Check in LLVM r95781.

1 files changed, 1438 insertions, 0 deletions

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
new file mode 100644
index 0000000000..f9331e76d0
--- /dev/null
+++ b/lib/Analysis/ValueTracking.cpp
@@ -0,0 +1,1438 @@
+//===- ValueTracking.cpp - Walk computations to compute properties --------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains routines that help analyze properties that chains of
+// computations have.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/Constants.h"
+#include "llvm/Instructions.h"
+#include "llvm/GlobalVariable.h"
+#include "llvm/GlobalAlias.h"
+#include "llvm/IntrinsicInst.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Operator.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Support/GetElementPtrTypeIterator.h"
+#include "llvm/Support/MathExtras.h"
+#include <cstring>
+using namespace llvm;
+
+/// ComputeMaskedBits - Determine which of the bits specified in Mask are
+/// known to be either zero or one and return them in the KnownZero/KnownOne
+/// bit sets.  This code only analyzes bits in Mask, in order to short-circuit
+/// processing.
+/// NOTE: we cannot consider 'undef' to be "IsZero" here.  The problem is that
+/// we cannot optimize based on the assumption that it is zero without changing
+/// it to be an explicit zero.  If we don't change it to zero, other code could
+/// optimized based on the contradictory assumption that it is non-zero.
+/// Because instcombine aggressively folds operations with undef args anyway,
+/// this won't lose us code quality.
+///
+/// This function is defined on values with integer type, values with pointer
+/// type (but only if TD is non-null), and vectors of integers.  In the case
+/// where V is a vector, the mask, known zero, and known one values are the
+/// same width as the vector element, and the bit is set only if it is true
+/// for all of the elements in the vector.
+void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
+                             APInt &KnownZero, APInt &KnownOne,
+                             const TargetData *TD, unsigned Depth) {
+  const unsigned MaxDepth = 6;
+  assert(V && "No Value?");
+  assert(Depth <= MaxDepth && "Limit Search Depth");
+  unsigned BitWidth = Mask.getBitWidth();
+  assert((V->getType()->isIntOrIntVector() || isa<PointerType>(V->getType())) &&
+         "Not integer or pointer type!");
+  assert((!TD ||
+          TD->getTypeSizeInBits(V->getType()->getScalarType()) == BitWidth) &&
+         (!V->getType()->isIntOrIntVector() ||
+          V->getType()->getScalarSizeInBits() == BitWidth) &&
+         KnownZero.getBitWidth() == BitWidth && 
+         KnownOne.getBitWidth() == BitWidth &&
+         "V, Mask, KnownOne and KnownZero should have same BitWidth");
+
+  if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
+    // We know all of the bits for a constant!
+    KnownOne = CI->getValue() & Mask;
+    KnownZero = ~KnownOne & Mask;
+    return;
+  }
+  // Null and aggregate-zero are all-zeros.
+  if (isa<ConstantPointerNull>(V) ||
+      isa<ConstantAggregateZero>(V)) {
+    KnownOne.clear();
+    KnownZero = Mask;
+    return;
+  }
+  // Handle a constant vector by taking the intersection of the known bits of
+  // each element.
+  if (ConstantVector *CV = dyn_cast<ConstantVector>(V)) {
+    KnownZero.set(); KnownOne.set();
+    for (unsigned i = 0, e = CV->getNumOperands(); i != e; ++i) {
+      APInt KnownZero2(BitWidth, 0), KnownOne2(BitWidth, 0);
+      ComputeMaskedBits(CV->getOperand(i), Mask, KnownZero2, KnownOne2,
+                        TD, Depth);
+      KnownZero &= KnownZero2;
+      KnownOne &= KnownOne2;
+    }
+    return;
+  }
+  // The address of an aligned GlobalValue has trailing zeros.
+  if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
+    unsigned Align = GV->getAlignment();
+    if (Align == 0 && TD && GV->getType()->getElementType()->isSized()) {
+      const Type *ObjectType = GV->getType()->getElementType();
+      // If the object is defined in the current Module, we'll be giving
+      // it the preferred alignment. Otherwise, we have to assume that it
+      // may only have the minimum ABI alignment.
+      if (!GV->isDeclaration() && !GV->mayBeOverridden())
+        Align = TD->getPrefTypeAlignment(ObjectType);
+      else
+        Align = TD->getABITypeAlignment(ObjectType);
+    }
+    if (Align > 0)
+      KnownZero = Mask & APInt::getLowBitsSet(BitWidth,
+                                              CountTrailingZeros_32(Align));
+    else
+      KnownZero.clear();
+    KnownOne.clear();
+    return;
+  }
+  // A weak GlobalAlias is totally unknown. A non-weak GlobalAlias has
+  // the bits of its aliasee.
+  if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
+    if (GA->mayBeOverridden()) {
+      KnownZero.clear(); KnownOne.clear();
+    } else {
+      ComputeMaskedBits(GA->getAliasee(), Mask, KnownZero, KnownOne,
+                        TD, Depth+1);
+    }
+    return;
+  }
+
+  KnownZero.clear(); KnownOne.clear();   // Start out not knowing anything.
+
+  if (Depth == MaxDepth || Mask == 0)
+    return;  // Limit search depth.
+
+  Operator *I = dyn_cast<Operator>(V);
+  if (!I) return;
+
+  APInt KnownZero2(KnownZero), KnownOne2(KnownOne);
+  switch (I->getOpcode()) {
+  default: break;
+  case Instruction::And: {
+    // If either the LHS or the RHS are Zero, the result is zero.
+    ComputeMaskedBits(I->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1);
+    APInt Mask2(Mask & ~KnownZero);
+    ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
+                      Depth+1);
+    assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+    assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); 
+    
+    // Output known-1 bits are only known if set in both the LHS & RHS.
+    KnownOne &= KnownOne2;
+    // Output known-0 are known to be clear if zero in either the LHS | RHS.
+    KnownZero |= KnownZero2;
+    return;
+  }
+  case Instruction::Or: {
+    ComputeMaskedBits(I->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1);
+    APInt Mask2(Mask & ~KnownOne);
+    ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
+                      Depth+1);
+    assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+    assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); 
+    
+    // Output known-0 bits are only known if clear in both the LHS & RHS.
+    KnownZero &= KnownZero2;
+    // Output known-1 are known to be set if set in either the LHS | RHS.
+    KnownOne |= KnownOne2;
+    return;
+  }
+  case Instruction::Xor: {
+    ComputeMaskedBits(I->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1);
+    ComputeMaskedBits(I->getOperand(0), Mask, KnownZero2, KnownOne2, TD,
+                      Depth+1);
+    assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+    assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); 
+    
+    // Output known-0 bits are known if clear or set in both the LHS & RHS.
+    APInt KnownZeroOut = (KnownZero & KnownZero2) | (KnownOne & KnownOne2);
+    // Output known-1 are known to be set if set in only one of the LHS, RHS.
+    KnownOne = (KnownZero & KnownOne2) | (KnownOne & KnownZero2);
+    KnownZero = KnownZeroOut;
+    return;
+  }
+  case Instruction::Mul: {
+    APInt Mask2 = APInt::getAllOnesValue(BitWidth);
+    ComputeMaskedBits(I->getOperand(1), Mask2, KnownZero, KnownOne, TD,Depth+1);
+    ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
+                      Depth+1);
+    assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+    assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); 
+    
+    // If low bits are zero in either operand, output low known-0 bits.
+    // Also compute a conserative estimate for high known-0 bits.
+    // More trickiness is possible, but this is sufficient for the
+    // interesting case of alignment computation.
+    KnownOne.clear();
+    unsigned TrailZ = KnownZero.countTrailingOnes() +
+                      KnownZero2.countTrailingOnes();
+    unsigned LeadZ =  std::max(KnownZero.countLeadingOnes() +
+                               KnownZero2.countLeadingOnes(),
+                               BitWidth) - BitWidth;
+
+    TrailZ = std::min(TrailZ, BitWidth);
+    LeadZ = std::min(LeadZ, BitWidth);
+    KnownZero = APInt::getLowBitsSet(BitWidth, TrailZ) |
+                APInt::getHighBitsSet(BitWidth, LeadZ);
+    KnownZero &= Mask;
+    return;
+  }
+  case Instruction::UDiv: {
+    // For the purposes of computing leading zeros we can conservatively
+    // treat a udiv as a logical right shift by the power of 2 known to
+    // be less than the denominator.
+    APInt AllOnes = APInt::getAllOnesValue(BitWidth);
+    ComputeMaskedBits(I->getOperand(0),
+                      AllOnes, KnownZero2, KnownOne2, TD, Depth+1);
+    unsigned LeadZ = KnownZero2.countLeadingOnes();
+
+    KnownOne2.clear();
+    KnownZero2.clear();
+    ComputeMaskedBits(I->getOperand(1),
+                      AllOnes, KnownZero2, KnownOne2, TD, Depth+1);
+    unsigned RHSUnknownLeadingOnes = KnownOne2.countLeadingZeros();
+    if (RHSUnknownLeadingOnes != BitWidth)
+      LeadZ = std::min(BitWidth,
+                       LeadZ + BitWidth - RHSUnknownLeadingOnes - 1);
+
+    KnownZero = APInt::getHighBitsSet(BitWidth, LeadZ) & Mask;
+    return;
+  }
+  case Instruction::Select:
+    ComputeMaskedBits(I->getOperand(2), Mask, KnownZero, KnownOne, TD, Depth+1);
+    ComputeMaskedBits(I->getOperand(1), Mask, KnownZero2, KnownOne2, TD,
+                      Depth+1);
+    assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+    assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); 
+
+    // Only known if known in both the LHS and RHS.
+    KnownOne &= KnownOne2;
+    KnownZero &= KnownZero2;
+    return;
+  case Instruction::FPTrunc:
+  case Instruction::FPExt:
+  case Instruction::FPToUI:
+  case Instruction::FPToSI:
+  case Instruction::SIToFP:
+  case Instruction::UIToFP:
+    return; // Can't work with floating point.
+  case Instruction::PtrToInt:
+  case Instruction::IntToPtr:
+    // We can't handle these if we don't know the pointer size.
+    if (!TD) return;
+    // FALL THROUGH and handle them the same as zext/trunc.
+  case Instruction::ZExt:
+  case Instruction::Trunc: {
+    const Type *SrcTy = I->getOperand(0)->getType();
+    
+    unsigned SrcBitWidth;
+    // Note that we handle pointer operands here because of inttoptr/ptrtoint
+    // which fall through here.
+    if (isa<PointerType>(SrcTy))
+      SrcBitWidth = TD->getTypeSizeInBits(SrcTy);
+    else
+      SrcBitWidth = SrcTy->getScalarSizeInBits();
+    
+    APInt MaskIn(Mask);
+    MaskIn.zextOrTrunc(SrcBitWidth);
+    KnownZero.zextOrTrunc(SrcBitWidth);
+    KnownOne.zextOrTrunc(SrcBitWidth);
+    ComputeMaskedBits(I->getOperand(0), MaskIn, KnownZero, KnownOne, TD,
+                      Depth+1);
+    KnownZero.zextOrTrunc(BitWidth);
+    KnownOne.zextOrTrunc(BitWidth);
+    // Any top bits are known to be zero.
+    if (BitWidth > SrcBitWidth)
+      KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth);
+    return;
+  }
+  case Instruction::BitCast: {
+    const Type *SrcTy = I->getOperand(0)->getType();
+    if ((SrcTy->isInteger() || isa<PointerType>(SrcTy)) &&
+        // TODO: For now, not handling conversions like:
+        // (bitcast i64 %x to <2 x i32>)
+        !isa<VectorType>(I->getType())) {
+      ComputeMaskedBits(I->getOperand(0), Mask, KnownZero, KnownOne, TD,
+                        Depth+1);
+      return;
+    }
+    break;
+  }
+  case Instruction::SExt: {
+    // Compute the bits in the result that are not present in the input.
+    unsigned SrcBitWidth = I->getOperand(0)->getType()->getScalarSizeInBits();
+      
+    APInt MaskIn(Mask); 
+    MaskIn.trunc(SrcBitWidth);
+    KnownZero.trunc(SrcBitWidth);
+    KnownOne.trunc(SrcBitWidth);
+    ComputeMaskedBits(I->getOperand(0), MaskIn, KnownZero, KnownOne, TD,
+                      Depth+1);
+    assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+    KnownZero.zext(BitWidth);
+    KnownOne.zext(BitWidth);
+
+    // If the sign bit of the input is known set or clear, then we know the
+    // top bits of the result.
+    if (KnownZero[SrcBitWidth-1])             // Input sign bit known zero
+      KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth);
+    else if (KnownOne[SrcBitWidth-1])           // Input sign bit known set
+      KnownOne |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth);
+    return;
+  }
+  case Instruction::Shl:
+    // (shl X, C1) & C2 == 0   iff   (X & C2 >>u C1) == 0
+    if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
+      uint64_t ShiftAmt = SA->getLimitedValue(BitWidth);
+      APInt Mask2(Mask.lshr(ShiftAmt));
+      ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero, KnownOne, TD,
+                        Depth+1);
+      assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+      KnownZero <<= ShiftAmt;
+      KnownOne  <<= ShiftAmt;
+      KnownZero |= APInt::getLowBitsSet(BitWidth, ShiftAmt); // low bits known 0
+      return;
+    }
+    break;
+  case Instruction::LShr:
+    // (ushr X, C1) & C2 == 0   iff  (-1 >> C1) & C2 == 0
+    if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
+      // Compute the new bits that are at the top now.
+      uint64_t ShiftAmt = SA->getLimitedValue(BitWidth);
+      
+      // Unsigned shift right.
+      APInt Mask2(Mask.shl(ShiftAmt));
+      ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero,KnownOne, TD,
+                        Depth+1);
+      assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+      KnownZero = APIntOps::lshr(KnownZero, ShiftAmt);
+      KnownOne  = APIntOps::lshr(KnownOne, ShiftAmt);
+      // high bits known zero.
+      KnownZero |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
+      return;
+    }
+    break;
+  case Instruction::AShr:
+    // (ashr X, C1) & C2 == 0   iff  (-1 >> C1) & C2 == 0
+    if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
+      // Compute the new bits that are at the top now.
+      uint64_t ShiftAmt = SA->getLimitedValue(BitWidth);
+      
+      // Signed shift right.
+      APInt Mask2(Mask.shl(ShiftAmt));
+      ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero, KnownOne, TD,
+                        Depth+1);
+      assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+      KnownZero = APIntOps::lshr(KnownZero, ShiftAmt);
+      KnownOne  = APIntOps::lshr(KnownOne, ShiftAmt);
+        
+      APInt HighBits(APInt::getHighBitsSet(BitWidth, ShiftAmt));
+      if (KnownZero[BitWidth-ShiftAmt-1])    // New bits are known zero.
+        KnownZero |= HighBits;
+      else if (KnownOne[BitWidth-ShiftAmt-1])  // New bits are known one.
+        KnownOne |= HighBits;
+      return;
+    }
+    break;
+  case Instruction::Sub: {
+    if (ConstantInt *CLHS = dyn_cast<ConstantInt>(I->getOperand(0))) {
+      // We know that the top bits of C-X are clear if X contains less bits
+      // than C (i.e. no wrap-around can happen).  For example, 20-X is
+      // positive if we can prove that X is >= 0 and < 16.
+      if (!CLHS->getValue().isNegative()) {
+        unsigned NLZ = (CLHS->getValue()+1).countLeadingZeros();
+        // NLZ can't be BitWidth with no sign bit
+        APInt MaskV = APInt::getHighBitsSet(BitWidth, NLZ+1);
+        ComputeMaskedBits(I->getOperand(1), MaskV, KnownZero2, KnownOne2,
+                          TD, Depth+1);
+    
+        // If all of the MaskV bits are known to be zero, then we know the
+        // output top bits are zero, because we now know that the output is
+        // from [0-C].
+        if ((KnownZero2 & MaskV) == MaskV) {
+          unsigned NLZ2 = CLHS->getValue().countLeadingZeros();
+          // Top bits known zero.
+          KnownZero = APInt::getHighBitsSet(BitWidth, NLZ2) & Mask;
+        }
+      }        
+    }
+  }
+  // fall through
+  case Instruction::Add: {
+    // If one of the operands has trailing zeros, then the bits that the
+    // other operand has in those bit positions will be preserved in the
+    // result. For an add, this works with either operand. For a subtract,
+    // this only works if the known zeros are in the right operand.
+    APInt LHSKnownZero(BitWidth, 0), LHSKnownOne(BitWidth, 0);
+    APInt Mask2 = APInt::getLowBitsSet(BitWidth,
+                                       BitWidth - Mask.countLeadingZeros());
+    ComputeMaskedBits(I->getOperand(0), Mask2, LHSKnownZero, LHSKnownOne, TD,
+                      Depth+1);
+    assert((LHSKnownZero & LHSKnownOne) == 0 &&
+           "Bits known to be one AND zero?");
+    unsigned LHSKnownZeroOut = LHSKnownZero.countTrailingOnes();
+
+    ComputeMaskedBits(I->getOperand(1), Mask2, KnownZero2, KnownOne2, TD, 
+                      Depth+1);
+    assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); 
+    unsigned RHSKnownZeroOut = KnownZero2.countTrailingOnes();
+
+    // Determine which operand has more trailing zeros, and use that
+    // many bits from the other operand.
+    if (LHSKnownZeroOut > RHSKnownZeroOut) {
+      if (I->getOpcode() == Instruction::Add) {
+        APInt Mask = APInt::getLowBitsSet(BitWidth, LHSKnownZeroOut);
+        KnownZero |= KnownZero2 & Mask;
+        KnownOne  |= KnownOne2 & Mask;
+      } else {
+        // If the known zeros are in the left operand for a subtract,
+        // fall back to the minimum known zeros in both operands.
+        KnownZero |= APInt::getLowBitsSet(BitWidth,
+                                          std::min(LHSKnownZeroOut,
+                                                   RHSKnownZeroOut));
+      }
+    } else if (RHSKnownZeroOut >= LHSKnownZeroOut) {
+      APInt Mask = APInt::getLowBitsSet(BitWidth, RHSKnownZeroOut);
+      KnownZero |= LHSKnownZero & Mask;
+      KnownOne  |= LHSKnownOne & Mask;
+    }
+    return;
+  }
+  case Instruction::SRem:
+    if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {
+      APInt RA = Rem->getValue().abs();
+      if (RA.isPowerOf2()) {
+        APInt LowBits = RA - 1;
+        APInt Mask2 = LowBits | APInt::getSignBit(BitWidth);
+        ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD, 
+                          Depth+1);
+
+        // The low bits of the first operand are unchanged by the srem.
+        KnownZero = KnownZero2 & LowBits;
+        KnownOne = KnownOne2 & LowBits;
+
+        // If the first operand is non-negative or has all low bits zero, then
+        // the upper bits are all zero.
+        if (KnownZero2[BitWidth-1] || ((KnownZero2 & LowBits) == LowBits))
+          KnownZero |= ~LowBits;
+
+        // If the first operand is negative and not all low bits are zero, then
+        // the upper bits are all one.
+        if (KnownOne2[BitWidth-1] && ((KnownOne2 & LowBits) != 0))
+          KnownOne |= ~LowBits;
+
+        KnownZero &= Mask;
+        KnownOne &= Mask;
+
+        assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+      }
+    }
+    break;
+  case Instruction::URem: {
+    if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {
+      APInt RA = Rem->getValue();
+      if (RA.isPowerOf2()) {
+        APInt LowBits = (RA - 1);
+        APInt Mask2 = LowBits & Mask;
+        KnownZero |= ~LowBits & Mask;
+        ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero, KnownOne, TD,
+                          Depth+1);
+        assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
+        break;
+      }
+    }
+
+    // Since the result is less than or equal to either operand, any leading
+    // zero bits in either operand must also exist in the result.
+    APInt AllOnes = APInt::getAllOnesValue(BitWidth);
+    ComputeMaskedBits(I->getOperand(0), AllOnes, KnownZero, KnownOne,
+                      TD, Depth+1);
+    ComputeMaskedBits(I->getOperand(1), AllOnes, KnownZero2, KnownOne2,
+                      TD, Depth+1);
+
+    unsigned Leaders = std::max(KnownZero.countLeadingOnes(),
+                                KnownZero2.countLeadingOnes());
+    KnownOne.clear();
+    KnownZero = APInt::getHighBitsSet(BitWidth, Leaders) & Mask;
+    break;
+  }
+
+  case Instruction::Alloca: {
+    AllocaInst *AI = cast<AllocaInst>(V);
+    unsigned Align = AI->getAlignment();
+    if (Align == 0 && TD)
+      Align = TD->getABITypeAlignment(AI->getType()->getElementType());
+    
+    if (Align > 0)
+      KnownZero = Mask & APInt::getLowBitsSet(BitWidth,
+                                              CountTrailingZeros_32(Align));
+    break;
+  }
+  case Instruction::GetElementPtr: {
+    // Analyze all of the subscripts of this getelementptr instruction
+    // to determine if we can prove known low zero bits.
+    APInt LocalMask = APInt::getAllOnesValue(BitWidth);
+    APInt LocalKnownZero(BitWidth, 0), LocalKnownOne(BitWidth, 0);
+    ComputeMaskedBits(I->getOperand(0), LocalMask,
+                      LocalKnownZero, LocalKnownOne, TD, Depth+1);
+    unsigned TrailZ = LocalKnownZero.countTrailingOnes();
+
+    gep_type_iterator GTI = gep_type_begin(I);
+    for (unsigned i = 1, e = I->getNumOperands(); i != e; ++i, ++GTI) {
+      Value *Index = I->getOperand(i);
+      if (const StructType *STy = dyn_cast<StructType>(*GTI)) {
+        // Handle struct member offset arithmetic.
+        if (!TD) return;
+        const StructLayout *SL = TD->getStructLayout(STy);
+        unsigned Idx = cast<ConstantInt>(Index)->getZExtValue();
+        uint64_t Offset = SL->getElementOffset(Idx);
+        TrailZ = std::min(TrailZ,
+                          CountTrailingZeros_64(Offset));
+      } else {
+        // Handle array index arithmetic.
+        const Type *IndexedTy = GTI.getIndexedType();
+        if (!IndexedTy->isSized()) return;
+        unsigned GEPOpiBits = Index->getType()->getScalarSizeInBits();
+        uint64_t TypeSize = TD ? TD->getTypeAllocSize(IndexedTy) : 1;
+        LocalMask = APInt::getAllOnesValue(GEPOpiBits);
+        LocalKnownZero = LocalKnownOne = APInt(GEPOpiBits, 0);
+        ComputeMaskedBits(Index, LocalMask,
+                          LocalKnownZero, LocalKnownOne, TD, Depth+1);
+        TrailZ = std::min(TrailZ,
+                          unsigned(CountTrailingZeros_64(TypeSize) +
+                                   LocalKnownZero.countTrailingOnes()));
+      }
+    }
+    
+    KnownZero = APInt::getLowBitsSet(BitWidth, TrailZ) & Mask;
+    break;
+  }
+  case Instruction::PHI: {
+    PHINode *P = cast<PHINode>(I);
+    // Handle the case of a simple two-predecessor recurrence PHI.
+    // There's a lot more that could theoretically be done here, but
+    // this is sufficient to catch some interesting cases.
+    if (P->getNumIncomingValues() == 2) {
+      for (unsigned i = 0; i != 2; ++i) {
+        Value *L = P->getIncomingValue(i);
+        Value *R = P->getIncomingValue(!i);
+        Operator *LU = dyn_cast<Operator>(L);
+        if (!LU)
+          continue;
+        unsigned Opcode = LU->getOpcode();
+        // Check for operations that have the property that if
+        // both their operands have low zero bits, the result
+        // will have low zero bits.
+        if (Opcode == Instruction::Add ||
+            Opcode == Instruction::Sub ||
+            Opcode == Instruction::And ||
+            Opcode == Instruction::Or ||
+            Opcode == Instruction::Mul) {
+          Value *LL = LU->getOperand(0);
+          Value *LR = LU->getOperand(1);
+          // Find a recurrence.
+          if (LL == I)
+            L = LR;
+          else if (LR == I)
+            L = LL;
+          else
+            break;
+          // Ok, we have a PHI of the form L op= R. Check for low
+          // zero bits.
+          APInt Mask2 = APInt::getAllOnesValue(BitWidth);
+          ComputeMaskedBits(R, Mask2, KnownZero2, KnownOne2, TD, Depth+1);
+          Mask2 = APInt::getLowBitsSet(BitWidth,
+                                       KnownZero2.countTrailingOnes());
+
+          // We need to take the minimum number of known bits
+          APInt KnownZero3(KnownZero), KnownOne3(KnownOne);
+          ComputeMaskedBits(L, Mask2, KnownZero3, KnownOne3, TD, Depth+1);
+
+          KnownZero = Mask &
+                      APInt::getLowBitsSet(BitWidth,
+                                           std::min(KnownZero2.countTrailingOnes(),
+                                                    KnownZero3.countTrailingOnes()));
+          break;
+        }
+      }
+    }
+
+    // Otherwise take the unions of the known bit sets of the operands,
+    // taking conservative care to avoid excessive recursion.
+    if (Depth < MaxDepth - 1 && !KnownZero && !KnownOne) {
+      KnownZero = APInt::getAllOnesValue(BitWidth);
+      KnownOne = APInt::getAllOnesValue(BitWidth);
+      for (unsigned i = 0, e = P->getNumIncomingValues(); i != e; ++i) {
+        // Skip direct self references.
+        if (P->getIncomingValue(i) == P) continue;
+
+        KnownZero2 = APInt(BitWidth, 0);
+        KnownOne2 = APInt(BitWidth, 0);
+        // Recurse, but cap the recursion to one level, because we don't
+        // want to waste time spinning around in loops.
+        ComputeMaskedBits(P->getIncomingValue(i), KnownZero | KnownOne,
+                          KnownZero2, KnownOne2, TD, MaxDepth-1);
+        KnownZero &= KnownZero2;
+        KnownOne &= KnownOne2;
+        // If all bits have been ruled out, there's no need to check
+        // more operands.
+        if (!KnownZero && !KnownOne)
+          break;
+      }
+    }
+    break;
+  }
+  case Instruction::Call:
+    if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
+      switch (II->getIntrinsicID()) {
+      default: break;
+      case Intrinsic::ctpop:
+      case Intrinsic::ctlz:
+      case Intrinsic::cttz: {
+        unsigned LowBits = Log2_32(BitWidth)+1;
+        KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
+        break;
+      }
+      }
+    }
+    break;
+  }
+}
+
+/// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero.  We use
+/// this predicate to simplify operations downstream.  Mask is known to be zero
+/// for bits that V cannot have.
+///
+/// This function is defined on values with integer type, values with pointer
+/// type (but only if TD is non-null), and vectors of integers.  In the case
+/// where V is a vector, the mask, known zero, and known one values are the
+/// same width as the vector element, and the bit is set only if it is true
+/// for all of the elements in the vector.
+bool llvm::MaskedValueIsZero(Value *V, const APInt &Mask,
+                             const TargetData *TD, unsigned Depth) {
+  APInt KnownZero(Mask.getBitWidth(), 0), KnownOne(Mask.getBitWidth(), 0);
+  ComputeMaskedBits(V, Mask, KnownZero, KnownOne, TD, Depth);
+  assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
+  return (KnownZero & Mask) == Mask;
+}
+
+
+
+/// ComputeNumSignBits - Return the number of times the sign bit of the
+/// register is replicated into the other bits.  We know that at least 1 bit
+/// is always equal to the sign bit (itself), but other cases can give us
+/// information.  For example, immediately after an "ashr X, 2", we know that
+/// the top 3 bits are all equal to each other, so we return 3.
+///
+/// 'Op' must have a scalar integer type.
+///
+unsigned llvm::ComputeNumSignBits(Value *V, const TargetData *TD,
+                                  unsigned Depth) {
+  assert((TD || V->getType()->isIntOrIntVector()) &&
+         "ComputeNumSignBits requires a TargetData object to operate "
+         "on non-integer values!");
+  const Type *Ty = V->getType();
+  unsigned TyBits = TD ? TD->getTypeSizeInBits(V->getType()->getScalarType()) :
+                         Ty->getScalarSizeInBits();
+  unsigned Tmp, Tmp2;
+  unsigned FirstAnswer = 1;
+
+  // Note that ConstantInt is handled by the general ComputeMaskedBits case
+  // below.
+
+  if (Depth == 6)
+    return 1;  // Limit search depth.
+  
+  Operator *U = dyn_cast<Operator>(V);
+  switch (Operator::getOpcode(V)) {
+  default: break;
+  case Instruction::SExt:
+    Tmp = TyBits - U->getOperand(0)->getType()->getScalarSizeInBits();
+    return ComputeNumSignBits(U->getOperand(0), TD, Depth+1) + Tmp;
+    
+  case Instruction::AShr:
+    Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
+    // ashr X, C   -> adds C sign bits.
+    if (ConstantInt *C = dyn_cast<ConstantInt>(U->getOperand(1))) {
+      Tmp += C->getZExtValue();
+      if (Tmp > TyBits) Tmp = TyBits;
+    }
+    return Tmp;
+  case Instruction::Shl:
+    if (ConstantInt *C = dyn_cast<ConstantInt>(U->getOperand(1))) {
+      // shl destroys sign bits.
+      Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
+      if (C->getZExtValue() >= TyBits ||      // Bad shift.
+          C->getZExtValue() >= Tmp) break;    // Shifted all sign bits out.
+      return Tmp - C->getZExtValue();
+    }
+    break;
+  case Instruction::And:
+  case Instruction::Or:
+  case Instruction::Xor:    // NOT is handled here.
+    // Logical binary ops preserve the number of sign bits at the worst.
+    Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
+    if (Tmp != 1) {
+      Tmp2 = ComputeNumSignBits(U->getOperand(1), TD, Depth+1);
+      FirstAnswer = std::min(Tmp, Tmp2);
+      // We computed what we know about the sign bits as our first
+      // answer. Now proceed to the generic code that uses
+      // ComputeMaskedBits, and pick whichever answer is better.
+    }
+    break;
+
+  case Instruction::Select:
+    Tmp = ComputeNumSignBits(U->getOperand(1), TD, Depth+1);
+    if (Tmp == 1) return 1;  // Early out.
+    Tmp2 = ComputeNumSignBits(U->getOperand(2), TD, Depth+1);
+    return std::min(Tmp, Tmp2);
+    
+  case Instruction::Add:
+    // Add can have at most one carry bit.  Thus we know that the output
+    // is, at worst, one more bit than the inputs.
+    Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
+    if (Tmp == 1) return 1;  // Early out.
+      
+    // Special case decrementing a value (ADD X, -1):
+    if (ConstantInt *CRHS = dyn_cast<ConstantInt>(U->getOperand(1)))
+      if (CRHS->isAllOnesValue()) {
+        APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
+        APInt Mask = APInt::getAllOnesValue(TyBits);
+        ComputeMaskedBits(U->getOperand(0), Mask, KnownZero, KnownOne, TD,
+                          Depth+1);
+        
+        // If the input is known to be 0 or 1, the output is 0/-1, which is all
+        // sign bits set.
+        if ((KnownZero | APInt(TyBits, 1)) == Mask)
+          return TyBits;
+        
+        // If we are subtracting one from a positive number, there is no carry
+        // out of the result.
+        if (KnownZero.isNegative())
+          return Tmp;
+      }
+      
+    Tmp2 = ComputeNumSignBits(U->getOperand(1), TD, Depth+1);
+    if (Tmp2 == 1) return 1;
+    return std::min(Tmp, Tmp2)-1;
+    
+  case Instruction::Sub:
+    Tmp2 = ComputeNumSignBits(U->getOperand(1), TD, Depth+1);
+    if (Tmp2 == 1) return 1;
+      
+    // Handle NEG.
+    if (ConstantInt *CLHS = dyn_cast<ConstantInt>(U->getOperand(0)))
+      if (CLHS->isNullValue()) {
+        APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
+        APInt Mask = APInt::getAllOnesValue(TyBits);
+        ComputeMaskedBits(U->getOperand(1), Mask, KnownZero, KnownOne, 
+                          TD, Depth+1);
+        // If the input is known to be 0 or 1, the output is 0/-1, which is all
+        // sign bits set.
+        if ((KnownZero | APInt(TyBits, 1)) == Mask)
+          return TyBits;
+        
+        // If the input is known to be positive (the sign bit is known clear),
+        // the output of the NEG has the same number of sign bits as the input.
+        if (KnownZero.isNegative())
+          return Tmp2;
+        
+        // Otherwise, we treat this like a SUB.
+      }
+    
+    // Sub can have at most one carry bit.  Thus we know that the output
+    // is, at worst, one more bit than the inputs.
+    Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
+    if (Tmp == 1) return 1;  // Early out.
+    return std::min(Tmp, Tmp2)-1;
+      
+  case Instruction::PHI: {
+    PHINode *PN = cast<PHINode>(U);
+    // Don't analyze large in-degree PHIs.
+    if (PN->getNumIncomingValues() > 4) break;
+    
+    // Take the minimum of all incoming values.  This can't infinitely loop
+    // because of our depth threshold.
+    Tmp = ComputeNumSignBits(PN->getIncomingValue(0), TD, Depth+1);
+    for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
+      if (Tmp == 1) return Tmp;
+      Tmp = std::min(Tmp,
+                     ComputeNumSignBits(PN->getIncomingValue(1), TD, Depth+1));
+    }
+    return Tmp;
+  }
+
+  case Instruction::Trunc:
+    // FIXME: it's tricky to do anything useful for this, but it is an important
+    // case for targets like X86.
+    break;
+  }
+  
+  // Finally, if we can prove that the top bits of the result are 0's or 1's,
+  // use this information.
+  APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
+  APInt Mask = APInt::getAllOnesValue(TyBits);
+  ComputeMaskedBits(V, Mask, KnownZero, KnownOne, TD, Depth);
+  
+  if (KnownZero.isNegative()) {        // sign bit is 0
+    Mask = KnownZero;
+  } else if (KnownOne.isNegative()) {  // sign bit is 1;
+    Mask = KnownOne;
+  } else {
+    // Nothing known.
+    return FirstAnswer;
+  }
+  
+  // Okay, we know that the sign bit in Mask is set.  Use CLZ to determine
+  // the number of identical bits in the top of the input value.
+  Mask = ~Mask;
+  Mask <<= Mask.getBitWidth()-TyBits;
+  // Return # leading zeros.  We use 'min' here in case Val was zero before
+  // shifting.  We don't want to return '64' as for an i32 "0".
+  return std::max(FirstAnswer, std::min(TyBits, Mask.countLeadingZeros()));
+}
+
+/// ComputeMultiple - This function computes the integer multiple of Base that
+/// equals V.  If successful, it returns true and returns the multiple in
+/// Multiple.  If unsuccessful, it returns false. It looks
+/// through SExt instructions only if LookThroughSExt is true.
+bool llvm::ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
+                           bool LookThroughSExt, unsigned Depth) {
+  const unsigned MaxDepth = 6;
+
+  assert(V && "No Value?");
+  assert(Depth <= MaxDepth && "Limit Search Depth");
+  assert(V->getType()->isInteger() && "Not integer or pointer type!");
+
+  const Type *T = V->getType();
+
+  ConstantInt *CI = dyn_cast<ConstantInt>(V);
+
+  if (Base == 0)
+    return false;
+    
+  if (Base == 1) {
+    Multiple = V;
+    return true;
+  }
+
+  ConstantExpr *CO = dyn_cast<ConstantExpr>(V);
+  Constant *BaseVal = ConstantInt::get(T, Base);
+  if (CO && CO == BaseVal) {
+    // Multiple is 1.
+    Multiple = ConstantInt::get(T, 1);
+    return true;
+  }
+
+  if (CI && CI->getZExtValue() % Base == 0) {
+    Multiple = ConstantInt::get(T, CI->getZExtValue() / Base);
+    return true;  
+  }
+  
+  if (Depth == MaxDepth) return false;  // Limit search depth.
+        
+  Operator *I = dyn_cast<Operator>(V);
+  if (!I) return false;
+
+  switch (I->getOpcode()) {
+  default: break;
+  case Instruction::SExt:
+    if (!LookThroughSExt) return false;
+    // otherwise fall through to ZExt
+  case Instruction::ZExt:
+    return ComputeMultiple(I->getOperand(0), Base, Multiple,
+                           LookThroughSExt, Depth+1);
+  case Instruction::Shl:
+  case Instruction::Mul: {
+    Value *Op0 = I->getOperand(0);
+    Value *Op1 = I->getOperand(1);
+
+    if (I->getOpcode() == Instruction::Shl) {
+      ConstantInt *Op1CI = dyn_cast<ConstantInt>(Op1);
+      if (!Op1CI) return false;
+      // Turn Op0 << Op1 into Op0 * 2^Op1
+      APInt Op1Int = Op1CI->getValue();
+      uint64_t BitToSet = Op1Int.getLimitedValue(Op1Int.getBitWidth() - 1);
+      Op1 = ConstantInt::get(V->getContext(), 
+                             APInt(Op1Int.getBitWidth(), 0).set(BitToSet));
+    }
+
+    Value *Mul0 = NULL;
+    Value *Mul1 = NULL;
+    bool M0 = ComputeMultiple(Op0, Base, Mul0,
+                              LookThroughSExt, Depth+1);
+    bool M1 = ComputeMultiple(Op1, Base, Mul1,
+                              LookThroughSExt, Depth+1);
+
+    if (M0) {
+      if (isa<Constant>(Op1) && isa<Constant>(Mul0)) {
+        // V == Base * (Mul0 * Op1), so return (Mul0 * Op1)
+        Multiple = ConstantExpr::getMul(cast<Constant>(Mul0),
+                                        cast<Constant>(Op1));
+        return true;
+      }
+
+      if (ConstantInt *Mul0CI = dyn_cast<ConstantInt>(Mul0))
+        if (Mul0CI->getValue() == 1) {
+          // V == Base * Op1, so return Op1
+          Multiple = Op1;
+          return true;
+        }
+    }
+
+    if (M1) {
+      if (isa<Constant>(Op0) && isa<Constant>(Mul1)) {
+        // V == Base * (Mul1 * Op0), so return (Mul1 * Op0)
+        Multiple = ConstantExpr::getMul(cast<Constant>(Mul1),
+                                        cast<Constant>(Op0));
+        return true;
+      }
+
+      if (ConstantInt *Mul1CI = dyn_cast<ConstantInt>(Mul1))
+        if (Mul1CI->getValue() == 1) {
+          // V == Base * Op0, so return Op0
+          Multiple = Op0;
+          return true;
+        }
+    }
+  }
+  }
+
+  // We could not determine if V is a multiple of Base.
+  return false;
+}
+
+/// CannotBeNegativeZero - Return true if we can prove that the specified FP 
+/// value is never equal to -0.0.
+///
+/// NOTE: this function will need to be revisited when we support non-default
+/// rounding modes!
+///
+bool llvm::CannotBeNegativeZero(const Value *V, unsigned Depth) {
+  if (const ConstantFP *CFP = dyn_cast<ConstantFP>(V))
+    return !CFP->getValueAPF().isNegZero();
+  
+  if (Depth == 6)
+    return 1;  // Limit search depth.
+
+  const Operator *I = dyn_cast<Operator>(V);
+  if (I == 0) return false;
+  
+  // (add x, 0.0) is guaranteed to return +0.0, not -0.0.
+  if (I->getOpcode() == Instruction::FAdd &&
+      isa<ConstantFP>(I->getOperand(1)) && 
+      cast<ConstantFP>(I->getOperand(1))->isNullValue())
+    return true;
+    
+  // sitofp and uitofp turn into +0.0 for zero.
+  if (isa<SIToFPInst>(I) || isa<UIToFPInst>(I))
+    return true;
+  
+  if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
+    // sqrt(-0.0) = -0.0, no other negative results are possible.
+    if (II->getIntrinsicID() == Intrinsic::sqrt)
+      return CannotBeNegativeZero(II->getOperand(1), Depth+1);
+  
+  if (const CallInst *CI = dyn_cast<CallInst>(I))
+    if (const Function *F = CI->getCalledFunction()) {
+      if (F->isDeclaration()) {
+        // abs(x) != -0.0
+        if (F->getName() == "abs") return true;
+        // fabs[lf](x) != -0.0
+        if (F->getName() == "fabs") return true;
+        if (F->getName() == "fabsf") return true;
+        if (F->getName() == "fabsl") return true;
+        if (F->getName() == "sqrt" || F->getName() == "sqrtf" ||
+            F->getName() == "sqrtl")
+          return CannotBeNegativeZero(CI->getOperand(1), Depth+1);
+      }
+    }
+  
+  return false;
+}
+
+
+/// GetLinearExpression - Analyze the specified value as a linear expression:
+/// "A*V + B", where A and B are constant integers.  Return the scale and offset
+/// values as APInts and return V as a Value*.  The incoming Value is known to
+/// have IntegerType.  Note that this looks through extends, so the high bits
+/// may not be represented in the result.
+static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
+                                  const TargetData *TD, unsigned Depth) {
+  assert(isa<IntegerType>(V->getType()) && "Not an integer value");
+
+  // Limit our recursion depth.
+  if (Depth == 6) {
+    Scale = 1;
+    Offset = 0;
+    return V;
+  }
+  
+  if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
+    if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
+      switch (BOp->getOpcode()) {
+      default: break;
+      case Instruction::Or:
+        // X|C == X+C if all the bits in C are unset in X.  Otherwise we can't
+        // analyze it.
+        if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), TD))
+          break;
+        // FALL THROUGH.
+      case Instruction::Add:
+        V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, TD, Depth+1);
+        Offset += RHSC->getValue();
+        return V;
+      case Instruction::Mul:
+        V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, TD, Depth+1);
+        Offset *= RHSC->getValue();
+        Scale *= RHSC->getValue();
+        return V;
+      case Instruction::Shl:
+        V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, TD, Depth+1);
+        Offset <<= RHSC->getValue().getLimitedValue();
+        Scale <<= RHSC->getValue().getLimitedValue();
+        return V;
+      }
+    }
+  }
+  
+  // Since clients don't care about the high bits of the value, just scales and
+  // offsets, we can look through extensions.
+  if (isa<SExtInst>(V) || isa<ZExtInst>(V)) {
+    Value *CastOp = cast<CastInst>(V)->getOperand(0);
+    unsigned OldWidth = Scale.getBitWidth();
+    unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
+    Scale.trunc(SmallWidth);
+    Offset.trunc(SmallWidth);
+    Value *Result = GetLinearExpression(CastOp, Scale, Offset, TD, Depth+1);
+    Scale.zext(OldWidth);
+    Offset.zext(OldWidth);
+    return Result;
+  }
+  
+  Scale = 1;
+  Offset = 0;
+  return V;
+}
+
+/// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
+/// into a base pointer with a constant offset and a number of scaled symbolic
+/// offsets.
+///
+/// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
+/// the VarIndices vector) are Value*'s that are known to be scaled by the
+/// specified amount, but which may have other unrepresented high bits. As such,
+/// the gep cannot necessarily be reconstructed from its decomposed form.
+///
+/// When TargetData is around, this function is capable of analyzing everything
+/// that Value::getUnderlyingObject() can look through.  When not, it just looks
+/// through pointer casts.
+///
+const Value *llvm::DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
+                 SmallVectorImpl<std::pair<const Value*, int64_t> > &VarIndices,
+                                          const TargetData *TD) {
+  // Limit recursion depth to limit compile time in crazy cases.
+  unsigned MaxLookup = 6;
+  
+  BaseOffs = 0;
+  do {
+    // See if this is a bitcast or GEP.
+    const Operator *Op = dyn_cast<Operator>(V);
+    if (Op == 0) {
+      // The only non-operator case we can handle are GlobalAliases.
+      if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
+        if (!GA->mayBeOverridden()) {
+          V = GA->getAliasee();
+          continue;
+        }
+      }
+      return V;
+    }
+    
+    if (Op->getOpcode() == Instruction::BitCast) {
+      V = Op->getOperand(0);
+      continue;
+    }
+    
+    const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
+    if (GEPOp == 0)
+      return V;
+    
+    // Don't attempt to analyze GEPs over unsized objects.
+    if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
+        ->getElementType()->isSized())
+      return V;
+    
+    // If we are lacking TargetData information, we can't compute the offets of
+    // elements computed by GEPs.  However, we can handle bitcast equivalent
+    // GEPs.
+    if (!TD) {
+      if (!GEPOp->hasAllZeroIndices())
+        return V;
+      V = GEPOp->getOperand(0);
+      continue;
+    }
+    
+    // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
+    gep_type_iterator GTI = gep_type_begin(GEPOp);
+    for (User::const_op_iterator I = GEPOp->op_begin()+1,
+         E = GEPOp->op_end(); I != E; ++I) {
+      Value *Index = *I;
+      // Compute the (potentially symbolic) offset in bytes for this index.
+      if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
+        // For a struct, add the member offset.
+        unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
+        if (FieldNo == 0) continue;
+        
+        BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
+        continue;
+      }
+      
+      // For an array/pointer, add the element offset, explicitly scaled.
+      if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
+        if (CIdx->isZero()) continue;
+        BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
+        continue;
+      }
+      
+      uint64_t Scale = TD->getTypeAllocSize(*GTI);
+      
+      // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
+      unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
+      APInt IndexScale(Width, 0), IndexOffset(Width, 0);
+      Index = GetLinearExpression(Index, IndexScale, IndexOffset, TD, 0);
+      
+      // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
+      // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
+      BaseOffs += IndexOffset.getZExtValue()*Scale;
+      Scale *= IndexScale.getZExtValue();
+      
+      
+      // If we already had an occurrance of this index variable, merge this
+      // scale into it.  For example, we want to handle:
+      //   A[x][x] -> x*16 + x*4 -> x*20
+      // This also ensures that 'x' only appears in the index list once.
+      for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
+        if (VarIndices[i].first == Index) {
+          Scale += VarIndices[i].second;
+          VarIndices.erase(VarIndices.begin()+i);
+          break;
+        }
+      }
+      
+      // Make sure that we have a scale that makes sense for this target's
+      // pointer size.
+      if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
+        Scale <<= ShiftBits;
+        Scale >>= ShiftBits;
+      }
+      
+      if (Scale)
+        VarIndices.push_back(std::make_pair(Index, Scale));
+    }
+    
+    // Analyze the base pointer next.
+    V = GEPOp->getOperand(0);
+  } while (--MaxLookup);
+  
+  // If the chain of expressions is too deep, just return early.
+  return V;
+}
+
+
+// This is the recursive version of BuildSubAggregate. It takes a few different
+// arguments. Idxs is the index within the nested struct From that we are
+// looking at now (which is of type IndexedType). IdxSkip is the number of
+// indices from Idxs that should be left out when inserting into the resulting
+// struct. To is the result struct built so far, new insertvalue instructions
+// build on that.
+static Value *BuildSubAggregate(Value *From, Value* To, const Type *IndexedType,
+                                SmallVector<unsigned, 10> &Idxs,
+                                unsigned IdxSkip,
+                                Instruction *InsertBefore) {
+  const llvm::StructType *STy = llvm::dyn_cast<llvm::StructType>(IndexedType);
+  if (STy) {
+    // Save the original To argument so we can modify it
+    Value *OrigTo = To;
+    // General case, the type indexed by Idxs is a struct
+    for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
+      // Process each struct element recursively
+      Idxs.push_back(i);
+      Value *PrevTo = To;
+      To = BuildSubAggregate(From, To, STy->getElementType(i), Idxs, IdxSkip,
+                             InsertBefore);
+      Idxs.pop_back();
+      if (!To) {
+        // Couldn't find any inserted value for this index? Cleanup
+        while (PrevTo != OrigTo) {
+          InsertValueInst* Del = cast<InsertValueInst>(PrevTo);
+          PrevTo = Del->getAggregateOperand();
+          Del->eraseFromParent();
+        }
+        // Stop processing elements
+        break;
+      }
+    }
+    // If we succesfully found a value for each of our subaggregates 
+    if (To)
+      return To;
+  }
+  // Base case, the type indexed by SourceIdxs is not a struct, or not all of
+  // the struct's elements had a value that was inserted directly. In the latter
+  // case, perhaps we can't determine each of the subelements individually, but
+  // we might be able to find the complete struct somewhere.
+  
+  // Find the value that is at that particular spot
+  Value *V = FindInsertedValue(From, Idxs.begin(), Idxs.end());
+
+  if (!V)
+    return NULL;
+
+  // Insert the value in the new (sub) aggregrate
+  return llvm::InsertValueInst::Create(To, V, Idxs.begin() + IdxSkip,
+                                       Idxs.end(), "tmp", InsertBefore);
+}
+
+// This helper takes a nested struct and extracts a part of it (which is again a
+// struct) into a new value. For example, given the struct:
+// { a, { b, { c, d }, e } }
+// and the indices "1, 1" this returns
+// { c, d }.
+//
+// It does this by inserting an insertvalue for each element in the resulting
+// struct, as opposed to just inserting a single struct. This will only work if
+// each of the elements of the substruct are known (ie, inserted into From by an
+// insertvalue instruction somewhere).
+//
+// All inserted insertvalue instructions are inserted before InsertBefore
+static Value *BuildSubAggregate(Value *From, const unsigned *idx_begin,
+                                const unsigned *idx_end,
+                                Instruction *InsertBefore) {
+  assert(InsertBefore && "Must have someplace to insert!");
+  const Type *IndexedType = ExtractValueInst::getIndexedType(From->getType(),
+                                                             idx_begin,
+                                                             idx_end);
+  Value *To = UndefValue::get(IndexedType);
+  SmallVector<unsigned, 10> Idxs(idx_begin, idx_end);
+  unsigned IdxSkip = Idxs.size();
+
+  return BuildSubAggregate(From, To, IndexedType, Idxs, IdxSkip, InsertBefore);
+}
+
+/// FindInsertedValue - Given an aggregrate and an sequence of indices, see if
+/// the scalar value indexed is already around as a register, for example if it
+/// were inserted directly into the aggregrate.
+///
+/// If InsertBefore is not null, this function will duplicate (modified)
+/// insertvalues when a part of a nested struct is extracted.
+Value *llvm::FindInsertedValue(Value *V, const unsigned *idx_begin,
+                         const unsigned *idx_end, Instruction *InsertBefore) {
+  // Nothing to index? Just return V then (this is useful at the end of our
+  // recursion)
+  if (idx_begin == idx_end)
+    return V;
+  // We have indices, so V should have an indexable type
+  assert((isa<StructType>(V->getType()) || isa<ArrayType>(V->getType()))
+         && "Not looking at a struct or array?");
+  assert(ExtractValueInst::getIndexedType(V->getType(), idx_begin, idx_end)
+         && "Invalid indices for type?");
+  const CompositeType *PTy = cast<CompositeType>(V->getType());
+
+  if (isa<UndefValue>(V))
+    return UndefValue::get(ExtractValueInst::getIndexedType(PTy,
+                                                              idx_begin,
+                                                              idx_end));
+  else if (isa<ConstantAggregateZero>(V))
+    return Constant::getNullValue(ExtractValueInst::getIndexedType(PTy, 
+                                                                  idx_begin,
+                                                                  idx_end));
+  else if (Constant *C = dyn_cast<Constant>(V)) {
+    if (isa<ConstantArray>(C) || isa<ConstantStruct>(C))
+      // Recursively process this constant
+      return FindInsertedValue(C->getOperand(*idx_begin), idx_begin + 1,
+                               idx_end, InsertBefore);
+  } else if (InsertValueInst *I = dyn_cast<InsertValueInst>(V)) {
+    // Loop the indices for the insertvalue instruction in parallel with the
+    // requested indices
+    const unsigned *req_idx = idx_begin;
+    for (const unsigned *i = I->idx_begin(), *e = I->idx_end();
+         i != e; ++i, ++req_idx) {
+      if (req_idx == idx_end) {
+        if (InsertBefore)
+          // The requested index identifies a part of a nested aggregate. Handle
+          // this specially. For example,
+          // %A = insertvalue { i32, {i32, i32 } } undef, i32 10, 1, 0
+          // %B = insertvalue { i32, {i32, i32 } } %A, i32 11, 1, 1
+          // %C = extractvalue {i32, { i32, i32 } } %B, 1
+          // This can be changed into
+          // %A = insertvalue {i32, i32 } undef, i32 10, 0
+          // %C = insertvalue {i32, i32 } %A, i32 11, 1
+          // which allows the unused 0,0 element from the nested struct to be
+          // removed.
+          return BuildSubAggregate(V, idx_begin, req_idx, InsertBefore);
+        else
+          // We can't handle this without inserting insertvalues
+          return 0;
+      }
+      
+      // This insert value inserts something else than what we are looking for.
+      // See if the (aggregrate) value inserted into has the value we are
+      // looking for, then.
+      if (*req_idx != *i)
+        return FindInsertedValue(I->getAggregateOperand(), idx_begin, idx_end,
+                                 InsertBefore);
+    }
+    // If we end up here, the indices of the insertvalue match with those
+    // requested (though possibly only partially). Now we recursively look at
+    // the inserted value, passing any remaining indices.
+    return FindInsertedValue(I->getInsertedValueOperand(), req_idx, idx_end,
+                             InsertBefore);
+  } else if (ExtractValueInst *I = dyn_cast<ExtractValueInst>(V)) {
+    // If we're extracting a value from an aggregrate that was extracted from
+    // something else, we can extract from that something else directly instead.
+    // However, we will need to chain I's indices with the requested indices.
+   
+    // Calculate the number of indices required 
+    unsigned size = I->getNumIndices() + (idx_end - idx_begin);
+    // Allocate some space to put the new indices in
+    SmallVector<unsigned, 5> Idxs;
+    Idxs.reserve(size);
+    // Add indices from the extract value instruction
+    for (const unsigned *i = I->idx_begin(), *e = I->idx_end();
+         i != e; ++i)
+      Idxs.push_back(*i);
+    
+    // Add requested indices
+    for (const unsigned *i = idx_begin, *e = idx_end; i != e; ++i)
+      Idxs.push_back(*i);
+
+    assert(Idxs.size() == size 
+           && "Number of indices added not correct?");
+    
+    return FindInsertedValue(I->getAggregateOperand(), Idxs.begin(), Idxs.end(),
+                             InsertBefore);
+  }
+  // Otherwise, we don't know (such as, extracting from a function return value
+  // or load instruction)
+  return 0;
+}
+
+/// GetConstantStringInfo - This function computes the length of a
+/// null-terminated C string pointed to by V.  If successful, it returns true
+/// and returns the string in Str.  If unsuccessful, it returns false.
+bool llvm::GetConstantStringInfo(Value *V, std::string &Str, uint64_t Offset,
+                                 bool StopAtNul) {
+  // If V is NULL then return false;
+  if (V == NULL) return false;
+
+  // Look through bitcast instructions.
+  if (BitCastInst *BCI = dyn_cast<BitCastInst>(V))
+    return GetConstantStringInfo(BCI->getOperand(0), Str, Offset, StopAtNul);
+  
+  // If the value is not a GEP instruction nor a constant expression with a
+  // GEP instruction, then return false because ConstantArray can't occur
+  // any other way
+  User *GEP = 0;
+  if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(V)) {
+    GEP = GEPI;
+  } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
+    if (CE->getOpcode() == Instruction::BitCast)
+      return GetConstantStringInfo(CE->getOperand(0), Str, Offset, StopAtNul);
+    if (CE->getOpcode() != Instruction::GetElementPtr)
+      return false;
+    GEP = CE;
+  }
+  
+  if (GEP) {
+    // Make sure the GEP has exactly three arguments.
+    if (GEP->getNumOperands() != 3)
+      return false;
+    
+    // Make sure the index-ee is a pointer to array of i8.
+    const PointerType *PT = cast<PointerType>(GEP->getOperand(0)->getType());
+    const ArrayType *AT = dyn_cast<ArrayType>(PT->getElementType());
+    if (AT == 0 || !AT->getElementType()->isInteger(8))
+      return false;
+    
+    // Check to make sure that the first operand of the GEP is an integer and
+    // has value 0 so that we are sure we're indexing into the initializer.
+    ConstantInt *FirstIdx = dyn_cast<ConstantInt>(GEP->getOperand(1));
+    if (FirstIdx == 0 || !FirstIdx->isZero())
+      return false;
+    
+    // If the second index isn't a ConstantInt, then this is a variable index
+    // into the array.  If this occurs, we can't say anything meaningful about
+    // the string.
+    uint64_t StartIdx = 0;
+    if (ConstantInt *CI = dyn_cast<ConstantInt>(GEP->getOperand(2)))
+      StartIdx = CI->getZExtValue();
+    else
+      return false;
+    return GetConstantStringInfo(GEP->getOperand(0), Str, StartIdx+Offset,
+                                 StopAtNul);
+  }
+  
+  // The GEP instruction, constant or instruction, must reference a global
+  // variable that is a constant and is initialized. The referenced constant
+  // initializer is the array that we'll use for optimization.
+  GlobalVariable* GV = dyn_cast<GlobalVariable>(V);
+  if (!GV || !GV->isConstant() || !GV->hasDefinitiveInitializer())
+    return false;
+  Constant *GlobalInit = GV->getInitializer();
+  
+  // Handle the ConstantAggregateZero case
+  if (isa<ConstantAggregateZero>(GlobalInit)) {
+    // This is a degenerate case. The initializer is constant zero so the
+    // length of the string must be zero.
+    Str.clear();
+    return true;
+  }
+  
+  // Must be a Constant Array
+  ConstantArray *Array = dyn_cast<ConstantArray>(GlobalInit);
+  if (Array == 0 || !Array->getType()->getElementType()->isInteger(8))
+    return false;
+  
+  // Get the number of elements in the array
+  uint64_t NumElts = Array->getType()->getNumElements();
+  
+  if (Offset > NumElts)
+    return false;
+  
+  // Traverse the constant array from 'Offset' which is the place the GEP refers
+  // to in the array.
+  Str.reserve(NumElts-Offset);
+  for (unsigned i = Offset; i != NumElts; ++i) {
+    Constant *Elt = Array->getOperand(i);
+    ConstantInt *CI = dyn_cast<ConstantInt>(Elt);
+    if (!CI) // This array isn't suitable, non-int initializer.
+      return false;
+    if (StopAtNul && CI->isZero())
+      return true; // we found end of string, success!
+    Str += (char)CI->getZExtValue();
+  }
+  
+  // The array isn't null terminated, but maybe this is a memcpy, not a strcpy.
+  return true;
+}