move ComputeMaskedBits, MaskedValueIsZero, and ComputeNumSignBits

out of instcombine into a new file in libanalysis.  This also teaches
ComputeNumSignBits about the number of sign bits in a constantint.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51863 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 709 insertions, 0 deletions

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
new file mode 100644
index 0000000..a35d625
--- /dev/null
+++ b/lib/Analysis/ValueTracking.cpp
@@ -0,0 +1,709 @@
+//===- 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/IntrinsicInst.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Support/GetElementPtrTypeIterator.h"
+#include "llvm/Support/MathExtras.h"
+using namespace llvm;
+
+/// getOpcode - If this is an Instruction or a ConstantExpr, return the
+/// opcode value. Otherwise return UserOp1.
+static unsigned getOpcode(const Value *V) {
+  if (const Instruction *I = dyn_cast<Instruction>(V))
+    return I->getOpcode();
+  if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
+    return CE->getOpcode();
+  // Use UserOp1 to mean there's no opcode.
+  return Instruction::UserOp1;
+}
+
+
+/// 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.
+void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
+                             APInt &KnownZero, APInt &KnownOne,
+                             TargetData *TD, unsigned Depth) {
+  assert(V && "No Value?");
+  assert(Depth <= 6 && "Limit Search Depth");
+  uint32_t BitWidth = Mask.getBitWidth();
+  assert((V->getType()->isInteger() || isa<PointerType>(V->getType())) &&
+         "Not integer or pointer type!");
+  assert((!TD || TD->getTypeSizeInBits(V->getType()) == BitWidth) &&
+         (!isa<IntegerType>(V->getType()) ||
+          V->getType()->getPrimitiveSizeInBits() == 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 is all-zeros.
+  if (isa<ConstantPointerNull>(V)) {
+    KnownOne.clear();
+    KnownZero = Mask;
+    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()) 
+      Align = TD->getPrefTypeAlignment(GV->getType()->getElementType());
+    if (Align > 0)
+      KnownZero = Mask & APInt::getLowBitsSet(BitWidth,
+                                              CountTrailingZeros_32(Align));
+    else
+      KnownZero.clear();
+    KnownOne.clear();
+    return;
+  }
+
+  KnownZero.clear(); KnownOne.clear();   // Start out not knowing anything.
+
+  if (Depth == 6 || Mask == 0)
+    return;  // Limit search depth.
+
+  User *I = dyn_cast<User>(V);
+  if (!I) return;
+
+  APInt KnownZero2(KnownZero), KnownOne2(KnownOne);
+  switch (getOpcode(I)) {
+  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: {
+    // Note that we handle pointer operands here because of inttoptr/ptrtoint
+    // which fall through here.
+    const Type *SrcTy = I->getOperand(0)->getType();
+    uint32_t SrcBitWidth = TD ?
+      TD->getTypeSizeInBits(SrcTy) :
+      SrcTy->getPrimitiveSizeInBits();
+    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)) {
+      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.
+    const IntegerType *SrcTy = cast<IntegerType>(I->getOperand(0)->getType());
+    uint32_t SrcBitWidth = SrcTy->getBitWidth();
+      
+    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: {
+    // Output known-0 bits are known if clear or set in both the low clear bits
+    // common to both LHS & RHS.  For example, 8+(X<<3) is known to have the
+    // low 3 bits clear.
+    APInt Mask2 = APInt::getLowBitsSet(BitWidth, Mask.countTrailingOnes());
+    ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
+                      Depth+1);
+    assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); 
+    unsigned KnownZeroOut = KnownZero2.countTrailingOnes();
+
+    ComputeMaskedBits(I->getOperand(1), Mask2, KnownZero2, KnownOne2, TD, 
+                      Depth+1);
+    assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); 
+    KnownZeroOut = std::min(KnownZeroOut, 
+                            KnownZero2.countTrailingOnes());
+
+    KnownZero |= APInt::getLowBitsSet(BitWidth, KnownZeroOut);
+    return;
+  }
+  case Instruction::SRem:
+    if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {
+      APInt RA = Rem->getValue();
+      if (RA.isPowerOf2() || (-RA).isPowerOf2()) {
+        APInt LowBits = RA.isStrictlyPositive() ? (RA - 1) : ~RA;
+        APInt Mask2 = LowBits | APInt::getSignBit(BitWidth);
+        ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD, 
+                          Depth+1);
+
+        // The sign of a remainder is equal to the sign of the first
+        // operand (zero being positive).
+        if (KnownZero2[BitWidth-1] || ((KnownZero2 & LowBits) == LowBits))
+          KnownZero2 |= ~LowBits;
+        else if (KnownOne2[BitWidth-1])
+          KnownOne2 |= ~LowBits;
+
+        KnownZero |= KnownZero2 & Mask;
+        KnownOne |= KnownOne2 & 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);
+
+    uint32_t Leaders = std::max(KnownZero.countLeadingOnes(),
+                                KnownZero2.countLeadingOnes());
+    KnownOne.clear();
+    KnownZero = APInt::getHighBitsSet(BitWidth, Leaders) & Mask;
+    break;
+  }
+
+  case Instruction::Alloca:
+  case Instruction::Malloc: {
+    AllocationInst *AI = cast<AllocationInst>(V);
+    unsigned Align = AI->getAlignment();
+    if (Align == 0 && TD) {
+      if (isa<AllocaInst>(AI))
+        Align = TD->getPrefTypeAlignment(AI->getType()->getElementType());
+      else if (isa<MallocInst>(AI)) {
+        // Malloc returns maximally aligned memory.
+        Align = TD->getABITypeAlignment(AI->getType()->getElementType());
+        Align =
+          std::max(Align,
+                   (unsigned)TD->getABITypeAlignment(Type::DoubleTy));
+        Align =
+          std::max(Align,
+                   (unsigned)TD->getABITypeAlignment(Type::Int64Ty));
+      }
+    }
+    
+    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()->getPrimitiveSizeInBits();
+        uint64_t TypeSize = TD ? TD->getABITypeSize(IndexedTy) : 1;
+        LocalMask = APInt::getAllOnesValue(GEPOpiBits);
+        LocalKnownZero = LocalKnownOne = APInt(GEPOpiBits, 0);
+        ComputeMaskedBits(Index, LocalMask,
+                          LocalKnownZero, LocalKnownOne, TD, Depth+1);
+        TrailZ = std::min(TrailZ,
+                          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);
+        User *LU = dyn_cast<User>(L);
+        if (!LU)
+          continue;
+        unsigned Opcode = getOpcode(LU);
+        // 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());
+          KnownOne2.clear();
+          KnownZero2.clear();
+          ComputeMaskedBits(L, Mask2, KnownZero2, KnownOne2, TD, Depth+1);
+          KnownZero = Mask &
+                      APInt::getLowBitsSet(BitWidth,
+                                           KnownZero2.countTrailingOnes());
+          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.
+bool llvm::MaskedValueIsZero(Value *V, const APInt &Mask,
+                             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, TargetData *TD, unsigned Depth) {
+  const IntegerType *Ty = cast<IntegerType>(V->getType());
+  unsigned TyBits = Ty->getBitWidth();
+  unsigned Tmp, Tmp2;
+  unsigned FirstAnswer = 1;
+
+  if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
+    if (CI->getValue().isNegative())
+      return CI->getValue().countLeadingOnes();
+    return CI->getValue().countLeadingZeros();
+  }
+  
+  if (Depth == 6)
+    return 1;  // Limit search depth.
+  
+  User *U = dyn_cast<User>(V);
+  switch (getOpcode(V)) {
+  default: break;
+  case Instruction::SExt:
+    Tmp = TyBits-cast<IntegerType>(U->getOperand(0)->getType())->getBitWidth();
+    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(0)))
+      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;
+    break;
+    
+  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;
+    break;
+  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()));
+}