- // If the entry node is dead, insert a new entry node to eliminate the entry
- // node as a special case.
- //
- if (!AliveBlocks.count(&Func->front())) {
- BasicBlock *NewEntry = new BasicBlock();
- new BranchInst(&Func->front(), NewEntry);
- Func->getBasicBlockList().push_front(NewEntry);
- AliveBlocks.insert(NewEntry); // This block is always alive!
- LiveSet.insert(NewEntry->getTerminator()); // The branch is live
- }
-
- // Loop over all of the alive blocks in the function. If any successor
- // blocks are not alive, we adjust the outgoing branches to branch to the
- // first live postdominator of the live block, adjusting any PHI nodes in
- // the block to reflect this.
- //
- for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
- if (AliveBlocks.count(I)) {
- BasicBlock *BB = I;
- TerminatorInst *TI = BB->getTerminator();
-
- // If the terminator instruction is alive, but the block it is contained
- // in IS alive, this means that this terminator is a conditional branch on
- // a condition that doesn't matter. Make it an unconditional branch to
- // ONE of the successors. This has the side effect of dropping a use of
- // the conditional value, which may also be dead.
- if (!LiveSet.count(TI))
- TI = convertToUnconditionalBranch(TI);
-
- // Loop over all of the successors, looking for ones that are not alive.
- // We cannot save the number of successors in the terminator instruction
- // here because we may remove them if we don't have a postdominator.
- //
- for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
- if (!AliveBlocks.count(TI->getSuccessor(i))) {
- // Scan up the postdominator tree, looking for the first
- // postdominator that is alive, and the last postdominator that is
- // dead...
- //
- PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
- PostDominatorTree::Node *NextNode = 0;
-
- if (LastNode) {
- NextNode = LastNode->getIDom();
- while (!AliveBlocks.count(NextNode->getBlock())) {
- LastNode = NextNode;
- NextNode = NextNode->getIDom();
- if (NextNode == 0) {
- LastNode = 0;
- break;
- }
- }
- }
-
- // There is a special case here... if there IS no post-dominator for
- // the block we have nowhere to point our branch to. Instead, convert
- // it to a return. This can only happen if the code branched into an
- // infinite loop. Note that this may not be desirable, because we
- // _are_ altering the behavior of the code. This is a well known
- // drawback of ADCE, so in the future if we choose to revisit the
- // decision, this is where it should be.
- //
- if (LastNode == 0) { // No postdominator!
- if (!isa<InvokeInst>(TI)) {
- // Call RemoveSuccessor to transmogrify the terminator instruction
- // to not contain the outgoing branch, or to create a new
- // terminator if the form fundamentally changes (i.e.,
- // unconditional branch to return). Note that this will change a
- // branch into an infinite loop into a return instruction!
- //
- RemoveSuccessor(TI, i);
-
- // RemoveSuccessor may replace TI... make sure we have a fresh
- // pointer.
- //
- TI = BB->getTerminator();
-
- // Rescan this successor...
- --i;
- } else {
-
- }
- } else {
- // Get the basic blocks that we need...
- BasicBlock *LastDead = LastNode->getBlock();
- BasicBlock *NextAlive = NextNode->getBlock();
-
- // Make the conditional branch now go to the next alive block...
- TI->getSuccessor(i)->removePredecessor(BB);
- TI->setSuccessor(i, NextAlive);
-
- // If there are PHI nodes in NextAlive, we need to add entries to
- // the PHI nodes for the new incoming edge. The incoming values
- // should be identical to the incoming values for LastDead.
- //
- for (BasicBlock::iterator II = NextAlive->begin();
- isa<PHINode>(II); ++II) {
- PHINode *PN = cast<PHINode>(II);
- if (LiveSet.count(PN)) { // Only modify live phi nodes
- // Get the incoming value for LastDead...
- int OldIdx = PN->getBasicBlockIndex(LastDead);
- assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
- Value *InVal = PN->getIncomingValue(OldIdx);
-
- // Add an incoming value for BB now...
- PN->addIncoming(InVal, BB);
- }
- }
- }
- }
-
- // Now loop over all of the instructions in the basic block, deleting
- // dead instructions. This is so that the next sweep over the program
- // can safely delete dead instructions without other dead instructions
- // still referring to them.
- //
- deleteDeadInstructionsInLiveBlock(BB);
- }
-
- // Loop over all of the basic blocks in the function, dropping references of
- // the dead basic blocks. We must do this after the previous step to avoid
- // dropping references to PHIs which still have entries...
- //
- std::vector<BasicBlock*> DeadBlocks;
- for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
- if (!AliveBlocks.count(BB)) {
- // Remove PHI node entries for this block in live successor blocks.
- for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
- if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
- (*SI)->removePredecessor(BB);
-
- BB->dropAllReferences();
- MadeChanges = true;
- DeadBlocks.push_back(BB);
- }
-
- NumBlockRemoved += DeadBlocks.size();
-
- // Now loop through all of the blocks and delete the dead ones. We can safely
- // do this now because we know that there are no references to dead blocks
- // (because they have dropped all of their references).
- for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
- E = DeadBlocks.end(); I != E; ++I)
- Func->getBasicBlockList().erase(*I);
-
- return MadeChanges;