-/// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
-/// almost-empty BB ending in an unconditional branch to Succ, into succ.
-///
-/// Assumption: Succ is the single successor for BB.
-///
-static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
- assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
-
- DOUT << "Looking to fold " << BB->getNameStart() << " into "
- << Succ->getNameStart() << "\n";
- // Shortcut, if there is only a single predecessor it must be BB and merging
- // is always safe
- if (Succ->getSinglePredecessor()) return true;
-
- typedef SmallPtrSet<Instruction*, 16> InstrSet;
- InstrSet BBPHIs;
-
- // Make a list of all phi nodes in BB
- BasicBlock::iterator BBI = BB->begin();
- while (isa<PHINode>(*BBI)) BBPHIs.insert(BBI++);
-
- // Make a list of the predecessors of BB
- typedef SmallPtrSet<BasicBlock*, 16> BlockSet;
- BlockSet BBPreds(pred_begin(BB), pred_end(BB));
-
- // Use that list to make another list of common predecessors of BB and Succ
- BlockSet CommonPreds;
- for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
- PI != PE; ++PI)
- if (BBPreds.count(*PI))
- CommonPreds.insert(*PI);
-
- // Shortcut, if there are no common predecessors, merging is always safe
- if (CommonPreds.empty())
- return true;
-
- // Look at all the phi nodes in Succ, to see if they present a conflict when
- // merging these blocks
- for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
- PHINode *PN = cast<PHINode>(I);
-
- // If the incoming value from BB is again a PHINode in
- // BB which has the same incoming value for *PI as PN does, we can
- // merge the phi nodes and then the blocks can still be merged
- PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB));
- if (BBPN && BBPN->getParent() == BB) {
- for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
- PI != PE; PI++) {
- if (BBPN->getIncomingValueForBlock(*PI)
- != PN->getIncomingValueForBlock(*PI)) {
- DOUT << "Can't fold, phi node " << *PN->getNameStart() << " in "
- << Succ->getNameStart() << " is conflicting with "
- << BBPN->getNameStart() << " with regard to common predecessor "
- << (*PI)->getNameStart() << "\n";
- return false;
- }
- }
- // Remove this phinode from the list of phis in BB, since it has been
- // handled.
- BBPHIs.erase(BBPN);
- } else {
- Value* Val = PN->getIncomingValueForBlock(BB);
- for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
- PI != PE; PI++) {
- // See if the incoming value for the common predecessor is equal to the
- // one for BB, in which case this phi node will not prevent the merging
- // of the block.
- if (Val != PN->getIncomingValueForBlock(*PI)) {
- DOUT << "Can't fold, phi node " << *PN->getNameStart() << " in "
- << Succ->getNameStart() << " is conflicting with regard to common "
- << "predecessor " << (*PI)->getNameStart() << "\n";
- return false;
- }
- }
- }
- }
-
- // If there are any other phi nodes in BB that don't have a phi node in Succ
- // to merge with, they must be moved to Succ completely. However, for any
- // predecessors of Succ, branches will be added to the phi node that just
- // point to itself. So, for any common predecessors, this must not cause
- // conflicts.
- for (InstrSet::iterator I = BBPHIs.begin(), E = BBPHIs.end();
- I != E; I++) {
- PHINode *PN = cast<PHINode>(*I);
- for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
- PI != PE; PI++)
- if (PN->getIncomingValueForBlock(*PI) != PN) {
- DOUT << "Can't fold, phi node " << *PN->getNameStart() << " in "
- << BB->getNameStart() << " is conflicting with regard to common "
- << "predecessor " << (*PI)->getNameStart() << "\n";
- return false;
- }
- }
-
- return true;
-}
-
-/// TryToSimplifyUncondBranchFromEmptyBlock - BB contains an unconditional
-/// branch to Succ, and contains no instructions other than PHI nodes and the
-/// branch. If possible, eliminate BB.
-static bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
- BasicBlock *Succ) {
- // Check to see if merging these blocks would cause conflicts for any of the
- // phi nodes in BB or Succ. If not, we can safely merge.
- if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
-
- DOUT << "Killing Trivial BB: \n" << *BB;
-
- if (isa<PHINode>(Succ->begin())) {
- // If there is more than one pred of succ, and there are PHI nodes in
- // the successor, then we need to add incoming edges for the PHI nodes
- //
- const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
-
- // Loop over all of the PHI nodes in the successor of BB.
- for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
- PHINode *PN = cast<PHINode>(I);
- Value *OldVal = PN->removeIncomingValue(BB, false);
- assert(OldVal && "No entry in PHI for Pred BB!");
-
- // If this incoming value is one of the PHI nodes in BB, the new entries
- // in the PHI node are the entries from the old PHI.
- if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
- PHINode *OldValPN = cast<PHINode>(OldVal);
- for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
- // Note that, since we are merging phi nodes and BB and Succ might
- // have common predecessors, we could end up with a phi node with
- // identical incoming branches. This will be cleaned up later (and
- // will trigger asserts if we try to clean it up now, without also
- // simplifying the corresponding conditional branch).
- PN->addIncoming(OldValPN->getIncomingValue(i),
- OldValPN->getIncomingBlock(i));
- } else {
- // Add an incoming value for each of the new incoming values.
- for (unsigned i = 0, e = BBPreds.size(); i != e; ++i)
- PN->addIncoming(OldVal, BBPreds[i]);
- }
- }
- }
-
- if (isa<PHINode>(&BB->front())) {
- SmallVector<BasicBlock*, 16>
- OldSuccPreds(pred_begin(Succ), pred_end(Succ));
-
- // Move all PHI nodes in BB to Succ if they are alive, otherwise
- // delete them.
- while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
- if (PN->use_empty()) {
- // Just remove the dead phi. This happens if Succ's PHIs were the only
- // users of the PHI nodes.
- PN->eraseFromParent();
- continue;
- }
-
- // The instruction is alive, so this means that BB must dominate all
- // predecessors of Succ (Since all uses of the PN are after its
- // definition, so in Succ or a block dominated by Succ. If a predecessor
- // of Succ would not be dominated by BB, PN would violate the def before
- // use SSA demand). Therefore, we can simply move the phi node to the
- // next block.
- Succ->getInstList().splice(Succ->begin(),
- BB->getInstList(), BB->begin());
-
- // We need to add new entries for the PHI node to account for
- // predecessors of Succ that the PHI node does not take into
- // account. At this point, since we know that BB dominated succ and all
- // of its predecessors, this means that we should any newly added
- // incoming edges should use the PHI node itself as the value for these
- // edges, because they are loop back edges.
- for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
- if (OldSuccPreds[i] != BB)
- PN->addIncoming(PN, OldSuccPreds[i]);
- }
- }
-
- // Everything that jumped to BB now goes to Succ.
- BB->replaceAllUsesWith(Succ);
- if (!Succ->hasName()) Succ->takeName(BB);
- BB->eraseFromParent(); // Delete the old basic block.
- return true;
-}