+ AvailableValsTy &AvailableVals = getAvailableVals(AV);
+
+ // Query AvailableVals by doing an insertion of null.
+ std::pair<AvailableValsTy::iterator, bool> InsertRes =
+ AvailableVals.insert(std::make_pair(BB, 0));
+
+ // Handle the case when the insertion fails because we have already seen BB.
+ if (!InsertRes.second) {
+ // If the insertion failed, there are two cases. The first case is that the
+ // value is already available for the specified block. If we get this, just
+ // return the value.
+ if (InsertRes.first->second != 0)
+ return InsertRes.first->second;
+
+ // Otherwise, if the value we find is null, then this is the value is not
+ // known but it is being computed elsewhere in our recursion. This means
+ // that we have a cycle. Handle this by inserting a PHI node and returning
+ // it. When we get back to the first instance of the recursion we will fill
+ // in the PHI node.
+ MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
+ MachineInstr *NewPHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
+ VRC, MRI,TII);
+ unsigned NewVR = NewPHI->getOperand(0).getReg();
+ InsertRes.first->second = NewVR;
+ return NewVR;
+ }
+
+ // If there are no predecessors, then we must have found an unreachable block
+ // just return 'undef'. Since there are no predecessors, InsertRes must not
+ // be invalidated.
+ if (BB->pred_empty()) {
+ // Insert an implicit_def to represent an undef value.
+ MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
+ BB, BB->getFirstTerminator(),
+ VRC, MRI, TII);
+ return InsertRes.first->second = NewDef->getOperand(0).getReg();
+ }
+
+ // Okay, the value isn't in the map and we just inserted a null in the entry
+ // to indicate that we're processing the block. Since we have no idea what
+ // value is in this block, we have to recurse through our predecessors.
+ //
+ // While we're walking our predecessors, we keep track of them in a vector,
+ // then insert a PHI node in the end if we actually need one. We could use a
+ // smallvector here, but that would take a lot of stack space for every level
+ // of the recursion, just use IncomingPredInfo as an explicit stack.
+ IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
+ unsigned FirstPredInfoEntry = IncomingPredInfo.size();
+
+ // As we're walking the predecessors, keep track of whether they are all
+ // producing the same value. If so, this value will capture it, if not, it
+ // will get reset to null. We distinguish the no-predecessor case explicitly
+ // below.
+ unsigned SingularValue = 0;
+ bool isFirstPred = true;
+ for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
+ E = BB->pred_end(); PI != E; ++PI) {
+ MachineBasicBlock *PredBB = *PI;
+ unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
+ IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
+
+ // Compute SingularValue.
+ if (isFirstPred) {
+ SingularValue = PredVal;
+ isFirstPred = false;
+ } else if (PredVal != SingularValue)
+ SingularValue = 0;
+ }
+
+ /// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If
+ /// this block is involved in a loop, a no-entry PHI node will have been
+ /// inserted as InsertedVal. Otherwise, we'll still have the null we inserted
+ /// above.
+ unsigned &InsertedVal = AvailableVals[BB];
+
+ // If all the predecessor values are the same then we don't need to insert a
+ // PHI. This is the simple and common case.
+ if (SingularValue) {
+ // If a PHI node got inserted, replace it with the singlar value and delete
+ // it.
+ if (InsertedVal) {
+ MachineInstr *OldVal = MRI->getVRegDef(InsertedVal);
+ // Be careful about dead loops. These RAUW's also update InsertedVal.
+ assert(InsertedVal != SingularValue && "Dead loop?");
+ ReplaceRegWith(InsertedVal, SingularValue);
+ OldVal->eraseFromParent();
+ }
+
+ InsertedVal = SingularValue;
+
+ // Drop the entries we added in IncomingPredInfo to restore the stack.
+ IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
+ IncomingPredInfo.end());
+ return InsertedVal;
+ }
+
+
+ // Otherwise, we do need a PHI: insert one now if we don't already have one.
+ MachineInstr *InsertedPHI;
+ if (InsertedVal == 0) {
+ MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
+ InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
+ VRC, MRI, TII);
+ InsertedVal = InsertedPHI->getOperand(0).getReg();
+ } else {
+ InsertedPHI = MRI->getVRegDef(InsertedVal);
+ }
+
+ // Fill in all the predecessors of the PHI.
+ MachineInstrBuilder MIB(InsertedPHI);
+ for (IncomingPredInfoTy::iterator I =
+ IncomingPredInfo.begin()+FirstPredInfoEntry,
+ E = IncomingPredInfo.end(); I != E; ++I)
+ MIB.addReg(I->second).addMBB(I->first);
+
+ // Drop the entries we added in IncomingPredInfo to restore the stack.
+ IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
+ IncomingPredInfo.end());
+
+ // See if the PHI node can be merged to a single value. This can happen in
+ // loop cases when we get a PHI of itself and one other value.
+ if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
+ MRI->replaceRegWith(InsertedVal, ConstVal);
+ InsertedPHI->eraseFromParent();
+ InsertedVal = ConstVal;
+ } else {
+ DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n");
+
+ // If the client wants to know about all new instructions, tell it.
+ if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
+ }
+
+ return InsertedVal;