示例#1
0
/// isFormingBranchFromSelectProfitable - Returns true if a SelectInst should be
/// turned into an explicit branch.
static bool isFormingBranchFromSelectProfitable(SelectInst *SI) {
  // FIXME: This should use the same heuristics as IfConversion to determine
  // whether a select is better represented as a branch.  This requires that
  // branch probability metadata is preserved for the select, which is not the
  // case currently.

  CmpInst *Cmp = dyn_cast<CmpInst>(SI->getCondition());

  // If the branch is predicted right, an out of order CPU can avoid blocking on
  // the compare.  Emit cmovs on compares with a memory operand as branches to
  // avoid stalls on the load from memory.  If the compare has more than one use
  // there's probably another cmov or setcc around so it's not worth emitting a
  // branch.
  if (!Cmp)
    return false;

  Value *CmpOp0 = Cmp->getOperand(0);
  Value *CmpOp1 = Cmp->getOperand(1);

  // We check that the memory operand has one use to avoid uses of the loaded
  // value directly after the compare, making branches unprofitable.
  return Cmp->hasOneUse() &&
         ((isa<LoadInst>(CmpOp0) && CmpOp0->hasOneUse()) ||
          (isa<LoadInst>(CmpOp1) && CmpOp1->hasOneUse()));
}
示例#2
0
/// Try to simplify cmp instruction.
bool UnrolledInstAnalyzer::visitCmpInst(CmpInst &I) {
  Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);

  // First try to handle simplified comparisons.
  if (!isa<Constant>(LHS))
    if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
      LHS = SimpleLHS;
  if (!isa<Constant>(RHS))
    if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
      RHS = SimpleRHS;

  if (!isa<Constant>(LHS) && !isa<Constant>(RHS)) {
    auto SimplifiedLHS = SimplifiedAddresses.find(LHS);
    if (SimplifiedLHS != SimplifiedAddresses.end()) {
      auto SimplifiedRHS = SimplifiedAddresses.find(RHS);
      if (SimplifiedRHS != SimplifiedAddresses.end()) {
        SimplifiedAddress &LHSAddr = SimplifiedLHS->second;
        SimplifiedAddress &RHSAddr = SimplifiedRHS->second;
        if (LHSAddr.Base == RHSAddr.Base) {
          LHS = LHSAddr.Offset;
          RHS = RHSAddr.Offset;
        }
      }
    }
  }

  if (Constant *CLHS = dyn_cast<Constant>(LHS)) {
    if (Constant *CRHS = dyn_cast<Constant>(RHS)) {
      if (Constant *C = ConstantExpr::getCompare(I.getPredicate(), CLHS, CRHS)) {
        SimplifiedValues[&I] = C;
        return true;
      }
    }
  }

  return Base::visitCmpInst(I);
}
// Predict that a comparison in which a register is an operand, the register is
// used before being defined in a successor block, and the successor block
// does not post-dominate will reach the successor block.
int BranchProbabilities::CheckGuardHeuristic()
{
    BranchInst *BI = dyn_cast<BranchInst>(_TI);
    bool bUses[2] = {false, false};

    // If we don't have a conditional branch, abandon
    if ((!BI) || (BI->isUnconditional()))
        return -1;

    // If the condition is not immediately dependent on a comparison, abandon
    CmpInst *cmp = dyn_cast<CmpInst>(BI->getCondition());
    if (!cmp)
        return -1;

    for (int i = 0; i < 2; i++)
    {
        if (_bPostDoms[i])
            continue;

        // Get the values being compared
        Value *v = cmp->getOperand(i);

        // For all uses of the first value check if the use post-dominates
        for (Value::use_iterator UI = v->use_begin(), UE = v->use_end();
                UI != UE; ++UI)
        {
            // if the use is not an instruction, skip it
            Instruction *I = dyn_cast<Instruction>(*UI);
            if (!I)
                continue;

            BasicBlock *UsingBlock = I->getParent();

            // Check if the use is in either successor
            for (int i = 0; i < 2; i++)
                if (UsingBlock == _Succ[i])
                    bUses[i] = true;
        }
    }

    if (bUses[0] == bUses[1])
        return -1;
    if (bUses[0])
        return 0;
    else
        return 1;
}
示例#4
0
void SystemZTDCPass::convertFCmp(CmpInst &I) {
  Value *Op0 = I.getOperand(0);
  auto *Const = dyn_cast<ConstantFP>(I.getOperand(1));
  auto Pred = I.getPredicate();
  // Only comparisons with consts are interesting.
  if (!Const)
    return;
  // Compute the smallest normal number (and its negation).
  auto &Sem = Op0->getType()->getFltSemantics();
  APFloat Smallest = APFloat::getSmallestNormalized(Sem);
  APFloat NegSmallest = Smallest;
  NegSmallest.changeSign();
  // Check if Const is one of our recognized consts.
  int WhichConst;
  if (Const->isZero()) {
    // All comparisons with 0 can be converted.
    WhichConst = 0;
  } else if (Const->isInfinity()) {
    // Likewise for infinities.
    WhichConst = Const->isNegative() ? 2 : 1;
  } else if (Const->isExactlyValue(Smallest)) {
    // For Smallest, we cannot do EQ separately from GT.
    if ((Pred & CmpInst::FCMP_OGE) != CmpInst::FCMP_OGE &&
        (Pred & CmpInst::FCMP_OGE) != 0)
      return;
    WhichConst = 3;
  } else if (Const->isExactlyValue(NegSmallest)) {
    // Likewise for NegSmallest, we cannot do EQ separately from LT.
    if ((Pred & CmpInst::FCMP_OLE) != CmpInst::FCMP_OLE &&
        (Pred & CmpInst::FCMP_OLE) != 0)
      return;
    WhichConst = 4;
  } else {
    // Not one of our special constants.
    return;
  }
  // Partial masks to use for EQ, GT, LT, UN comparisons, respectively.
  static const int Masks[][4] = {
    { // 0
      SystemZ::TDCMASK_ZERO,              // eq
      SystemZ::TDCMASK_POSITIVE,          // gt
      SystemZ::TDCMASK_NEGATIVE,          // lt
      SystemZ::TDCMASK_NAN,               // un
    },
    { // inf
      SystemZ::TDCMASK_INFINITY_PLUS,     // eq
      0,                                  // gt
      (SystemZ::TDCMASK_ZERO |
       SystemZ::TDCMASK_NEGATIVE |
       SystemZ::TDCMASK_NORMAL_PLUS |
       SystemZ::TDCMASK_SUBNORMAL_PLUS),  // lt
      SystemZ::TDCMASK_NAN,               // un
    },
    { // -inf
      SystemZ::TDCMASK_INFINITY_MINUS,    // eq
      (SystemZ::TDCMASK_ZERO |
       SystemZ::TDCMASK_POSITIVE |
       SystemZ::TDCMASK_NORMAL_MINUS |
       SystemZ::TDCMASK_SUBNORMAL_MINUS), // gt
      0,                                  // lt
      SystemZ::TDCMASK_NAN,               // un
    },
    { // minnorm
      0,                                  // eq (unsupported)
      (SystemZ::TDCMASK_NORMAL_PLUS |
       SystemZ::TDCMASK_INFINITY_PLUS),   // gt (actually ge)
      (SystemZ::TDCMASK_ZERO |
       SystemZ::TDCMASK_NEGATIVE |
       SystemZ::TDCMASK_SUBNORMAL_PLUS),  // lt
      SystemZ::TDCMASK_NAN,               // un
    },
    { // -minnorm
      0,                                  // eq (unsupported)
      (SystemZ::TDCMASK_ZERO |
       SystemZ::TDCMASK_POSITIVE |
       SystemZ::TDCMASK_SUBNORMAL_MINUS), // gt
      (SystemZ::TDCMASK_NORMAL_MINUS |
       SystemZ::TDCMASK_INFINITY_MINUS),  // lt (actually le)
      SystemZ::TDCMASK_NAN,               // un
    }
  };
  // Construct the mask as a combination of the partial masks.
  int Mask = 0;
  if (Pred & CmpInst::FCMP_OEQ)
    Mask |= Masks[WhichConst][0];
  if (Pred & CmpInst::FCMP_OGT)
    Mask |= Masks[WhichConst][1];
  if (Pred & CmpInst::FCMP_OLT)
    Mask |= Masks[WhichConst][2];
  if (Pred & CmpInst::FCMP_UNO)
    Mask |= Masks[WhichConst][3];
  // A lone fcmp is unworthy of tdc conversion on its own, but may become
  // worthy if combined with fabs.
  bool Worthy = false;
  if (CallInst *CI = dyn_cast<CallInst>(Op0)) {
    Function *F = CI->getCalledFunction();
    if (F && F->getIntrinsicID() == Intrinsic::fabs) {
      // Fold with fabs - adjust the mask appropriately.
      Mask &= SystemZ::TDCMASK_PLUS;
      Mask |= Mask >> 1;
      Op0 = CI->getArgOperand(0);
      // A combination of fcmp with fabs is a win, unless the constant
      // involved is 0 (which is handled by later passes).
      Worthy = WhichConst != 0;
      PossibleJunk.insert(CI);
    }
示例#5
0
bool CallAnalyzer::visitCmpInst(CmpInst &I) {
  Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
  // First try to handle simplified comparisons.
  if (!isa<Constant>(LHS))
    if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
      LHS = SimpleLHS;
  if (!isa<Constant>(RHS))
    if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
      RHS = SimpleRHS;
  if (Constant *CLHS = dyn_cast<Constant>(LHS)) {
    if (Constant *CRHS = dyn_cast<Constant>(RHS))
      if (Constant *C = ConstantExpr::getCompare(I.getPredicate(), CLHS, CRHS)) {
        SimplifiedValues[&I] = C;
        return true;
      }
  }

  if (I.getOpcode() == Instruction::FCmp)
    return false;

  // Otherwise look for a comparison between constant offset pointers with
  // a common base.
  Value *LHSBase, *RHSBase;
  APInt LHSOffset, RHSOffset;
  std::tie(LHSBase, LHSOffset) = ConstantOffsetPtrs.lookup(LHS);
  if (LHSBase) {
    std::tie(RHSBase, RHSOffset) = ConstantOffsetPtrs.lookup(RHS);
    if (RHSBase && LHSBase == RHSBase) {
      // We have common bases, fold the icmp to a constant based on the
      // offsets.
      Constant *CLHS = ConstantInt::get(LHS->getContext(), LHSOffset);
      Constant *CRHS = ConstantInt::get(RHS->getContext(), RHSOffset);
      if (Constant *C = ConstantExpr::getICmp(I.getPredicate(), CLHS, CRHS)) {
        SimplifiedValues[&I] = C;
        ++NumConstantPtrCmps;
        return true;
      }
    }
  }

  // If the comparison is an equality comparison with null, we can simplify it
  // for any alloca-derived argument.
  if (I.isEquality() && isa<ConstantPointerNull>(I.getOperand(1)))
    if (isAllocaDerivedArg(I.getOperand(0))) {
      // We can actually predict the result of comparisons between an
      // alloca-derived value and null. Note that this fires regardless of
      // SROA firing.
      bool IsNotEqual = I.getPredicate() == CmpInst::ICMP_NE;
      SimplifiedValues[&I] = IsNotEqual ? ConstantInt::getTrue(I.getType())
                                        : ConstantInt::getFalse(I.getType());
      return true;
    }

  // Finally check for SROA candidates in comparisons.
  Value *SROAArg;
  DenseMap<Value *, int>::iterator CostIt;
  if (lookupSROAArgAndCost(I.getOperand(0), SROAArg, CostIt)) {
    if (isa<ConstantPointerNull>(I.getOperand(1))) {
      accumulateSROACost(CostIt, InlineConstants::InstrCost);
      return true;
    }

    disableSROA(CostIt);
  }

  return false;
}
	void CGGraph::constructGraph() {

		std::vector<CGConstraint*>::iterator it;
		std::string nodeName;
		CGConstraint* curConstraint;
		CGNode* firstNode;
		CGNode* secondNode;
		std::vector<int>::iterator lengthIt;
		int curValue;
		ConstantInt* cInt;

		for (it = constraints.begin(); it != constraints.end(); ++it) {

			curConstraint = *it;
			Instruction* PP = curConstraint->programPoint;

			switch(curConstraint->type) {
				case CGConstraint::C1:
				{

					firstNode = getNode(getNameFromValue(curConstraint->programPoint->getOperand(0), PP));
					secondNode = getNode(getNameFromValue(curConstraint->programPoint->getOperand(1), PP));

					firstNode->connectTo(secondNode, 0);
					
					break;
				}
				case CGConstraint::C2:
				{
					if (isa<StoreInst>(PP)) {
						/*
						operand(0) = constant int or variable of int type
						operand(1)  = var being stored into
						*/
						firstNode = getNode(getNameFromValue(PP->getOperand(0), PP));
						secondNode = getNode(getNameFromValue(PP->getOperand(1), PP));
						firstNode->connectTo(secondNode, 0);
					}
					else if (isa<LoadInst>(PP)) {
						/*
						operand(0) = pointer being loaded from
						(Value*)PP  = var being loaded into
						*/
						firstNode = getNode(getNameFromValue(PP->getOperand(0), PP));
						secondNode = getNode(getNameFromValue(PP, PP));
						firstNode->connectTo(secondNode, 0);
					}
					else if (isa<CastInst>(PP)) {
						/*
						operand(0) = var being casted 
						(Value*)PP  = var getting the result of the cast
						*/
						firstNode = getNode(getNameFromValue(PP->getOperand(0), PP));
						secondNode = getNode(getNameFromValue(PP, PP));
						firstNode->connectTo(secondNode, 0);
					}

					break;
				}
				case CGConstraint::C3:
				{
				
					secondNode = getNode(getNameFromValue(PP, PP));
					if ((cInt = dyn_cast<ConstantInt>(curConstraint->programPoint->getOperand(0)))) {
						firstNode = getNode(getNameFromValue(curConstraint->programPoint->getOperand(1), PP));
					} else if ((cInt = dyn_cast<ConstantInt>(curConstraint->programPoint->getOperand(1)))) {
						firstNode = getNode(getNameFromValue(curConstraint->programPoint->getOperand(0), PP));
					} else {
						return;
					}
					curValue = cInt->getSExtValue();
					firstNode->connectTo(secondNode, curValue);
					break;
				}
				case CGConstraint::C4:
				{

					CmpInst* cmpInst;
					BranchInst* branchInst;
					
					if( !(branchInst = dyn_cast<BranchInst>(curConstraint->programPoint)) ) {
						errs() << "ERROR: BranchInst cast unsuccessful for C4 in CGGraph::constructGraph() \n";
						return;
					}
					if( !(cmpInst = dyn_cast<CmpInst>(owner->branchToCompare[branchInst])) ) {
						errs() << "ERROR: CmpInst not found for C4 in CGGraph::constructGraph() \n";
						return;
					}

					int size1, size2;
					size1 = curConstraint->piAssignments.size();
					size2 = curConstraint->piAssignments2.size();
					/*
					There are two possibilities here: (a) size1 = size2 = 2, or (b) size1 = size2 = 1;
					If (b), there will be one operand in the compare inst that is a literal value.
					That literal value still generates a constraint although it does not generate a pi assignment.
					We need to account for that.
					*/

					if (size1 != size2) {
						errs() << "ERROR: piAssignments not of equal length for C4 in CGGraph::constructGraph()\n"; 
						return;
					}
					if ( (size1 < 1) || (size1 > 2) ) {
						errs() << "ERROR: piAssignments.size() != 1 or 2 for C4 in CGGraph::constructGraph()\n"; 
						return;
					}
						
					//this takes care of the first two constraints for both cases (size = 1 or size = 2)
					//first branch
					for (int i = 0; i < size1; ++i) {
						firstNode = getNode(curConstraint->piAssignments[i]->getOperandName()); //vi - wr
						secondNode = getNode(curConstraint->piAssignments[i]->getAssignedName()); //vj - ws 
						firstNode->connectTo(secondNode, 0); //vi -> vj 0  -  wr -> ws 0
					}
					//second branch
					for (int i = 0; i < size2; ++i) {
						firstNode = getNode(curConstraint->piAssignments2[i]->getOperandName()); //vi - wr
						secondNode = getNode(curConstraint->piAssignments2[i]->getAssignedName()); //vk - wt 
						firstNode->connectTo(secondNode, 0); //vi -> vk 0  -  wr -> wt 0
					}
		
					/*
					- first and second op names for the third constraint for branches 1 and 2
					- each case is stored in the order of the pi assignments, which is also the 
					order of the cmp instruction operands
					*/
					std::string firstOpNameBr1, secondOpNameBr1, firstOpNameBr2, secondOpNameBr2;
					
					if (size1 == 1) {
						/*
						the 2nd constraint in the table will not exist in this case, but there will be a 3rd constraint
						that was missed by the above, e.g.
						if (x1 <= 10)
						1. x2 <= x1
						2. nothing
						3. x2 <= 10  <---- we need to add this here
						*/ 
						
						//prune the int from the CmpInst
						if (cmpInst->getNumOperands() != 2) {
							errs() << "ERROR: cmpInst->getNumOperands() != 2 in CGGraph::constructGraph()\n";
							return;
						}
						if ((cInt = dyn_cast<ConstantInt>(cmpInst->getOperand(0)))) {
							//int is first op
							firstOpNameBr1 = getNameFromValue(cInt, PP);
							secondOpNameBr1 = curConstraint->piAssignments[0]->getAssignedName();
							firstOpNameBr2 = firstOpNameBr1;
							secondOpNameBr2 = curConstraint->piAssignments2[0]->getAssignedName();
						} else if ((cInt = dyn_cast<ConstantInt>(cmpInst->getOperand(1)))) {
							//int is second op 
							firstOpNameBr1 = curConstraint->piAssignments[0]->getAssignedName();
							secondOpNameBr1 = getNameFromValue(cInt, PP);
							firstOpNameBr2 = curConstraint->piAssignments2[0]->getAssignedName();
							secondOpNameBr2 = secondOpNameBr1;
						} else {
							errs() << "ERROR: int not found in cmpInstr in CGGraph::constructGraph()\n";
							return;
						}
					}
					else if (size1 == 2) {
						//store in order of pi assignments
						firstOpNameBr1 = curConstraint->piAssignments[0]->getAssignedName();
						secondOpNameBr1 = curConstraint->piAssignments[1]->getAssignedName();
						firstOpNameBr2 = curConstraint->piAssignments2[0]->getAssignedName();
						secondOpNameBr2 = curConstraint->piAssignments2[1]->getAssignedName();
					}
	
					CGNode* firstNodeBr1 = getNode(firstOpNameBr1);
					CGNode* secondNodeBr1 = getNode(secondOpNameBr1);
					CGNode* firstNodeBr2 = getNode(firstOpNameBr2);
					CGNode* secondNodeBr2 = getNode(secondOpNameBr2);
					
					switch(cmpInst->getPredicate()) {
						case CmpInst::ICMP_SGT: // >
							firstNodeBr1->connectTo(secondNodeBr1, -1); //vj -> ws -1
							secondNodeBr2->connectTo(firstNodeBr2, 0); //wt -> vk 0
							break;
						case CmpInst::ICMP_SLT: // <
							secondNodeBr1->connectTo(firstNodeBr1, -1); //ws -> vj -1
							firstNodeBr2->connectTo(secondNodeBr2, 0); //vk -> wt 0
							break;
						case CmpInst::ICMP_SGE: // >=
							firstNodeBr1->connectTo(secondNodeBr1, 0); //vj -> ws 0
							secondNodeBr2->connectTo(firstNodeBr2, -1); //wt -> vk -1
							break;
						case CmpInst::ICMP_SLE: // <=
							secondNodeBr1->connectTo(firstNodeBr1, 0); //ws -> vj 0
							firstNodeBr2->connectTo(secondNodeBr2, -1); //vk -> wt -1						
							break;
						default:
							break;
					}

					break;
				}
				case CGConstraint::C5:
				{
					//operand 1 = array length
					firstNode = getNode(getNameFromValue(PP->getOperand(1), PP));
					secondNode = getNode(curConstraint->piAssignments[0]->getAssignedName());
					firstNode->connectTo(secondNode, -1);
					break;
				}
				case CGConstraint::CONTROL_FLOW:
				{
					//Done and ready to test

					firstNode = getNode(getNameFromValue(curConstraint->programPoint->getOperand(0), PP));
					secondNode = getNode(getNameFromValue(curConstraint->programPoint, PP));
					firstNode->connectTo(secondNode, 0);
					firstNode = getNode(getNameFromValue(curConstraint->programPoint->getOperand(1), PP));
					firstNode->connectTo(secondNode, 0);
					break;
				}
			} //end switch
		} //end for	
	} //end constructGraph()
示例#7
0
// Compute the unlikely successors to the block BB in the loop L, specifically
// those that are unlikely because this is a loop, and add them to the
// UnlikelyBlocks set.
static void
computeUnlikelySuccessors(const BasicBlock *BB, Loop *L,
                          SmallPtrSetImpl<const BasicBlock*> &UnlikelyBlocks) {
  // Sometimes in a loop we have a branch whose condition is made false by
  // taking it. This is typically something like
  //  int n = 0;
  //  while (...) {
  //    if (++n >= MAX) {
  //      n = 0;
  //    }
  //  }
  // In this sort of situation taking the branch means that at the very least it
  // won't be taken again in the next iteration of the loop, so we should
  // consider it less likely than a typical branch.
  //
  // We detect this by looking back through the graph of PHI nodes that sets the
  // value that the condition depends on, and seeing if we can reach a successor
  // block which can be determined to make the condition false.
  //
  // FIXME: We currently consider unlikely blocks to be half as likely as other
  // blocks, but if we consider the example above the likelyhood is actually
  // 1/MAX. We could therefore be more precise in how unlikely we consider
  // blocks to be, but it would require more careful examination of the form
  // of the comparison expression.
  const BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
  if (!BI || !BI->isConditional())
    return;

  // Check if the branch is based on an instruction compared with a constant
  CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
  if (!CI || !isa<Instruction>(CI->getOperand(0)) ||
      !isa<Constant>(CI->getOperand(1)))
    return;

  // Either the instruction must be a PHI, or a chain of operations involving
  // constants that ends in a PHI which we can then collapse into a single value
  // if the PHI value is known.
  Instruction *CmpLHS = dyn_cast<Instruction>(CI->getOperand(0));
  PHINode *CmpPHI = dyn_cast<PHINode>(CmpLHS);
  Constant *CmpConst = dyn_cast<Constant>(CI->getOperand(1));
  // Collect the instructions until we hit a PHI
  SmallVector<BinaryOperator *, 1> InstChain;
  while (!CmpPHI && CmpLHS && isa<BinaryOperator>(CmpLHS) &&
         isa<Constant>(CmpLHS->getOperand(1))) {
    // Stop if the chain extends outside of the loop
    if (!L->contains(CmpLHS))
      return;
    InstChain.push_back(cast<BinaryOperator>(CmpLHS));
    CmpLHS = dyn_cast<Instruction>(CmpLHS->getOperand(0));
    if (CmpLHS)
      CmpPHI = dyn_cast<PHINode>(CmpLHS);
  }
  if (!CmpPHI || !L->contains(CmpPHI))
    return;

  // Trace the phi node to find all values that come from successors of BB
  SmallPtrSet<PHINode*, 8> VisitedInsts;
  SmallVector<PHINode*, 8> WorkList;
  WorkList.push_back(CmpPHI);
  VisitedInsts.insert(CmpPHI);
  while (!WorkList.empty()) {
    PHINode *P = WorkList.back();
    WorkList.pop_back();
    for (BasicBlock *B : P->blocks()) {
      // Skip blocks that aren't part of the loop
      if (!L->contains(B))
        continue;
      Value *V = P->getIncomingValueForBlock(B);
      // If the source is a PHI add it to the work list if we haven't
      // already visited it.
      if (PHINode *PN = dyn_cast<PHINode>(V)) {
        if (VisitedInsts.insert(PN).second)
          WorkList.push_back(PN);
        continue;
      }
      // If this incoming value is a constant and B is a successor of BB, then
      // we can constant-evaluate the compare to see if it makes the branch be
      // taken or not.
      Constant *CmpLHSConst = dyn_cast<Constant>(V);
      if (!CmpLHSConst ||
          std::find(succ_begin(BB), succ_end(BB), B) == succ_end(BB))
        continue;
      // First collapse InstChain
      for (Instruction *I : llvm::reverse(InstChain)) {
        CmpLHSConst = ConstantExpr::get(I->getOpcode(), CmpLHSConst,
                                        cast<Constant>(I->getOperand(1)), true);
        if (!CmpLHSConst)
          break;
      }
      if (!CmpLHSConst)
        continue;
      // Now constant-evaluate the compare
      Constant *Result = ConstantExpr::getCompare(CI->getPredicate(),
                                                  CmpLHSConst, CmpConst, true);
      // If the result means we don't branch to the block then that block is
      // unlikely.
      if (Result &&
          ((Result->isZeroValue() && B == BI->getSuccessor(0)) ||
           (Result->isOneValue() && B == BI->getSuccessor(1))))
        UnlikelyBlocks.insert(B);
    }
  }
}
/// MatchGuardHeuristic - Predict that a comparison in which a register is
/// an operand, the register is used before being defined in a successor
/// block, and the successor block does not post-dominate will reach the
/// successor block.
/// @returns a Prediction that is a pair in which the first element is the
/// successor taken, and the second the successor not taken.
Prediction BranchHeuristicsInfo::MatchGuardHeuristic(BasicBlock *root) const {
  bool matched = false;
  Prediction pred;

  // Last instruction of basic block.
  TerminatorInst *TI = root->getTerminator();

  // Basic block successors. True and False branches.
  BasicBlock *trueSuccessor = TI->getSuccessor(0);
  BasicBlock *falseSuccessor = TI->getSuccessor(1);

  // Is the last instruction a Branch Instruction?
  BranchInst *BI = dyn_cast<BranchInst>(TI);
  if (!BI || !BI->isConditional())
    return empty;

  // Conditional instruction.
  Value *cond = BI->getCondition();

  // Find if the variable used in the branch instruction is
  // in fact a comparison instruction.
  CmpInst *CI = dyn_cast<CmpInst>(cond);
  if (!CI)
    return empty;

  // Seek over all of the operands of this comparison instruction.
  for (unsigned ops = 0; ops < CI->getNumOperands(); ++ops) {
    // Find the operand.
    Value *operand = CI->getOperand(ops);

    // Check if the operand is neither a function argument or a value.
    if (!isa<Argument>(operand) && !isa<User>(operand))
      continue;

    // Check if this variable was used in the true successor and
    // does not post dominate.
    // Since LLVM is in SSA form, it's impossible for a variable being used
    // before being defined, so that statement is skipped.
    if (operand->isUsedInBasicBlock(trueSuccessor) &&
        !PDT->dominates(trueSuccessor, root)) {
      // If a heuristic was already matched, predict none and abort immediately.
      if (matched)
        return empty;

      matched = true;
      pred = std::make_pair(trueSuccessor, falseSuccessor);
    }

    // Check if this variable was used in the false successor and
    // does not post dominate.
    if (operand->isUsedInBasicBlock(falseSuccessor) &&
        !PDT->dominates(falseSuccessor, root)) {
      // If a heuristic was already matched, predict none and abort immediately.
      if (matched)
        return empty;

      matched = true;
      pred = std::make_pair(falseSuccessor, trueSuccessor);
    }
  }

  return (matched ? pred : empty);
}