-
- // 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)];
-
- // There is a special case here... if there IS no post-dominator for
- // the block we have no owhere 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!
- // 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... and e variable.
- //
- TI = BB->getTerminator();
-
- // Rescan this successor...
- --i;
- } else {
- PostDominatorTree::Node *NextNode = LastNode->getIDom();
-
- while (!AliveBlocks.count(NextNode->getBlock())) {
- LastNode = NextNode;
- NextNode = NextNode->getIDom();
- }
-
- // 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();
- PHINode *PN = dyn_cast<PHINode>(II); ++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, telling
- // dead instructions to drop their references. This is so that the next
- // sweep over the program can safely delete dead instructions without
- // other dead instructions still referring to them.
- //
- dropReferencesOfDeadInstructionsInLiveBlock(BB);
- }