lib/Analysis/ConstantFolding.cpp

//===-- ConstantFolding.cpp - Analyze constant folding possibilities ------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This family of functions determines the possibility of performing constant
// folding.
//
//===----------------------------------------------------------------------===//

#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/LLVMContext.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/MathExtras.h"
#include <cerrno>
#include <cmath>
using namespace llvm;

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

/// 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->getZExtValue() == 0) 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;
}


/// 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,
                                           LLVMContext *Context){
  // 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 Context->getConstantInt(Op0->getType(), Offs1-Offs2);
      }
  }
    
  return 0;
}

/// SymbolicallyEvaluateGEP - If we can symbolically evaluate the specified GEP
/// constant expression, do so.
static Constant *SymbolicallyEvaluateGEP(Constant* const* Ops, unsigned NumOps,
                                         const Type *ResultTy,
                                         LLVMContext *Context,
                                         const TargetData *TD) {
  Constant *Ptr = Ops[0];
  if (!TD || !cast<PointerType>(Ptr->getType())->getElementType()->isSized())
    return 0;
  
  uint64_t BasePtr = 0;
  if (!Ptr->isNullValue()) {
    // If this is a inttoptr from a constant int, we can fold this as the base,
    // otherwise we can't.
    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
      if (CE->getOpcode() == Instruction::IntToPtr)
        if (ConstantInt *Base = dyn_cast<ConstantInt>(CE->getOperand(0)))
          BasePtr = Base->getZExtValue();
    
    if (BasePtr == 0)
      return 0;
  }

  // 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 false;
  
  uint64_t Offset = TD->getIndexedOffset(Ptr->getType(),
                                         (Value**)Ops+1, NumOps-1);
  Constant *C = Context->getConstantInt(TD->getIntPtrType(), Offset+BasePtr);
  return Context->getConstantExprIntToPtr(C, ResultTy);
}

/// FoldBitCast - Constant fold bitcast, symbolically evaluating it with 
/// targetdata.  Return 0 if unfoldable.
static Constant *FoldBitCast(Constant *C, const Type *DestTy,
                             const TargetData &TD, LLVMContext *Context) {
  // If this is a bitcast from constant vector -> vector, fold it.
  if (ConstantVector *CV = dyn_cast<ConstantVector>(C)) {
    if (const VectorType *DestVTy = dyn_cast<VectorType>(DestTy)) {
      // If the element types match, VMCore can fold it.
      unsigned NumDstElt = DestVTy->getNumElements();
      unsigned NumSrcElt = CV->getNumOperands();
      if (NumDstElt == NumSrcElt)
        return 0;
      
      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->isFloatingPoint()) {
        // Fold to an vector of integers with same size as our FP type.
        unsigned FPWidth = DstEltTy->getPrimitiveSizeInBits();
        const Type *DestIVTy = Context->getVectorType(
                                   Context->getIntegerType(FPWidth), NumDstElt);
        // Recursively handle this integer conversion, if possible.
        C = FoldBitCast(C, DestIVTy, TD, Context);
        if (!C) return 0;
        
        // Finally, VMCore can handle this now that #elts line up.
        return Context->getConstantExprBitCast(C, DestTy);
      }
      
      // Okay, we know the destination is integer, if the input is FP, convert
      // it to integer first.
      if (SrcEltTy->isFloatingPoint()) {
        unsigned FPWidth = SrcEltTy->getPrimitiveSizeInBits();
        const Type *SrcIVTy = Context->getVectorType(
                                   Context->getIntegerType(FPWidth), NumSrcElt);
        // Ask VMCore to do the conversion now that #elts line up.
        C = Context->getConstantExprBitCast(C, SrcIVTy);
        CV = dyn_cast<ConstantVector>(C);
        if (!CV) return 0;  // If VMCore wasn't able to fold it, bail out.
      }
      
      // 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 = Context->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) return 0;  // Reject constantexpr elements.
            
            // Zero extend the element to the right size.
            Src = Context->getConstantExprZExt(Src, Elt->getType());
            
            // Shift it to the right place, depending on endianness.
            Src = Context->getConstantExprShl(Src, 
                             Context->getConstantInt(Src->getType(), ShiftAmt));
            ShiftAmt += isLittleEndian ? SrcBitSize : -SrcBitSize;
            
            // Mix it in.
            Elt = Context->getConstantExprOr(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) return 0;  // Reject constantexpr elements.

          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 = Context->getConstantExprLShr(Src, 
                            Context->getConstantInt(Src->getType(), ShiftAmt));
            ShiftAmt += isLittleEndian ? DstBitSize : -DstBitSize;

            // Truncate and remember this piece.
            Result.push_back(Context->getConstantExprTrunc(Elt, DstEltTy));
          }
        }
      }
      
      return Context->getConstantVector(Result.data(), Result.size());
    }
  }
  
  return 0;
}


//===----------------------------------------------------------------------===//
// 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, LLVMContext *Context,
                                        const TargetData *TD) {
  if (PHINode *PN = dyn_cast<PHINode>(I)) {
    if (PN->getNumIncomingValues() == 0)
      return Context->getUndef(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.data(), Ops.size(), 
                                           Context, TD);
  else
    return ConstantFoldInstOperands(I->getOpcode(), I->getType(),
                                    Ops.data(), Ops.size(), Context, 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(ConstantExpr *CE,
                                               LLVMContext *Context,
                                               const TargetData *TD) {
  SmallVector<Constant*, 8> Ops;
  for (User::op_iterator i = CE->op_begin(), e = CE->op_end(); i != e; ++i)
    Ops.push_back(cast<Constant>(*i));

  if (CE->isCompare())
    return ConstantFoldCompareInstOperands(CE->getPredicate(),
                                           Ops.data(), Ops.size(), 
                                           Context, TD);
  else 
    return ConstantFoldInstOperands(CE->getOpcode(), CE->getType(),
                                    Ops.data(), Ops.size(), Context, 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.
///
Constant *llvm::ConstantFoldInstOperands(unsigned Opcode, const Type *DestTy, 
                                         Constant* const* Ops, unsigned NumOps,
                                         LLVMContext *Context,
                                         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,
                                                  Context))
        return C;
    
    return Context->getConstantExpr(Opcode, Ops[0], Ops[1]);
  }
  
  switch (Opcode) {
  default: return 0;
  case Instruction::Call:
    if (Function *F = dyn_cast<Function>(Ops[0]))
      if (canConstantFoldCallTo(F))
        return ConstantFoldCall(F, Ops+1, NumOps-1);
    return 0;
  case Instruction::ICmp:
  case Instruction::FCmp:
    llvm_unreachable("This function is invalid for compares: no predicate specified");
  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 = 
            Context->getConstantInt(APInt::getLowBitsSet(InWidth,
                                                  TD->getPointerSizeInBits()));
          Input = Context->getConstantExprAnd(Input, Mask);
        }
        // Do a zext or trunc to get to the dest size.
        return Context->getConstantExprIntegerCast(Input, DestTy, false);
      }
    }
    return Context->getConstantExprCast(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()) {
        if (CE->getOpcode() == Instruction::PtrToInt) {
          Constant *Input = CE->getOperand(0);
          Constant *C = FoldBitCast(Input, DestTy, *TD, Context);
          return C ? C : Context->getConstantExprBitCast(Input, DestTy);
        }
        // If there's a constant offset added to the integer value before
        // it is casted back to a pointer, see if the expression can be
        // converted into a GEP.
        if (CE->getOpcode() == Instruction::Add)
          if (ConstantInt *L = dyn_cast<ConstantInt>(CE->getOperand(0)))
            if (ConstantExpr *R = dyn_cast<ConstantExpr>(CE->getOperand(1)))
              if (R->getOpcode() == Instruction::PtrToInt)
                if (GlobalVariable *GV =
                      dyn_cast<GlobalVariable>(R->getOperand(0))) {
                  const PointerType *GVTy = cast<PointerType>(GV->getType());
                  if (const ArrayType *AT =
                        dyn_cast<ArrayType>(GVTy->getElementType())) {
                    const Type *ElTy = AT->getElementType();
                    uint64_t AllocSize = TD->getTypeAllocSize(ElTy);
                    APInt PSA(L->getValue().getBitWidth(), AllocSize);
                    if (ElTy == cast<PointerType>(DestTy)->getElementType() &&
                        L->getValue().urem(PSA) == 0) {
                      APInt ElemIdx = L->getValue().udiv(PSA);
                      if (ElemIdx.ult(APInt(ElemIdx.getBitWidth(),
                                            AT->getNumElements()))) {
                        Constant *Index[] = {
                          Context->getNullValue(CE->getType()),
                          Context->getConstantInt(ElemIdx)
                        };
                        return
                        Context->getConstantExprGetElementPtr(GV, &Index[0], 2);
                      }
                    }
                  }
                }
      }
    }
    return Context->getConstantExprCast(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 Context->getConstantExprCast(Opcode, Ops[0], DestTy);
  case Instruction::BitCast:
    if (TD)
      if (Constant *C = FoldBitCast(Ops[0], DestTy, *TD, Context))
        return C;
    return Context->getConstantExprBitCast(Ops[0], DestTy);
  case Instruction::Select:
    return Context->getConstantExprSelect(Ops[0], Ops[1], Ops[2]);
  case Instruction::ExtractElement:
    return Context->getConstantExprExtractElement(Ops[0], Ops[1]);
  case Instruction::InsertElement:
    return Context->getConstantExprInsertElement(Ops[0], Ops[1], Ops[2]);
  case Instruction::ShuffleVector:
    return Context->getConstantExprShuffleVector(Ops[0], Ops[1], Ops[2]);
  case Instruction::GetElementPtr:
    if (Constant *C = SymbolicallyEvaluateGEP(Ops, NumOps, DestTy, Context, TD))
      return C;
    
    return Context->getConstantExprGetElementPtr(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*const * Ops, 
                                                unsigned NumOps,
                                                LLVMContext *Context,
                                                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>(Ops[0])) {
    if (TD && Ops[1]->isNullValue()) {
      const Type *IntPtrTy = TD->getIntPtrType();
      if (CE0->getOpcode() == Instruction::IntToPtr) {
        // Convert the integer value to the right size to ensure we get the
        // proper extension or truncation.
        Constant *C = Context->getConstantExprIntegerCast(CE0->getOperand(0),
                                                   IntPtrTy, false);
        Constant *NewOps[] = { C, Context->getNullValue(C->getType()) };
        return ConstantFoldCompareInstOperands(Predicate, NewOps, 2,
                                               Context, 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 *NewOps[] = { C, Context->getNullValue(C->getType()) };
        // FIXME!
        return ConstantFoldCompareInstOperands(Predicate, NewOps, 2,
                                               Context, TD);
      }
    }
    
    if (ConstantExpr *CE1 = dyn_cast<ConstantExpr>(Ops[1])) {
      if (TD && CE0->getOpcode() == CE1->getOpcode()) {
        const Type *IntPtrTy = TD->getIntPtrType();

        if (CE0->getOpcode() == Instruction::IntToPtr) {
          // Convert the integer value to the right size to ensure we get the
          // proper extension or truncation.
          Constant *C0 = Context->getConstantExprIntegerCast(CE0->getOperand(0),
                                                      IntPtrTy, false);
          Constant *C1 = Context->getConstantExprIntegerCast(CE1->getOperand(0),
                                                      IntPtrTy, false);
          Constant *NewOps[] = { C0, C1 };
          return ConstantFoldCompareInstOperands(Predicate, NewOps, 2, 
                                                 Context, 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())) {
          Constant *NewOps[] = { 
            CE0->getOperand(0), CE1->getOperand(0) 
          };
          return ConstantFoldCompareInstOperands(Predicate, NewOps, 2, 
                                                 Context, TD);
        }
      }
    }
  }
  return Context->getConstantExprCompare(Predicate, Ops[0], Ops[1]);
}


/// 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,
                                                       LLVMContext *Context) {
  if (CE->getOperand(1) != Context->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 = Context->getNullValue(STy->getElementType(El));
      } else if (isa<UndefValue>(C)) {
        C = Context->getUndef(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 = Context->getNullValue(ATy->getElementType());
        else if (isa<UndefValue>(C))
          C = Context->getUndef(ATy->getElementType());
        else
          return 0;
      } else if (const VectorType *PTy = dyn_cast<VectorType>(*I)) {
        if (CI->getZExtValue() >= PTy->getNumElements())
          return 0;
        if (ConstantVector *CP = dyn_cast<ConstantVector>(C))
          C = CP->getOperand(CI->getZExtValue());
        else if (isa<ConstantAggregateZero>(C))
          C = Context->getNullValue(PTy->getElementType());
        else if (isa<UndefValue>(C))
          C = Context->getUndef(PTy->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:
    return true;
  default: break;
  }

  if (!F->hasName()) return false;
  const char *Str = F->getNameStart();
  unsigned Len = F->getNameLen();
  
  // In these cases, the check of the length is required.  We don't want to
  // return true for a name like "cos\0blah" which strcmp would return equal to
  // "cos", but has length 8.
  switch (Str[0]) {
  default: return false;
  case 'a':
    if (Len == 4)
      return !strcmp(Str, "acos") || !strcmp(Str, "asin") ||
             !strcmp(Str, "atan");
    else if (Len == 5)
      return !strcmp(Str, "atan2");
    return false;
  case 'c':
    if (Len == 3)
      return !strcmp(Str, "cos");
    else if (Len == 4)
      return !strcmp(Str, "ceil") || !strcmp(Str, "cosf") ||
             !strcmp(Str, "cosh");
    return false;
  case 'e':
    if (Len == 3)
      return !strcmp(Str, "exp");
    return false;
  case 'f':
    if (Len == 4)
      return !strcmp(Str, "fabs") || !strcmp(Str, "fmod");
    else if (Len == 5)
      return !strcmp(Str, "floor");
    return false;
    break;
  case 'l':
    if (Len == 3 && !strcmp(Str, "log"))
      return true;
    if (Len == 5 && !strcmp(Str, "log10"))
      return true;
    return false;
  case 'p':
    if (Len == 3 && !strcmp(Str, "pow"))
      return true;
    return false;
  case 's':
    if (Len == 3)
      return !strcmp(Str, "sin");
    if (Len == 4)
      return !strcmp(Str, "sinh") || !strcmp(Str, "sqrt") ||
             !strcmp(Str, "sinf");
    if (Len == 5)
      return !strcmp(Str, "sqrtf");
    return false;
  case 't':
    if (Len == 3 && !strcmp(Str, "tan"))
      return true;
    else if (Len == 4 && !strcmp(Str, "tanh"))
      return true;
    return false;
  }
}

static Constant *ConstantFoldFP(double (*NativeFP)(double), double V, 
                                const Type *Ty, LLVMContext *Context) {
  errno = 0;
  V = NativeFP(V);
  if (errno != 0) {
    errno = 0;
    return 0;
  }
  
  if (Ty == Type::FloatTy)
    return Context->getConstantFP(APFloat((float)V));
  if (Ty == Type::DoubleTy)
    return Context->getConstantFP(APFloat(V));
  llvm_unreachable("Can only constant fold float/double");
  return 0; // dummy return to suppress warning
}

static Constant *ConstantFoldBinaryFP(double (*NativeFP)(double, double),
                                      double V, double W,
                                      const Type *Ty,
                                      LLVMContext *Context) {
  errno = 0;
  V = NativeFP(V, W);
  if (errno != 0) {
    errno = 0;
    return 0;
  }
  
  if (Ty == Type::FloatTy)
    return Context->getConstantFP(APFloat((float)V));
  if (Ty == Type::DoubleTy)
    return Context->getConstantFP(APFloat(V));
  llvm_unreachable("Can only constant fold float/double");
  return 0; // dummy return to suppress warning
}

/// ConstantFoldCall - Attempt to constant fold a call to the specified function
/// with the specified arguments, returning null if unsuccessful.

Constant *
llvm::ConstantFoldCall(Function *F, 
                       Constant* const* Operands, unsigned NumOperands) {
  if (!F->hasName()) return 0;
  LLVMContext *Context = F->getContext();
  const char *Str = F->getNameStart();
  unsigned Len = F->getNameLen();
  
  const Type *Ty = F->getReturnType();
  if (NumOperands == 1) {
    if (ConstantFP *Op = dyn_cast<ConstantFP>(Operands[0])) {
      if (Ty!=Type::FloatTy && Ty!=Type::DoubleTy)
        return 0;
      /// Currently APFloat versions of these functions do not exist, so we use
      /// the host native double versions.  Float versions are not called
      /// directly but for all these it is true (float)(f((double)arg)) ==
      /// f(arg).  Long double not supported yet.
      double V = Ty==Type::FloatTy ? (double)Op->getValueAPF().convertToFloat():
                                     Op->getValueAPF().convertToDouble();
      switch (Str[0]) {
      case 'a':
        if (Len == 4 && !strcmp(Str, "acos"))
          return ConstantFoldFP(acos, V, Ty, Context);
        else if (Len == 4 && !strcmp(Str, "asin"))
          return ConstantFoldFP(asin, V, Ty, Context);
        else if (Len == 4 && !strcmp(Str, "atan"))
          return ConstantFoldFP(atan, V, Ty, Context);
        break;
      case 'c':
        if (Len == 4 && !strcmp(Str, "ceil"))
          return ConstantFoldFP(ceil, V, Ty, Context);
        else if (Len == 3 && !strcmp(Str, "cos"))
          return ConstantFoldFP(cos, V, Ty, Context);
        else if (Len == 4 && !strcmp(Str, "cosh"))
          return ConstantFoldFP(cosh, V, Ty, Context);
        else if (Len == 4 && !strcmp(Str, "cosf"))
          return ConstantFoldFP(cos, V, Ty, Context);
        break;
      case 'e':
        if (Len == 3 && !strcmp(Str, "exp"))
          return ConstantFoldFP(exp, V, Ty, Context);
        break;
      case 'f':
        if (Len == 4 && !strcmp(Str, "fabs"))
          return ConstantFoldFP(fabs, V, Ty, Context);
        else if (Len == 5 && !strcmp(Str, "floor"))
          return ConstantFoldFP(floor, V, Ty, Context);
        break;
      case 'l':
        if (Len == 3 && !strcmp(Str, "log") && V > 0)
          return ConstantFoldFP(log, V, Ty, Context);
        else if (Len == 5 && !strcmp(Str, "log10") && V > 0)
          return ConstantFoldFP(log10, V, Ty, Context);
        else if (!strcmp(Str, "llvm.sqrt.f32") ||
                 !strcmp(Str, "llvm.sqrt.f64")) {
          if (V >= -0.0)
            return ConstantFoldFP(sqrt, V, Ty, Context);
          else // Undefined
            return Context->getNullValue(Ty);
        }
        break;
      case 's':
        if (Len == 3 && !strcmp(Str, "sin"))
          return ConstantFoldFP(sin, V, Ty, Context);
        else if (Len == 4 && !strcmp(Str, "sinh"))
          return ConstantFoldFP(sinh, V, Ty, Context);
        else if (Len == 4 && !strcmp(Str, "sqrt") && V >= 0)
          return ConstantFoldFP(sqrt, V, Ty, Context);
        else if (Len == 5 && !strcmp(Str, "sqrtf") && V >= 0)
          return ConstantFoldFP(sqrt, V, Ty, Context);
        else if (Len == 4 && !strcmp(Str, "sinf"))
          return ConstantFoldFP(sin, V, Ty, Context);
        break;
      case 't':
        if (Len == 3 && !strcmp(Str, "tan"))
          return ConstantFoldFP(tan, V, Ty, Context);
        else if (Len == 4 && !strcmp(Str, "tanh"))
          return ConstantFoldFP(tanh, V, Ty, Context);
        break;
      default:
        break;
      }
    } else if (ConstantInt *Op = dyn_cast<ConstantInt>(Operands[0])) {
      if (Len > 11 && !memcmp(Str, "llvm.bswap", 10))
        return Context->getConstantInt(Op->getValue().byteSwap());
      else if (Len > 11 && !memcmp(Str, "llvm.ctpop", 10))
        return Context->getConstantInt(Ty, Op->getValue().countPopulation());
      else if (Len > 10 && !memcmp(Str, "llvm.cttz", 9))
        return Context->getConstantInt(Ty, Op->getValue().countTrailingZeros());
      else if (Len > 10 && !memcmp(Str, "llvm.ctlz", 9))
        return Context->getConstantInt(Ty, Op->getValue().countLeadingZeros());
    }
  } else if (NumOperands == 2) {
    if (ConstantFP *Op1 = dyn_cast<ConstantFP>(Operands[0])) {
      if (Ty!=Type::FloatTy && Ty!=Type::DoubleTy)
        return 0;
      double Op1V = Ty==Type::FloatTy ? 
                      (double)Op1->getValueAPF().convertToFloat():
                      Op1->getValueAPF().convertToDouble();
      if (ConstantFP *Op2 = dyn_cast<ConstantFP>(Operands[1])) {
        double Op2V = Ty==Type::FloatTy ? 
                      (double)Op2->getValueAPF().convertToFloat():
                      Op2->getValueAPF().convertToDouble();

        if (Len == 3 && !strcmp(Str, "pow")) {
          return ConstantFoldBinaryFP(pow, Op1V, Op2V, Ty, Context);
        } else if (Len == 4 && !strcmp(Str, "fmod")) {
          return ConstantFoldBinaryFP(fmod, Op1V, Op2V, Ty, Context);
        } else if (Len == 5 && !strcmp(Str, "atan2")) {
          return ConstantFoldBinaryFP(atan2, Op1V, Op2V, Ty, Context);
        }
      } else if (ConstantInt *Op2C = dyn_cast<ConstantInt>(Operands[1])) {
        if (!strcmp(Str, "llvm.powi.f32")) {
          return Context->getConstantFP(APFloat((float)std::pow((float)Op1V,
                                                 (int)Op2C->getZExtValue())));
        } else if (!strcmp(Str, "llvm.powi.f64")) {
          return Context->getConstantFP(APFloat((double)std::pow((double)Op1V,
                                                 (int)Op2C->getZExtValue())));
        }
      }
    }
  }
  return 0;
}