C++ BinaryOperator::moveBefore方法代码示例

// Given an expression of the form '(A+B)+(D+C)', turn it into '(((A+B)+C)+D)'.
// Note that if D is also part of the expression tree that we recurse to
// linearize it as well.  Besides that case, this does not recurse into A,B, or
// C.
void Reassociate::LinearizeExpr(BinaryOperator *I) {
  BinaryOperator *LHS = cast<BinaryOperator>(I->getOperand(0));
  BinaryOperator *RHS = cast<BinaryOperator>(I->getOperand(1));
  assert(isReassociableOp(LHS, I->getOpcode()) &&
         isReassociableOp(RHS, I->getOpcode()) &&
         "Not an expression that needs linearization?");

  DEBUG(dbgs() << "Linear" << *LHS << '\n' << *RHS << '\n' << *I << '\n');

  // Move the RHS instruction to live immediately before I, avoiding breaking
  // dominator properties.
  RHS->moveBefore(I);

  // Move operands around to do the linearization.
  I->setOperand(1, RHS->getOperand(0));
  RHS->setOperand(0, LHS);
  I->setOperand(0, RHS);

  // Conservatively clear all the optional flags, which may not hold
  // after the reassociation.
  I->clearSubclassOptionalData();
  LHS->clearSubclassOptionalData();
  RHS->clearSubclassOptionalData();

  ++NumLinear;
  MadeChange = true;
  DEBUG(dbgs() << "Linearized: " << *I << '\n');

  // If D is part of this expression tree, tail recurse.
  if (isReassociableOp(I->getOperand(1), I->getOpcode()))
    LinearizeExpr(I);
}

Value *ConstantOffsetExtractor::removeConstOffset(unsigned ChainIndex) {
  if (ChainIndex == 0) {
    assert(isa<ConstantInt>(UserChain[ChainIndex]));
    return ConstantInt::getNullValue(UserChain[ChainIndex]->getType());
  }

  BinaryOperator *BO = cast<BinaryOperator>(UserChain[ChainIndex]);
  unsigned OpNo = (BO->getOperand(0) == UserChain[ChainIndex - 1] ? 0 : 1);
  assert(BO->getOperand(OpNo) == UserChain[ChainIndex - 1]);
  Value *NextInChain = removeConstOffset(ChainIndex - 1);
  Value *TheOther = BO->getOperand(1 - OpNo);

  // If NextInChain is 0 and not the LHS of a sub, we can simplify the
  // sub-expression to be just TheOther.
  if (ConstantInt *CI = dyn_cast<ConstantInt>(NextInChain)) {
    if (CI->isZero() && !(BO->getOpcode() == Instruction::Sub && OpNo == 0))
      return TheOther;
  }

  if (BO->getOpcode() == Instruction::Or) {
    // Rebuild "or" as "add", because "or" may be invalid for the new
    // epxression.
    //
    // For instance, given
    //   a | (b + 5) where a and b + 5 have no common bits,
    // we can extract 5 as the constant offset.
    //
    // However, reusing the "or" in the new index would give us
    //   (a | b) + 5
    // which does not equal a | (b + 5).
    //
    // Replacing the "or" with "add" is fine, because
    //   a | (b + 5) = a + (b + 5) = (a + b) + 5
    if (OpNo == 0) {
      return BinaryOperator::CreateAdd(NextInChain, TheOther, BO->getName(),
                                       IP);
    } else {
      return BinaryOperator::CreateAdd(TheOther, NextInChain, BO->getName(),
                                       IP);
    }
  }

  // We can reuse BO in this case, because the new expression shares the same
  // instruction type and BO is used at most once.
  assert(BO->getNumUses() <= 1 &&
         "distributeExtsAndCloneChain clones each BinaryOperator in "
         "UserChain, so no one should be used more than "
         "once");
  BO->setOperand(OpNo, NextInChain);
  BO->setHasNoSignedWrap(false);
  BO->setHasNoUnsignedWrap(false);
  // Make sure it appears after all instructions we've inserted so far.
  BO->moveBefore(IP);
  return BO;
}

// RewriteExprTree - Now that the operands for this expression tree are
// linearized and optimized, emit them in-order.  This function is written to be
// tail recursive.
void Reassociate::RewriteExprTree(BinaryOperator *I,
                                  SmallVectorImpl<ValueEntry> &Ops,
                                  unsigned i) {
  if (i+2 == Ops.size()) {
    if (I->getOperand(0) != Ops[i].Op ||
        I->getOperand(1) != Ops[i+1].Op) {
      Value *OldLHS = I->getOperand(0);
      DEBUG(dbgs() << "RA: " << *I << '\n');
      I->setOperand(0, Ops[i].Op);
      I->setOperand(1, Ops[i+1].Op);

      // Clear all the optional flags, which may not hold after the
      // reassociation if the expression involved more than just this operation.
      if (Ops.size() != 2)
        I->clearSubclassOptionalData();

      DEBUG(dbgs() << "TO: " << *I << '\n');
      MadeChange = true;
      ++NumChanged;
      
      // If we reassociated a tree to fewer operands (e.g. (1+a+2) -> (a+3)
      // delete the extra, now dead, nodes.
      RemoveDeadBinaryOp(OldLHS);
    }
    return;
  }
  assert(i+2 < Ops.size() && "Ops index out of range!");

  if (I->getOperand(1) != Ops[i].Op) {
    DEBUG(dbgs() << "RA: " << *I << '\n');
    I->setOperand(1, Ops[i].Op);

    // Conservatively clear all the optional flags, which may not hold
    // after the reassociation.
    I->clearSubclassOptionalData();

    DEBUG(dbgs() << "TO: " << *I << '\n');
    MadeChange = true;
    ++NumChanged;
  }
  
  BinaryOperator *LHS = cast<BinaryOperator>(I->getOperand(0));
  assert(LHS->getOpcode() == I->getOpcode() &&
         "Improper expression tree!");
  
  // Compactify the tree instructions together with each other to guarantee
  // that the expression tree is dominated by all of Ops.
  LHS->moveBefore(I);
  RewriteExprTree(LHS, Ops, i+1);
}

// NegateValue - Insert instructions before the instruction pointed to by BI,
// that computes the negative version of the value specified.  The negative
// version of the value is returned, and BI is left pointing at the instruction
// that should be processed next by the reassociation pass.
//
static Value *NegateValue(Value *V, Instruction *BI) {
  if (Constant *C = dyn_cast<Constant>(V))
    return ConstantExpr::getNeg(C);
  
  // We are trying to expose opportunity for reassociation.  One of the things
  // that we want to do to achieve this is to push a negation as deep into an
  // expression chain as possible, to expose the add instructions.  In practice,
  // this means that we turn this:
  //   X = -(A+12+C+D)   into    X = -A + -12 + -C + -D = -12 + -A + -C + -D
  // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate
  // the constants.  We assume that instcombine will clean up the mess later if
  // we introduce tons of unnecessary negation instructions.
  //
  if (Instruction *I = dyn_cast<Instruction>(V))
    if (I->getOpcode() == Instruction::Add && I->hasOneUse()) {
      // Push the negates through the add.
      I->setOperand(0, NegateValue(I->getOperand(0), BI));
      I->setOperand(1, NegateValue(I->getOperand(1), BI));

      // We must move the add instruction here, because the neg instructions do
      // not dominate the old add instruction in general.  By moving it, we are
      // assured that the neg instructions we just inserted dominate the 
      // instruction we are about to insert after them.
      //
      I->moveBefore(BI);
      I->setName(I->getName()+".neg");
      return I;
    }
  
  // Okay, we need to materialize a negated version of V with an instruction.
  // Scan the use lists of V to see if we have one already.
  for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E;++UI){
    User *U = *UI;
    if (!BinaryOperator::isNeg(U)) continue;

    // We found one!  Now we have to make sure that the definition dominates
    // this use.  We do this by moving it to the entry block (if it is a
    // non-instruction value) or right after the definition.  These negates will
    // be zapped by reassociate later, so we don't need much finesse here.
    BinaryOperator *TheNeg = cast<BinaryOperator>(U);

    // Verify that the negate is in this function, V might be a constant expr.
    if (TheNeg->getParent()->getParent() != BI->getParent()->getParent())
      continue;
    
    BasicBlock::iterator InsertPt;
    if (Instruction *InstInput = dyn_cast<Instruction>(V)) {
      if (InvokeInst *II = dyn_cast<InvokeInst>(InstInput)) {
        InsertPt = II->getNormalDest()->begin();
      } else {
        InsertPt = InstInput;
        ++InsertPt;
      }
      while (isa<PHINode>(InsertPt)) ++InsertPt;
    } else {
      InsertPt = TheNeg->getParent()->getParent()->getEntryBlock().begin();
    }
    TheNeg->moveBefore(InsertPt);
    return TheNeg;
  }

  // Insert a 'neg' instruction that subtracts the value from zero to get the
  // negation.
  return BinaryOperator::CreateNeg(V, V->getName() + ".neg", BI);
}

/// LinearizeExprTree - Given an associative binary expression tree, traverse
/// all of the uses putting it into canonical form.  This forces a left-linear
/// form of the expression (((a+b)+c)+d), and collects information about the
/// rank of the non-tree operands.
///
/// NOTE: These intentionally destroys the expression tree operands (turning
/// them into undef values) to reduce #uses of the values.  This means that the
/// caller MUST use something like RewriteExprTree to put the values back in.
///
void Reassociate::LinearizeExprTree(BinaryOperator *I,
                                    SmallVectorImpl<ValueEntry> &Ops) {
  Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
  unsigned Opcode = I->getOpcode();

  // First step, linearize the expression if it is in ((A+B)+(C+D)) form.
  BinaryOperator *LHSBO = isReassociableOp(LHS, Opcode);
  BinaryOperator *RHSBO = isReassociableOp(RHS, Opcode);

  // If this is a multiply expression tree and it contains internal negations,
  // transform them into multiplies by -1 so they can be reassociated.
  if (I->getOpcode() == Instruction::Mul) {
    if (!LHSBO && LHS->hasOneUse() && BinaryOperator::isNeg(LHS)) {
      LHS = LowerNegateToMultiply(cast<Instruction>(LHS), ValueRankMap);
      LHSBO = isReassociableOp(LHS, Opcode);
    }
    if (!RHSBO && RHS->hasOneUse() && BinaryOperator::isNeg(RHS)) {
      RHS = LowerNegateToMultiply(cast<Instruction>(RHS), ValueRankMap);
      RHSBO = isReassociableOp(RHS, Opcode);
    }
  }

  if (!LHSBO) {
    if (!RHSBO) {
      // Neither the LHS or RHS as part of the tree, thus this is a leaf.  As
      // such, just remember these operands and their rank.
      Ops.push_back(ValueEntry(getRank(LHS), LHS));
      Ops.push_back(ValueEntry(getRank(RHS), RHS));
      
      // Clear the leaves out.
      I->setOperand(0, UndefValue::get(I->getType()));
      I->setOperand(1, UndefValue::get(I->getType()));
      return;
    }
    
    // Turn X+(Y+Z) -> (Y+Z)+X
    std::swap(LHSBO, RHSBO);
    std::swap(LHS, RHS);
    bool Success = !I->swapOperands();
    assert(Success && "swapOperands failed");
    (void)Success;
    MadeChange = true;
  } else if (RHSBO) {
    // Turn (A+B)+(C+D) -> (((A+B)+C)+D).  This guarantees the RHS is not
    // part of the expression tree.
    LinearizeExpr(I);
    LHS = LHSBO = cast<BinaryOperator>(I->getOperand(0));
    RHS = I->getOperand(1);
    RHSBO = 0;
  }

  // Okay, now we know that the LHS is a nested expression and that the RHS is
  // not.  Perform reassociation.
  assert(!isReassociableOp(RHS, Opcode) && "LinearizeExpr failed!");

  // Move LHS right before I to make sure that the tree expression dominates all
  // values.
  LHSBO->moveBefore(I);

  // Linearize the expression tree on the LHS.
  LinearizeExprTree(LHSBO, Ops);

  // Remember the RHS operand and its rank.
  Ops.push_back(ValueEntry(getRank(RHS), RHS));
  
  // Clear the RHS leaf out.
  I->setOperand(1, UndefValue::get(I->getType()));
}