C++ APInt::countLeadingZeros方法代码示例

std::string get_string(const APInt &api)
{
  std::ostringstream str;
  for (unsigned count = api.countLeadingZeros(); count > 0; count--)
    str << "0";

  if (api != 0)
    str << api.toString(2, false /* treat as  unsigned */);
  return str.str();
}

static SILInstruction *constantFoldIntrinsic(BuiltinInst *BI,
                                             llvm::Intrinsic::ID ID,
                                             Optional<bool> &ResultsInError) {
  switch (ID) {
  default: break;
  case llvm::Intrinsic::expect: {
    // An expect of an integral constant is the constant itself.
    assert(BI->getArguments().size() == 2 && "Expect should have 2 args.");
    auto *Op1 = dyn_cast<IntegerLiteralInst>(BI->getArguments()[0]);
    if (!Op1)
      return nullptr;
    return Op1;
  }

  case llvm::Intrinsic::ctlz: {
    assert(BI->getArguments().size() == 2 && "Ctlz should have 2 args.");
    OperandValueArrayRef Args = BI->getArguments();

    // Fold for integer constant arguments.
    auto *LHS = dyn_cast<IntegerLiteralInst>(Args[0]);
    if (!LHS) {
      return nullptr;
    }
    APInt LHSI = LHS->getValue();
    unsigned LZ = 0;
    // Check corner-case of source == zero
    if (LHSI == 0) {
      auto *RHS = dyn_cast<IntegerLiteralInst>(Args[1]);
      if (!RHS || RHS->getValue() != 0) {
        // Undefined
        return nullptr;
      }
      LZ = LHSI.getBitWidth();
    } else {
      LZ = LHSI.countLeadingZeros();
    }
    APInt LZAsAPInt = APInt(LHSI.getBitWidth(), LZ);
    SILBuilderWithScope B(BI);
    return B.createIntegerLiteral(BI->getLoc(), LHS->getType(), LZAsAPInt);
  }

  case llvm::Intrinsic::sadd_with_overflow:
  case llvm::Intrinsic::uadd_with_overflow:
  case llvm::Intrinsic::ssub_with_overflow:
  case llvm::Intrinsic::usub_with_overflow:
  case llvm::Intrinsic::smul_with_overflow:
  case llvm::Intrinsic::umul_with_overflow:
    return constantFoldBinaryWithOverflow(BI, ID,
                                          /* ReportOverflow */ false,
                                          ResultsInError);
  }
  return nullptr;
}

ConstantRange
ConstantRange::shl(const ConstantRange &Other) const {
  if (isEmptySet() || Other.isEmptySet())
    return ConstantRange(getBitWidth(), /*isFullSet=*/false);

  APInt max = getUnsignedMax();
  APInt Other_umax = Other.getUnsignedMax();

  // there's overflow!
  if (Other_umax.uge(max.countLeadingZeros()))
    return ConstantRange(getBitWidth(), /*isFullSet=*/true);

  // FIXME: implement the other tricky cases

  APInt min = getUnsignedMin();
  min <<= Other.getUnsignedMin();
  max <<= Other_umax;

  return ConstantRange(std::move(min), std::move(max) + 1);
}

void BDCE::determineLiveOperandBits(const Instruction *UserI,
                                    const Instruction *I, unsigned OperandNo,
                                    const APInt &AOut, APInt &AB,
                                    APInt &KnownZero, APInt &KnownOne,
                                    APInt &KnownZero2, APInt &KnownOne2) {
  unsigned BitWidth = AB.getBitWidth();

  // We're called once per operand, but for some instructions, we need to
  // compute known bits of both operands in order to determine the live bits of
  // either (when both operands are instructions themselves). We don't,
  // however, want to do this twice, so we cache the result in APInts that live
  // in the caller. For the two-relevant-operands case, both operand values are
  // provided here.
  auto ComputeKnownBits =
      [&](unsigned BitWidth, const Value *V1, const Value *V2) {
        const DataLayout &DL = I->getModule()->getDataLayout();
        KnownZero = APInt(BitWidth, 0);
        KnownOne = APInt(BitWidth, 0);
        computeKnownBits(const_cast<Value *>(V1), KnownZero, KnownOne, DL, 0,
                         AC, UserI, DT);

        if (V2) {
          KnownZero2 = APInt(BitWidth, 0);
          KnownOne2 = APInt(BitWidth, 0);
          computeKnownBits(const_cast<Value *>(V2), KnownZero2, KnownOne2, DL,
                           0, AC, UserI, DT);
        }
      };

  switch (UserI->getOpcode()) {
  default: break;
  case Instruction::Call:
  case Instruction::Invoke:
    if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(UserI))
      switch (II->getIntrinsicID()) {
      default: break;
      case Intrinsic::bswap:
        // The alive bits of the input are the swapped alive bits of
        // the output.
        AB = AOut.byteSwap();
        break;
      case Intrinsic::ctlz:
        if (OperandNo == 0) {
          // We need some output bits, so we need all bits of the
          // input to the left of, and including, the leftmost bit
          // known to be one.
          ComputeKnownBits(BitWidth, I, nullptr);
          AB = APInt::getHighBitsSet(BitWidth,
                 std::min(BitWidth, KnownOne.countLeadingZeros()+1));
        }
        break;
      case Intrinsic::cttz:
        if (OperandNo == 0) {
          // We need some output bits, so we need all bits of the
          // input to the right of, and including, the rightmost bit
          // known to be one.
          ComputeKnownBits(BitWidth, I, nullptr);
          AB = APInt::getLowBitsSet(BitWidth,
                 std::min(BitWidth, KnownOne.countTrailingZeros()+1));
        }
        break;
      }
    break;
  case Instruction::Add:
  case Instruction::Sub:
    // Find the highest live output bit. We don't need any more input
    // bits than that (adds, and thus subtracts, ripple only to the
    // left).
    AB = APInt::getLowBitsSet(BitWidth, AOut.getActiveBits());
    break;
  case Instruction::Shl:
    if (OperandNo == 0)
      if (ConstantInt *CI =
            dyn_cast<ConstantInt>(UserI->getOperand(1))) {
        uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
        AB = AOut.lshr(ShiftAmt);

        // If the shift is nuw/nsw, then the high bits are not dead
        // (because we've promised that they *must* be zero).
        const ShlOperator *S = cast<ShlOperator>(UserI);
        if (S->hasNoSignedWrap())
          AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt+1);
        else if (S->hasNoUnsignedWrap())
          AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
      }
    break;
  case Instruction::LShr:
    if (OperandNo == 0)
      if (ConstantInt *CI =
            dyn_cast<ConstantInt>(UserI->getOperand(1))) {
        uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
        AB = AOut.shl(ShiftAmt);

        // If the shift is exact, then the low bits are not dead
        // (they must be zero).
        if (cast<LShrOperator>(UserI)->isExact())
          AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
      }
    break;
  case Instruction::AShr:
//.........这里部分代码省略.........

//.........这里部分代码省略.........
      // 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;
    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()));
}

//.........这里部分代码省略.........
    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: {