLShr(Src, 
                                    ConstantInt::get(Src->getType(), ShiftAmt));
        ShiftAmt += isLittleEndian ? DstBitSize : -DstBitSize;
        
        // Truncate and remember this piece.
        Result.push_back(ConstantExpr::getTrunc(Elt, DstEltTy));
      }
    }
  }
  
  return ConstantVector::get(Result.data(), Result.size());
}


/// IsConstantOffsetFromGlobal - If this constant is actually a constant offset
/// from a global, return the global and the constant.  Because of
/// constantexprs, this function is recursive.
static bool IsConstantOffsetFromGlobal(Constant *C, GlobalValue *&GV,
                                       int64_t &Offset, const TargetData &TD) {
  // Trivial case, constant is the global.
  if ((GV = dyn_cast<GlobalValue>(C))) {
    Offset = 0;
    return true;
  }
  
  // Otherwise, if this isn't a constant expr, bail out.
  ConstantExpr *CE = dyn_cast<ConstantExpr>(C);
  if (!CE) return false;
  
  // Look through ptr->int and ptr->ptr casts.
  if (CE->getOpcode() == Instruction::PtrToInt ||
      CE->getOpcode() == Instruction::BitCast)
    return IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD);
  
  // i32* getelementptr ([5 x i32]* @a, i32 0, i32 5)    
  if (CE->getOpcode() == Instruction::GetElementPtr) {
    // Cannot compute this if the element type of the pointer is missing size
    // info.
    if (!cast<PointerType>(CE->getOperand(0)->getType())
                 ->getElementType()->isSized())
      return false;
    
    // If the base isn't a global+constant, we aren't either.
    if (!IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD))
      return false;
    
    // Otherwise, add any offset that our operands provide.
    gep_type_iterator GTI = gep_type_begin(CE);
    for (User::const_op_iterator i = CE->op_begin() + 1, e = CE->op_end();
         i != e; ++i, ++GTI) {
      ConstantInt *CI = dyn_cast<ConstantInt>(*i);
      if (!CI) return false;  // Index isn't a simple constant?
      if (CI->isZero()) continue;  // Not adding anything.
      
      if (const StructType *ST = dyn_cast<StructType>(*GTI)) {
        // N = N + Offset
        Offset += TD.getStructLayout(ST)->getElementOffset(CI->getZExtValue());
      } else {
        const SequentialType *SQT = cast<SequentialType>(*GTI);
        Offset += TD.getTypeAllocSize(SQT->getElementType())*CI->getSExtValue();
      }
    }
    return true;
  }
  
  return false;
}

/// ReadDataFromGlobal - Recursive helper to read bits out of global.  C is the
/// constant being copied out of. ByteOffset is an offset into C.  CurPtr is the
/// pointer to copy results into and BytesLeft is the number of bytes left in
/// the CurPtr buffer.  TD is the target data.
static bool ReadDataFromGlobal(Constant *C, uint64_t ByteOffset,
                               unsigned char *CurPtr, unsigned BytesLeft,
                               const TargetData &TD) {
  assert(ByteOffset <= TD.getTypeAllocSize(C->getType()) &&
         "Out of range access");
  
  // If this element is zero or undefined, we can just return since *CurPtr is
  // zero initialized.
  if (isa<ConstantAggregateZero>(C) || isa<UndefValue>(C))
    return true;
  
  if (ConstantInt *CI = dyn_cast<ConstantInt>(C)) {
    if (CI->getBitWidth() > 64 ||
        (CI->getBitWidth() & 7) != 0)
      return false;
    
    uint64_t Val = CI->getZExtValue();
    unsigned IntBytes = unsigned(CI->getBitWidth()/8);
    
    for (unsigned i = 0; i != BytesLeft && ByteOffset != IntBytes; ++i) {
      CurPtr[i] = (unsigned char)(Val >> (ByteOffset * 8));
      ++ByteOffset;
    }
    return true;
  }
  
  if (ConstantFP *CFP = dyn_cast<ConstantFP>(C)) {
    if (CFP->getType()->isDoubleTy()) {
      C = FoldBitCast(C, Type::getInt64Ty(C->getContext()), TD);
      return ReadDataFromGlobal(C, ByteOffset, CurPtr, BytesLeft, TD);
    }
    if (CFP->getType()->isFloatTy()){
      C = FoldBitCast(C, Type::getInt32Ty(C->getContext()), TD);
      return ReadDataFromGlobal(C, ByteOffset, CurPtr, BytesLeft, TD);
    }
    return false;
  }

  if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) {
    const StructLayout *SL = TD.getStructLayout(CS->getType());
    unsigned Index = SL->getElementContainingOffset(ByteOffset);
    uint64_t CurEltOffset = SL->getElementOffset(Index);
    ByteOffset -= CurEltOffset;
    
    while (1) {
      // If the element access is to the element itself and not to tail padding,
      // read the bytes from the element.
      uint64_t EltSize = TD.getTypeAllocSize(CS->getOperand(Index)->getType());

      if (ByteOffset < EltSize &&
          !ReadDataFromGlobal(CS->getOperand(Index), ByteOffset, CurPtr,
                              BytesLeft, TD))
        return false;
      
      ++Index;
      
      // Check to see if we read from the last struct element, if so we're done.
      if (Index == CS->getType()->getNumElements())
        return true;

      // If we read all of the bytes we needed from this element we're done.
      uint64_t NextEltOffset = SL->getElementOffset(Index);

      if (BytesLeft <= NextEltOffset-CurEltOffset-ByteOffset)
        return true;

      // Move to the next element of the struct.
      CurPtr += NextEltOffset-CurEltOffset-ByteOffset;
      BytesLeft -= NextEltOffset-CurEltOffset-ByteOffset;
      ByteOffset = 0;
      CurEltOffset = NextEltOffset;
    }
    // not reached.
  }

  if (ConstantArray *CA = dyn_cast<ConstantArray>(C)) {
    uint64_t EltSize = TD.getTypeAllocSize(CA->getType()->getElementType());
    uint64_t Index = ByteOffset / EltSize;
    uint64_t Offset = ByteOffset - Index * EltSize;
    for (; Index != CA->getType()->getNumElements(); ++Index) {
      if (!ReadDataFromGlobal(CA->getOperand(Index), Offset, CurPtr,
                              BytesLeft, TD))
        return false;
      if (EltSize >= BytesLeft)
        return true;
      
      Offset = 0;
      BytesLeft -= EltSize;
      CurPtr += EltSize;
    }
    return true;
  }
  
  if (ConstantVector *CV = dyn_cast<ConstantVector>(C)) {
    uint64_t EltSize = TD.getTypeAllocSize(CV->getType()->getElementType());
    uint64_t Index = ByteOffset / EltSize;
    uint64_t Offset = ByteOffset - Index * EltSize;
    for (; Index != CV->getType()->getNumElements(); ++Index) {
      if (!ReadDataFromGlobal(CV->getOperand(Index), Offset, CurPtr,
                              BytesLeft, TD))
        return false;
      if (EltSize >= BytesLeft)
        return true;
      
      Offset = 0;
      BytesLeft -= EltSize;
      CurPtr += EltSize;
    }
    return true;
  }
  
  // Otherwise, unknown initializer type.
  return false;
}

static Constant *FoldReinterpretLoadFromConstPtr(Constant *C,
                                                 const TargetData &TD) {
  const Type *LoadTy = cast<PointerType>(C->getType())->getElementType();
  const IntegerType *IntType = dyn_cast<IntegerType>(LoadTy);
  
  // If this isn't an integer load we can't fold it directly.
  if (!IntType) {
    // If this is a float/double load, we can try folding it as an int32/64 load
    // and then bitcast the result.  This can be useful for union cases.  Note
    // that address spaces don't matter here since we're not going to result in
    // an actual new load.
    const Type *MapTy;
    if (LoadTy->isFloatTy())
      MapTy = Type::getInt32PtrTy(C->getContext());
    else if (LoadTy->isDoubleTy())
      MapTy = Type::getInt64PtrTy(C->getContext());
    else if (LoadTy->isVectorTy()) {
      MapTy = IntegerType::get(C->getContext(),
                               TD.getTypeAllocSizeInBits(LoadTy));
      MapTy = PointerType::getUnqual(MapTy);
    } else
      return 0;

    C = FoldBitCast(C, MapTy, TD);
    if (Constant *Res = FoldReinterpretLoadFromConstPtr(C, TD))
      return FoldBitCast(Res, LoadTy, TD);
    return 0;
  }
  
  unsigned BytesLoaded = (IntType->getBitWidth() + 7) / 8;
  if (BytesLoaded > 32 || BytesLoaded == 0) return 0;
  
  GlobalValue *GVal;
  int64_t Offset;
  if (!IsConstantOffsetFromGlobal(C, GVal, Offset, TD))
    return 0;
  
  GlobalVariable *GV = dyn_cast<GlobalVariable>(GVal);
  if (!GV || !GV->isConstant() || !GV->hasDefinitiveInitializer() ||
      !GV->getInitializer()->getType()->isSized())
    return 0;

  // If we're loading off the beginning of the global, some bytes may be valid,
  // but we don't try to handle this.
  if (Offset < 0) return 0;
  
  // If we're not accessing anything in this constant, the result is undefined.
  if (uint64_t(Offset) >= TD.getTypeAllocSize(GV->getInitializer()->getType()))
    return UndefValue::get(IntType);
  
  unsigned char RawBytes[32] = {0};
  if (!ReadDataFromGlobal(GV->getInitializer(), Offset, RawBytes,
                          BytesLoaded, TD))
    return 0;

  APInt ResultVal = APInt(IntType->getBitWidth(), RawBytes[BytesLoaded-1]);
  for (unsigned i = 1; i != BytesLoaded; ++i) {
    ResultVal <<= 8;
    ResultVal |= RawBytes[BytesLoaded-1-i];
  }

  return ConstantInt::get(IntType->getContext(), ResultVal);
}

/// ConstantFoldLoadFromConstPtr - Return the value that a load from C would
/// produce if it is constant and determinable.  If this is not determinable,
/// return null.
Constant *llvm::ConstantFoldLoadFromConstPtr(Constant *C,
                                             const TargetData *TD) {
  // First, try the easy cases:
  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C))
    if (GV->isConstant() && GV->hasDefinitiveInitializer())
      return GV->getInitializer();

  // If the loaded value isn't a constant expr, we can't handle it.
  ConstantExpr *CE = dyn_cast<ConstantExpr>(C);
  if (!CE) return 0;
  
  if (CE->getOpcode() == Instruction::GetElementPtr) {
    if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getOperand(0)))
      if (GV->isConstant() && GV->hasDefinitiveInitializer())
        if (Constant *V = 
             ConstantFoldLoadThroughGEPConstantExpr(GV->getInitializer(), CE))
          return V;
  }
  
  // Instead of loading constant c string, use corresponding integer value
  // directly if string length is small enough.
  std::string Str;
  if (TD && GetConstantStringInfo(CE, Str) && !Str.empty()) {
    unsigned StrLen = Str.length();
    const Type *Ty = cast<PointerType>(CE->getType())->getElementType();
    unsigned NumBits = Ty->getPrimitiveSizeInBits();
    // Replace load with immediate integer if the result is an integer or fp
    // value.
    if ((NumBits >> 3) == StrLen + 1 && (NumBits & 7) == 0 &&
        (isa<IntegerType>(Ty) || Ty->isFloatingPointTy())) {
      APInt StrVal(NumBits, 0);
      APInt SingleChar(NumBits, 0);
      if (TD->isLittleEndian()) {
        for (signed i = StrLen-1; i >= 0; i--) {
          SingleChar = (uint64_t) Str[i] & UCHAR_MAX;
          StrVal = (StrVal << 8) | SingleChar;
        }
      } else {
        for (unsigned i = 0; i < StrLen; i++) {
          SingleChar = (uint64_t) Str[i] & UCHAR_MAX;
          StrVal = (StrVal << 8) | SingleChar;
        }
        // Append NULL at the end.
        SingleChar = 0;
        StrVal = (StrVal << 8) | SingleChar;
      }
      
      Constant *Res = ConstantInt::get(CE->getContext(), StrVal);
      if (Ty->isFloatingPointTy())
        Res = ConstantExpr::getBitCast(Res, Ty);
      return Res;
    }
  }
  
  // If this load comes from anywhere in a constant global, and if the global
  // is all undef or zero, we know what it loads.
  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getUnderlyingObject())){
    if (GV->isConstant() && GV->hasDefinitiveInitializer()) {
      const Type *ResTy = cast<PointerType>(C->getType())->getElementType();
      if (GV->getInitializer()->isNullValue())
        return Constant::getNullValue(ResTy);
      if (isa<UndefValue>(GV->getInitializer()))
        return UndefValue::get(ResTy);
    }
  }
  
  // Try hard to fold loads from bitcasted strange and non-type-safe things.  We
  // currently don't do any of this for big endian systems.  It can be
  // generalized in the future if someone is interested.
  if (TD && TD->isLittleEndian())
    return FoldReinterpretLoadFromConstPtr(CE, *TD);
  return 0;
}

static Constant *ConstantFoldLoadInst(const LoadInst *LI, const TargetData *TD){
  if (LI->isVolatile()) return 0;
  
  if (Constant *C = dyn_cast<Constant>(LI->getOperand(0)))
    return ConstantFoldLoadFromConstPtr(C, TD);

  return 0;
}

/// SymbolicallyEvaluateBinop - One of Op0/Op1 is a constant expression.
/// Attempt to symbolically evaluate the result of a binary operator merging
/// these together.  If target data info is available, it is provided as TD, 
/// otherwise TD is null.
static Constant *SymbolicallyEvaluateBinop(unsigned Opc, Constant *Op0,
                                           Constant *Op1, const TargetData *TD){
  // SROA
  
  // Fold (and 0xffffffff00000000, (shl x, 32)) -> shl.
  // Fold (lshr (or X, Y), 32) -> (lshr [X/Y], 32) if one doesn't contribute
  // bits.
  
  
  // If the constant expr is something like &A[123] - &A[4].f, fold this into a
  // constant.  This happens frequently when iterating over a global array.
  if (Opc == Instruction::Sub && TD) {
    GlobalValue *GV1, *GV2;
    int64_t Offs1, Offs2;
    
    if (IsConstantOffsetFromGlobal(Op0, GV1, Offs1, *TD))
      if (IsConstantOffsetFromGlobal(Op1, GV2, Offs2, *TD) &&
          GV1 == GV2) {
        // (&GV+C1) - (&GV+C2) -> C1-C2, pointer arithmetic cannot overflow.
        return ConstantInt::get(Op0->getType(), Offs1-Offs2);
      }
  }
    
  return 0;
}

/// CastGEPIndices - If array indices are not pointer-sized integers,
/// explicitly cast them so that they aren't implicitly casted by the
/// getelementptr.
static Constant *CastGEPIndices(Constant *const *Ops, unsigned NumOps,
                                const Type *ResultTy,
                                const TargetData *TD) {
  if (!TD) return 0;
  const Type *IntPtrTy = TD->getIntPtrType(ResultTy->getContext());

  bool Any = false;
  SmallVector<Constant*, 32> NewIdxs;
  for (unsigned i = 1; i != NumOps; ++i) {
    if ((i == 1 ||
         !isa<StructType>(GetElementPtrInst::getIndexedType(Ops[0]->getType(),
                                                            reinterpret_cast<Value *const *>(Ops+1),
                                                            i-1))) &&
        Ops[i]->getType() != IntPtrTy) {
      Any = true;
      NewIdxs.push_back(ConstantExpr::getCast(CastInst::getCastOpcode(Ops[i],
                                                                      true,
                                                                      IntPtrTy,
                                                                      true),
                                              Ops[i], IntPtrTy));
    } else
      NewIdxs.push_back(Ops[i]);
  }
  if (!Any) return 0;

  Constant *C =
    ConstantExpr::getGetElementPtr(Ops[0], &NewIdxs[0], NewIdxs.size());
  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C))
    if (Constant *Folded = ConstantFoldConstantExpression(CE, TD))
      C = Folded;
  return C;
}

/// SymbolicallyEvaluateGEP - If we can symbolically evaluate the specified GEP
/// constant expression, do so.
static Constant *SymbolicallyEvaluateGEP(Constant *const *Ops, unsigned NumOps,
                                         const Type *ResultTy,
                                         const TargetData *TD) {
  Constant *Ptr = Ops[0];
  if (!TD || !cast<PointerType>(Ptr->getType())->getElementType()->isSized())
    return 0;

  unsigned BitWidth =
    TD->getTypeSizeInBits(TD->getIntPtrType(Ptr->getContext()));

  // If this is a constant expr gep that is effectively computing an
  // "offsetof", fold it into 'cast int Size to T*' instead of 'gep 0, 0, 12'
  for (unsigned i = 1; i != NumOps; ++i)
    if (!isa<ConstantInt>(Ops[i]))
      return 0;
  
  APInt Offset = APInt(BitWidth,
                       TD->getIndexedOffset(Ptr->getType(),
                                            (Value**)Ops+1, NumOps-1));
  Ptr = cast<Constant>(Ptr->stripPointerCasts());

  // If this is a GEP of a GEP, fold it all into a single GEP.
  while (GEPOperator *GEP = dyn_cast<GEPOperator>(Ptr)) {
    SmallVector<Value *, 4> NestedOps(GEP->op_begin()+1, GEP->op_end());

    // Do not try the incorporate the sub-GEP if some index is not a number.
    bool AllConstantInt = true;
    for (unsigned i = 0, e = NestedOps.size(); i != e; ++i)
      if (!isa<ConstantInt>(NestedOps[i])) {
        AllConstantInt = false;
        break;
      }
    if (!AllConstantInt)
      break;

    Ptr = cast<Constant>(GEP->getOperand(0));
    Offset += APInt(BitWidth,
                    TD->getIndexedOffset(Ptr->getType(),
                                         (Value**)NestedOps.data(),
                                         NestedOps.size()));
    Ptr = cast<Constant>(Ptr->stripPointerCasts());
  }

  // If the base value for this address is a literal integer value, fold the
  // getelementptr to the resulting integer value casted to the pointer type.
  APInt BasePtr(BitWidth, 0);
  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
    if (CE->getOpcode() == Instruction::IntToPtr)
      if (ConstantInt *Base = dyn_cast<ConstantInt>(CE->getOperand(0))) {
        BasePtr = Base->getValue();
        BasePtr.zextOrTrunc(BitWidth);
      }
  if (Ptr->isNullValue() || BasePtr != 0) {
    Constant *C = ConstantInt::get(Ptr->getContext(), Offset+BasePtr);
    return ConstantExpr::getIntToPtr(C, ResultTy);
  }

  // Otherwise form a regular getelementptr. Recompute the indices so that
  // we eliminate over-indexing of the notional static type array bounds.
  // This makes it easy to determine if the getelementptr is "inbounds".
  // Also, this helps GlobalOpt do SROA on GlobalVariables.
  const Type *Ty = Ptr->getType();
  SmallVector<Constant*, 32> NewIdxs;
  do {
    if (const SequentialType *ATy = dyn_cast<SequentialType>(Ty)) {
      if (ATy->isPointerTy()) {
        // The only pointer indexing we'll do is on the first index of the GEP.
        if (!NewIdxs.empty())
          break;
       
        // Only handle pointers to sized types, not pointers to functions.
        if (!ATy->getElementType()->isSized())
          return 0;
      }
        
      // Determine which element of the array the offset points into.
      APInt ElemSize(BitWidth, TD->getTypeAllocSize(ATy->getElementType()));
      if (ElemSize == 0)
        return 0;
      APInt NewIdx = Offset.udiv(ElemSize);
      Offset -= NewIdx * ElemSize;
      NewIdxs.push_back(ConstantInt::get(TD->getIntPtrType(Ty->getContext()),
                                         NewIdx));
      Ty = ATy->getElementType();
    } else if (const StructType *STy = dyn_cast<StructType>(Ty)) {
      // Determine which field of the struct the offset points into. The
      // getZExtValue is at least as safe as the StructLayout API because we
      // know the offset is within the struct at this point.
      const StructLayout &SL = *TD->getStructLayout(STy);
      unsigned ElIdx = SL.getElementContainingOffset(Offset.getZExtValue());
      NewIdxs.push_back(ConstantInt::get(Type::getInt32Ty(Ty->getContext()),
                                         ElIdx));
      Offset -= APInt(BitWidth, SL.getElementOffset(ElIdx));
      Ty = STy->getTypeAtIndex(ElIdx);
    } else {
      // We've reached some non-indexable type.
      break;
    }
  } while (Ty != cast<PointerType>(ResultTy)->getElementType());

  // If we haven't used up the entire offset by descending the static
  // type, then the offset is pointing into the middle of an indivisible
  // member, so we can't simplify it.
  if (Offset != 0)
    return 0;

  // Create a GEP.
  Constant *C =
    ConstantExpr::getGetElementPtr(Ptr, &NewIdxs[0], NewIdxs.size());
  assert(cast<PointerType>(C->getType())->getElementType() == Ty &&
         "Computed GetElementPtr has unexpected type!");

  // If we ended up indexing a member with a type that doesn't match
  // the type of what the original indices indexed, add a cast.
  if (Ty != cast<PointerType>(ResultTy)->getElementType())
    C = FoldBitCast(C, ResultTy, *TD);

  return C;
}



//===----------------------------------------------------------------------===//
// Constant Folding public APIs
//===----------------------------------------------------------------------===//


/// ConstantFoldInstruction - Attempt to constant fold the specified
/// instruction.  If successful, the constant result is returned, if not, null
/// is returned.  Note that this function can only fail when attempting to fold
/// instructions like loads and stores, which have no constant expression form.
///
Constant *llvm::ConstantFoldInstruction(Instruction *I, const TargetData *TD) {
  if (PHINode *PN = dyn_cast<PHINode>(I)) {
    if (PN->getNumIncomingValues() == 0)
      return UndefValue::get(PN->getType());

    Constant *Result = dyn_cast<Constant>(PN->getIncomingValue(0));
    if (Result == 0) return 0;

    // Handle PHI nodes specially here...
    for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
      if (PN->getIncomingValue(i) != Result && PN->getIncomingValue(i) != PN)
        return 0;   // Not all the same incoming constants...

    // If we reach here, all incoming values are the same constant.
    return Result;
  }

  // Scan the operand list, checking to see if they are all constants, if so,
  // hand off to ConstantFoldInstOperands.
  SmallVector<Constant*, 8> Ops;
  for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i)
    if (Constant *Op = dyn_cast<Constant>(*i))
      Ops.push_back(Op);
    else
      return 0;  // All operands not constant!

  if (const CmpInst *CI = dyn_cast<CmpInst>(I))
    return ConstantFoldCompareInstOperands(CI->getPredicate(), Ops[0], Ops[1],
                                           TD);
  
  if (const LoadInst *LI = dyn_cast<LoadInst>(I))
    return ConstantFoldLoadInst(LI, TD);
  
  return ConstantFoldInstOperands(I->getOpcode(), I->getType(),
                                  Ops.data(), Ops.size(), TD);
}

/// ConstantFoldConstantExpression - Attempt to fold the constant expression
/// using the specified TargetData.  If successful, the constant result is
/// result is returned, if not, null is returned.
Constant *llvm::ConstantFoldConstantExpression(const ConstantExpr *CE,
                                               const TargetData *TD) {
  SmallVector<Constant*, 8> Ops;
  for (User::const_op_iterator i = CE->op_begin(), e = CE->op_end(); i != e; ++i) {
    Constant *NewC = cast<Constant>(*i);
    // Recursively fold the ConstantExpr's operands.
    if (ConstantExpr *NewCE = dyn_cast<ConstantExpr>(NewC))
      NewC = ConstantFoldConstantExpression(NewCE, TD);
    Ops.push_back(NewC);
  }

  if (CE->isCompare())
    return ConstantFoldCompareInstOperands(CE->getPredicate(), Ops[0], Ops[1],
                                           TD);
  return ConstantFoldInstOperands(CE->getOpcode(), CE->getType(),
                                  Ops.data(), Ops.size(), TD);
}

/// ConstantFoldInstOperands - Attempt to constant fold an instruction with the
/// specified opcode and operands.  If successful, the constant result is
/// returned, if not, null is returned.  Note that this function can fail when
/// attempting to fold instructions like loads and stores, which have no
/// constant expression form.
///
/// TODO: This function neither utilizes nor preserves nsw/nuw/inbounds/etc
/// information, due to only being passed an opcode and operands. Constant
/// folding using this function strips this information.
///
Constant *llvm::ConstantFoldInstOperands(unsigned Opcode, const Type *DestTy, 
                                         Constant* const* Ops, unsigned NumOps,
                                         const TargetData *TD) {
  // Handle easy binops first.
  if (Instruction::isBinaryOp(Opcode)) {
    if (isa<ConstantExpr>(Ops[0]) || isa<ConstantExpr>(Ops[1]))
      if (Constant *C = SymbolicallyEvaluateBinop(Opcode, Ops[0], Ops[1], TD))
        return C;
    
    return ConstantExpr::get(Opcode, Ops[0], Ops[1]);
  }
  
  switch (Opcode) {
  default: return 0;
  case Instruction::ICmp:
  case Instruction::FCmp: assert(0 && "Invalid for compares");
  case Instruction::Call:
    if (Function *F = dyn_cast<Function>(Ops[NumOps - 1]))
      if (canConstantFoldCallTo(F))
        return ConstantFoldCall(F, Ops, NumOps - 1);
    return 0;
  case Instruction::PtrToInt:
    // If the input is a inttoptr, eliminate the pair.  This requires knowing
    // the width of a pointer, so it can't be done in ConstantExpr::getCast.
    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ops[0])) {
      if (TD && CE->getOpcode() == Instruction::IntToPtr) {
        Constant *Input = CE->getOperand(0);
        unsigned InWidth = Input->getType()->getScalarSizeInBits();
        if (TD->getPointerSizeInBits() < InWidth) {
          Constant *Mask = 
            ConstantInt::get(CE->getContext(), APInt::getLowBitsSet(InWidth,
                                                  TD->getPointerSizeInBits()));
          Input = ConstantExpr::getAnd(Input, Mask);
        }
        // Do a zext or trunc to get to the dest size.
        return ConstantExpr::getIntegerCast(Input, DestTy, false);
      }
    }
    return ConstantExpr::getCast(Opcode, Ops[0], DestTy);
  case Instruction::IntToPtr:
    // If the input is a ptrtoint, turn the pair into a ptr to ptr bitcast if
    // the int size is >= the ptr size.  This requires knowing the width of a
    // pointer, so it can't be done in ConstantExpr::getCast.
    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ops[0]))
      if (TD &&
          TD->getPointerSizeInBits() <= CE->getType()->getScalarSizeInBits() &&
          CE->getOpcode() == Instruction::PtrToInt)
        return FoldBitCast(CE->getOperand(0), DestTy, *TD);

    return ConstantExpr::getCast(Opcode, Ops[0], DestTy);
  case Instruction::Trunc:
  case Instruction::ZExt:
  case Instruction::SExt:
  case Instruction::FPTrunc:
  case Instruction::FPExt:
  case Instruction::UIToFP:
  case Instruction::SIToFP:
  case Instruction::FPToUI:
  case Instruction::FPToSI:
      return ConstantExpr::getCast(Opcode, Ops[0], DestTy);
  case Instruction::BitCast:
    if (TD)
      return FoldBitCast(Ops[0], DestTy, *TD);
    return ConstantExpr::getBitCast(Ops[0], DestTy);
  case Instruction::Select:
    return ConstantExpr::getSelect(Ops[0], Ops[1], Ops[2]);
  case Instruction::ExtractElement:
    return ConstantExpr::getExtractElement(Ops[0], Ops[1]);
  case Instruction::InsertElement:
    return ConstantExpr::getInsertElement(Ops[0], Ops[1], Ops[2]);
  case Instruction::ShuffleVector:
    return ConstantExpr::getShuffleVector(Ops[0], Ops[1], Ops[2]);
  case Instruction::GetElementPtr:
    if (Constant *C = CastGEPIndices(Ops, NumOps, DestTy, TD))
      return C;
    if (Constant *C = SymbolicallyEvaluateGEP(Ops, NumOps, DestTy, TD))
      return C;
    
    return ConstantExpr::getGetElementPtr(Ops[0], Ops+1, NumOps-1);
  }
}

/// ConstantFoldCompareInstOperands - Attempt to constant fold a compare
/// instruction (icmp/fcmp) with the specified operands.  If it fails, it
/// returns a constant expression of the specified operands.
///
Constant *llvm::ConstantFoldCompareInstOperands(unsigned Predicate,
                                                Constant *Ops0, Constant *Ops1, 
                                                const TargetData *TD) {
  // fold: icmp (inttoptr x), null         -> icmp x, 0
  // fold: icmp (ptrtoint x), 0            -> icmp x, null
  // fold: icmp (inttoptr x), (inttoptr y) -> icmp trunc/zext x, trunc/zext y
  // fold: icmp (ptrtoint x), (ptrtoint y) -> icmp x, y
  //
  // ConstantExpr::getCompare cannot do this, because it doesn't have TD
  // around to know if bit truncation is happening.
  if (ConstantExpr *CE0 = dyn_cast<ConstantExpr>(Ops0)) {
    if (TD && Ops1->isNullValue()) {
      const Type *IntPtrTy = TD->getIntPtrType(CE0->getContext());
      if (CE0->getOpcode() == Instruction::IntToPtr) {
        // Convert the integer value to the right size to ensure we get the
        // proper extension or truncation.
        Constant *C = ConstantExpr::getIntegerCast(CE0->getOperand(0),
                                                   IntPtrTy, false);
        Constant *Null = Constant::getNullValue(C->getType());
        return ConstantFoldCompareInstOperands(Predicate, C, Null, TD);
      }
      
      // Only do this transformation if the int is intptrty in size, otherwise
      // there is a truncation or extension that we aren't modeling.
      if (CE0->getOpcode() == Instruction::PtrToInt && 
          CE0->getType() == IntPtrTy) {
        Constant *C = CE0->getOperand(0);
        Constant *Null = Constant::getNullValue(C->getType());
        return ConstantFoldCompareInstOperands(Predicate, C, Null, TD);
      }
    }
    
    if (ConstantExpr *CE1 = dyn_cast<ConstantExpr>(Ops1)) {
      if (TD && CE0->getOpcode() == CE1->getOpcode()) {
        const Type *IntPtrTy = TD->getIntPtrType(CE0->getContext());

        if (CE0->getOpcode() == Instruction::IntToPtr) {
          // Convert the integer value to the right size to ensure we get the
          // proper extension or truncation.
          Constant *C0 = ConstantExpr::getIntegerCast(CE0->getOperand(0),
                                                      IntPtrTy, false);
          Constant *C1 = ConstantExpr::getIntegerCast(CE1->getOperand(0),
                                                      IntPtrTy, false);
          return ConstantFoldCompareInstOperands(Predicate, C0, C1, TD);
        }

        // Only do this transformation if the int is intptrty in size, otherwise
        // there is a truncation or extension that we aren't modeling.
        if ((CE0->getOpcode() == Instruction::PtrToInt &&
             CE0->getType() == IntPtrTy &&
             CE0->getOperand(0)->getType() == CE1->getOperand(0)->getType()))
          return ConstantFoldCompareInstOperands(Predicate, CE0->getOperand(0),
                                                 CE1->getOperand(0), TD);
      }
    }
    
    // icmp eq (or x, y), 0 -> (icmp eq x, 0) & (icmp eq y, 0)
    // icmp ne (or x, y), 0 -> (icmp ne x, 0) | (icmp ne y, 0)
    if ((Predicate == ICmpInst::ICMP_EQ || Predicate == ICmpInst::ICMP_NE) &&
        CE0->getOpcode() == Instruction::Or && Ops1->isNullValue()) {
      Constant *LHS = 
        ConstantFoldCompareInstOperands(Predicate, CE0->getOperand(0), Ops1,TD);
      Constant *RHS = 
        ConstantFoldCompareInstOperands(Predicate, CE0->getOperand(1), Ops1,TD);
      unsigned OpC = 
        Predicate == ICmpInst::ICMP_EQ ? Instruction::And : Instruction::Or;
      Constant *Ops[] = { LHS, RHS };
      return ConstantFoldInstOperands(OpC, LHS->getType(), Ops, 2, TD);
    }
  }
  
  return ConstantExpr::getCompare(Predicate, Ops0, Ops1);
}


/// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a
/// getelementptr constantexpr, return the constant value being addressed by the
/// constant expression, or null if something is funny and we can't decide.
Constant *llvm::ConstantFoldLoadThroughGEPConstantExpr(Constant *C, 
                                                       ConstantExpr *CE) {
  if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
    return 0;  // Do not allow stepping over the value!
  
  // Loop over all of the operands, tracking down which value we are
  // addressing...
  gep_type_iterator I = gep_type_begin(CE), E = gep_type_end(CE);
  for (++I; I != E; ++I)
    if (const StructType *STy = dyn_cast<StructType>(*I)) {
      ConstantInt *CU = cast<ConstantInt>(I.getOperand());
      assert(CU->getZExtValue() < STy->getNumElements() &&
             "Struct index out of range!");
      unsigned El = (unsigned)CU->getZExtValue();
      if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) {
        C = CS->getOperand(El);
      } else if (isa<ConstantAggregateZero>(C)) {
        C = Constant::getNullValue(STy->getElementType(El));
      } else if (isa<UndefValue>(C)) {
        C = UndefValue::get(STy->getElementType(El));
      } else {
        return 0;
      }
    } else if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) {
      if (const ArrayType *ATy = dyn_cast<ArrayType>(*I)) {
        if (CI->getZExtValue() >= ATy->getNumElements())
         return 0;
        if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
          C = CA->getOperand(CI->getZExtValue());
        else if (isa<ConstantAggregateZero>(C))
          C = Constant::getNullValue(ATy->getElementType());
        else if (isa<UndefValue>(C))
          C = UndefValue::get(ATy->getElementType());
        else
          return 0;
      } else if (const VectorType *VTy = dyn_cast<VectorType>(*I)) {
        if (CI->getZExtValue() >= VTy->getNumElements())
          return 0;
        if (ConstantVector *CP = dyn_cast<ConstantVector>(C))
          C = CP->getOperand(CI->getZExtValue());
        else if (isa<ConstantAggregateZero>(C))
          C = Constant::getNullValue(VTy->getElementType());
        else if (isa<UndefValue>(C))
          C = UndefValue::get(VTy->getElementType());
        else
          return 0;
      } else {
        return 0;
      }
    } else {
      return 0;
    }
  return C;
}


//===----------------------------------------------------------------------===//
//  Constant Folding for Calls
//

/// canConstantFoldCallTo - Return true if its even possible to fold a call to
/// the specified function.
bool
llvm::canConstantFoldCallTo(const Function *F) {
  switch (F->getIntrinsicID()) {
  case Intrinsic::sqrt:
  case Intrinsic::powi:
  case Intrinsic::bswap:
  case Intrinsic::ctpop:
  case Intrinsic::ctlz:
  case Intrinsic::cttz:
  case Intrinsic::uadd_with_overflow: