1 //===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass performs several transformations to transform natural loops into a
11 // simpler form, which makes subsequent analyses and transformations simpler and
14 // Loop pre-header insertion guarantees that there is a single, non-critical
15 // entry edge from outside of the loop to the loop header. This simplifies a
16 // number of analyses and transformations, such as LICM.
18 // Loop exit-block insertion guarantees that all exit blocks from the loop
19 // (blocks which are outside of the loop that have predecessors inside of the
20 // loop) only have predecessors from inside of the loop (and are thus dominated
21 // by the loop header). This simplifies transformations such as store-sinking
22 // that are built into LICM.
24 // This pass also guarantees that loops will have exactly one backedge.
26 // Note that the simplifycfg pass will clean up blocks which are split out but
27 // end up being unnecessary, so usage of this pass should not pessimize
30 // This pass obviously modifies the CFG, but updates loop information and
31 // dominator information.
33 //===----------------------------------------------------------------------===//
35 #include "llvm/Transforms/Scalar.h"
36 #include "llvm/Analysis/Dominators.h"
37 #include "llvm/Analysis/LoopInfo.h"
38 #include "llvm/Function.h"
39 #include "llvm/iTerminators.h"
40 #include "llvm/iPHINode.h"
41 #include "llvm/Constant.h"
42 #include "llvm/Support/CFG.h"
43 #include "Support/SetOperations.h"
44 #include "Support/Statistic.h"
45 #include "Support/DepthFirstIterator.h"
50 NumInserted("loopsimplify", "Number of pre-header or exit blocks inserted");
52 struct LoopSimplify : public FunctionPass {
53 virtual bool runOnFunction(Function &F);
55 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56 // We need loop information to identify the loops...
57 AU.addRequired<LoopInfo>();
58 AU.addRequired<DominatorSet>();
60 AU.addPreserved<LoopInfo>();
61 AU.addPreserved<DominatorSet>();
62 AU.addPreserved<ImmediateDominators>();
63 AU.addPreserved<DominatorTree>();
64 AU.addPreserved<DominanceFrontier>();
65 AU.addPreservedID(BreakCriticalEdgesID); // No crit edges added....
68 bool ProcessLoop(Loop *L);
69 BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
70 const std::vector<BasicBlock*> &Preds);
71 void RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
72 void InsertPreheaderForLoop(Loop *L);
73 void InsertUniqueBackedgeBlock(Loop *L);
75 void UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB,
76 std::vector<BasicBlock*> &PredBlocks);
79 RegisterOpt<LoopSimplify>
80 X("loopsimplify", "Canonicalize natural loops", true);
83 // Publically exposed interface to pass...
84 const PassInfo *llvm::LoopSimplifyID = X.getPassInfo();
85 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
87 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
88 /// it in any convenient order) inserting preheaders...
90 bool LoopSimplify::runOnFunction(Function &F) {
92 LoopInfo &LI = getAnalysis<LoopInfo>();
94 for (unsigned i = 0, e = LI.getTopLevelLoops().size(); i != e; ++i)
95 Changed |= ProcessLoop(LI.getTopLevelLoops()[i]);
101 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
102 /// all loops have preheaders.
104 bool LoopSimplify::ProcessLoop(Loop *L) {
105 bool Changed = false;
107 // Does the loop already have a preheader? If so, don't modify the loop...
108 if (L->getLoopPreheader() == 0) {
109 InsertPreheaderForLoop(L);
114 // Next, check to make sure that all exit nodes of the loop only have
115 // predecessors that are inside of the loop. This check guarantees that the
116 // loop preheader/header will dominate the exit blocks. If the exit block has
117 // predecessors from outside of the loop, split the edge now.
118 for (unsigned i = 0, e = L->getExitBlocks().size(); i != e; ++i) {
119 BasicBlock *ExitBlock = L->getExitBlocks()[i];
120 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
122 if (!L->contains(*PI)) {
123 RewriteLoopExitBlock(L, ExitBlock);
130 // The preheader may have more than two predecessors at this point (from the
131 // preheader and from the backedges). To simplify the loop more, insert an
132 // extra back-edge block in the loop so that there is exactly one backedge.
133 if (L->getNumBackEdges() != 1) {
134 InsertUniqueBackedgeBlock(L);
139 const std::vector<Loop*> &SubLoops = L->getSubLoops();
140 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
141 Changed |= ProcessLoop(SubLoops[i]);
145 /// SplitBlockPredecessors - Split the specified block into two blocks. We want
146 /// to move the predecessors specified in the Preds list to point to the new
147 /// block, leaving the remaining predecessors pointing to BB. This method
148 /// updates the SSA PHINode's, but no other analyses.
150 BasicBlock *LoopSimplify::SplitBlockPredecessors(BasicBlock *BB,
152 const std::vector<BasicBlock*> &Preds) {
154 // Create new basic block, insert right before the original block...
155 BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB);
157 // The preheader first gets an unconditional branch to the loop header...
158 BranchInst *BI = new BranchInst(BB, NewBB);
160 // For every PHI node in the block, insert a PHI node into NewBB where the
161 // incoming values from the out of loop edges are moved to NewBB. We have two
162 // possible cases here. If the loop is dead, we just insert dummy entries
163 // into the PHI nodes for the new edge. If the loop is not dead, we move the
164 // incoming edges in BB into new PHI nodes in NewBB.
166 if (!Preds.empty()) { // Is the loop not obviously dead?
167 if (Preds.size() == 1) {
168 // No need to insert one operand PHI nodes! Instead, just update the
169 // incoming block ID's.
170 for (BasicBlock::iterator I = BB->begin();
171 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
172 unsigned i = PN->getBasicBlockIndex(Preds[0]);
173 PN->setIncomingBlock(i, NewBB);
176 for (BasicBlock::iterator I = BB->begin();
177 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
179 // Create the new PHI node, insert it into NewBB at the end of the block
180 PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
182 // Move all of the edges from blocks outside the loop to the new PHI
183 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
184 Value *V = PN->removeIncomingValue(Preds[i]);
185 NewPHI->addIncoming(V, Preds[i]);
188 // Add an incoming value to the PHI node in the loop for the preheader
190 PN->addIncoming(NewPHI, NewBB);
194 // Now that the PHI nodes are updated, actually move the edges from
195 // Preds to point to NewBB instead of BB.
197 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
198 TerminatorInst *TI = Preds[i]->getTerminator();
199 for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
200 if (TI->getSuccessor(s) == BB)
201 TI->setSuccessor(s, NewBB);
204 } else { // Otherwise the loop is dead...
205 for (BasicBlock::iterator I = BB->begin();
206 PHINode *PN = dyn_cast<PHINode>(I); ++I)
207 // Insert dummy values as the incoming value...
208 PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
213 // ChangeExitBlock - This recursive function is used to change any exit blocks
214 // that use OldExit to use NewExit instead. This is recursive because children
215 // may need to be processed as well.
217 static void ChangeExitBlock(Loop *L, BasicBlock *OldExit, BasicBlock *NewExit) {
218 if (L->hasExitBlock(OldExit)) {
219 L->changeExitBlock(OldExit, NewExit);
220 const std::vector<Loop*> &SubLoops = L->getSubLoops();
221 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
222 ChangeExitBlock(SubLoops[i], OldExit, NewExit);
227 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
228 /// preheader, this method is called to insert one. This method has two phases:
229 /// preheader insertion and analysis updating.
231 void LoopSimplify::InsertPreheaderForLoop(Loop *L) {
232 BasicBlock *Header = L->getHeader();
234 // Compute the set of predecessors of the loop that are not in the loop.
235 std::vector<BasicBlock*> OutsideBlocks;
236 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
238 if (!L->contains(*PI)) // Coming in from outside the loop?
239 OutsideBlocks.push_back(*PI); // Keep track of it...
241 // Split out the loop pre-header
243 SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
245 //===--------------------------------------------------------------------===//
246 // Update analysis results now that we have performed the transformation
249 // We know that we have loop information to update... update it now.
250 if (Loop *Parent = L->getParentLoop())
251 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
253 // If the header for the loop used to be an exit node for another loop, then
254 // we need to update this to know that the loop-preheader is now the exit
255 // node. Note that the only loop that could have our header as an exit node
256 // is a sibling loop, ie, one with the same parent loop, or one if it's
259 const std::vector<Loop*> *ParentSubLoops;
260 if (Loop *Parent = L->getParentLoop())
261 ParentSubLoops = &Parent->getSubLoops();
262 else // Must check top-level loops...
263 ParentSubLoops = &getAnalysis<LoopInfo>().getTopLevelLoops();
265 // Loop over all sibling loops, performing the substitution (recursively to
266 // include child loops)...
267 for (unsigned i = 0, e = ParentSubLoops->size(); i != e; ++i)
268 ChangeExitBlock((*ParentSubLoops)[i], Header, NewBB);
270 DominatorSet &DS = getAnalysis<DominatorSet>(); // Update dominator info
272 // The blocks that dominate NewBB are the blocks that dominate Header,
273 // minus Header, plus NewBB.
274 DominatorSet::DomSetType DomSet = DS.getDominators(Header);
275 DomSet.insert(NewBB); // We dominate ourself
276 DomSet.erase(Header); // Header does not dominate us...
277 DS.addBasicBlock(NewBB, DomSet);
279 // The newly created basic block dominates all nodes dominated by Header.
280 for (Function::iterator I = Header->getParent()->begin(),
281 E = Header->getParent()->end(); I != E; ++I)
282 if (DS.dominates(Header, I))
283 DS.addDominator(I, NewBB);
286 // Update immediate dominator information if we have it...
287 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
288 // Whatever i-dominated the header node now immediately dominates NewBB
289 ID->addNewBlock(NewBB, ID->get(Header));
291 // The preheader now is the immediate dominator for the header node...
292 ID->setImmediateDominator(Header, NewBB);
295 // Update DominatorTree information if it is active.
296 if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
297 // The immediate dominator of the preheader is the immediate dominator of
300 DominatorTree::Node *HeaderNode = DT->getNode(Header);
301 DominatorTree::Node *PHNode = DT->createNewNode(NewBB,
302 HeaderNode->getIDom());
304 // Change the header node so that PNHode is the new immediate dominator
305 DT->changeImmediateDominator(HeaderNode, PHNode);
308 // Update dominance frontier information...
309 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
310 // The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates
311 // everything that Header does, and it strictly dominates Header in
313 assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?");
314 DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second;
315 NewDFSet.erase(Header);
316 DF->addBasicBlock(NewBB, NewDFSet);
318 // Now we must loop over all of the dominance frontiers in the function,
319 // replacing occurrences of Header with NewBB in some cases. If a block
320 // dominates a (now) predecessor of NewBB, but did not strictly dominate
321 // Header, it will have Header in it's DF set, but should now have NewBB in
323 for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
324 // Get all of the dominators of the predecessor...
325 const DominatorSet::DomSetType &PredDoms =
326 DS.getDominators(OutsideBlocks[i]);
327 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
328 PDE = PredDoms.end(); PDI != PDE; ++PDI) {
329 BasicBlock *PredDom = *PDI;
330 // If the loop header is in DF(PredDom), then PredDom didn't dominate
331 // the header but did dominate a predecessor outside of the loop. Now
332 // we change this entry to include the preheader in the DF instead of
334 DominanceFrontier::iterator DFI = DF->find(PredDom);
335 assert(DFI != DF->end() && "No dominance frontier for node?");
336 if (DFI->second.count(Header)) {
337 DF->removeFromFrontier(DFI, Header);
338 DF->addToFrontier(DFI, NewBB);
345 void LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
346 DominatorSet &DS = getAnalysis<DominatorSet>();
347 assert(std::find(L->getExitBlocks().begin(), L->getExitBlocks().end(), Exit)
348 != L->getExitBlocks().end() && "Not a current exit block!");
350 std::vector<BasicBlock*> LoopBlocks;
351 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
353 LoopBlocks.push_back(*I);
355 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
356 BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);
358 // Update Loop Information - we know that the new block will be in the parent
360 if (Loop *Parent = L->getParentLoop())
361 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
363 // Replace any instances of Exit with NewBB in this and any nested loops...
364 for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
365 if (I->hasExitBlock(Exit))
366 I->changeExitBlock(Exit, NewBB); // Update exit block information
368 // Update dominator information (set, immdom, domtree, and domfrontier)
369 UpdateDomInfoForRevectoredPreds(NewBB, LoopBlocks);
372 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
373 /// has more than one backedge in it. If this occurs, revector all of these
374 /// backedges to target a new basic block and have that block branch to the loop
375 /// header. This ensures that loops have exactly one backedge.
377 void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
378 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
380 // Get information about the loop
381 BasicBlock *Preheader = L->getLoopPreheader();
382 BasicBlock *Header = L->getHeader();
383 Function *F = Header->getParent();
385 // Figure out which basic blocks contain back-edges to the loop header.
386 std::vector<BasicBlock*> BackedgeBlocks;
387 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
388 if (*I != Preheader) BackedgeBlocks.push_back(*I);
390 // Create and insert the new backedge block...
391 BasicBlock *BEBlock = new BasicBlock(Header->getName()+".backedge", F);
392 BranchInst *BETerminator = new BranchInst(Header, BEBlock);
394 // Move the new backedge block to right after the last backedge block.
395 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
396 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
398 // Now that the block has been inserted into the function, create PHI nodes in
399 // the backedge block which correspond to any PHI nodes in the header block.
400 for (BasicBlock::iterator I = Header->begin();
401 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
402 PHINode *NewPN = new PHINode(PN->getType(), PN->getName()+".be",
404 NewPN->op_reserve(2*BackedgeBlocks.size());
406 // Loop over the PHI node, moving all entries except the one for the
407 // preheader over to the new PHI node.
408 unsigned PreheaderIdx = ~0U;
409 bool HasUniqueIncomingValue = true;
410 Value *UniqueValue = 0;
411 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
412 BasicBlock *IBB = PN->getIncomingBlock(i);
413 Value *IV = PN->getIncomingValue(i);
414 if (IBB == Preheader) {
417 NewPN->addIncoming(IV, IBB);
418 if (HasUniqueIncomingValue) {
419 if (UniqueValue == 0)
421 else if (UniqueValue != IV)
422 HasUniqueIncomingValue = false;
427 // Delete all of the incoming values from the old PN except the preheader's
428 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
429 if (PreheaderIdx != 0) {
430 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
431 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
433 PN->op_erase(PN->op_begin()+2, PN->op_end());
435 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
436 PN->addIncoming(NewPN, BEBlock);
438 // As an optimization, if all incoming values in the new PhiNode (which is a
439 // subset of the incoming values of the old PHI node) have the same value,
440 // eliminate the PHI Node.
441 if (HasUniqueIncomingValue) {
442 NewPN->replaceAllUsesWith(UniqueValue);
443 BEBlock->getInstList().erase(NewPN);
447 // Now that all of the PHI nodes have been inserted and adjusted, modify the
448 // backedge blocks to just to the BEBlock instead of the header.
449 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
450 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
451 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
452 if (TI->getSuccessor(Op) == Header)
453 TI->setSuccessor(Op, BEBlock);
456 //===--- Update all analyses which we must preserve now -----------------===//
458 // Update Loop Information - we know that this block is now in the current
459 // loop and all parent loops.
460 L->addBasicBlockToLoop(BEBlock, getAnalysis<LoopInfo>());
462 // Replace any instances of Exit with NewBB in this and any nested loops...
463 for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
464 if (I->hasExitBlock(Header))
465 I->changeExitBlock(Header, BEBlock); // Update exit block information
467 // Update dominator information (set, immdom, domtree, and domfrontier)
468 UpdateDomInfoForRevectoredPreds(BEBlock, BackedgeBlocks);
471 /// UpdateDomInfoForRevectoredPreds - This method is used to update the four
472 /// different kinds of dominator information (dominator sets, immediate
473 /// dominators, dominator trees, and dominance frontiers) after a new block has
474 /// been added to the CFG.
476 /// This only supports the case when an existing block (known as "Exit"), had
477 /// some of its predecessors factored into a new basic block. This
478 /// transformation inserts a new basic block ("NewBB"), with a single
479 /// unconditional branch to Exit, and moves some predecessors of "Exit" to now
480 /// branch to NewBB. These predecessors are listed in PredBlocks, even though
481 /// they are the same as pred_begin(NewBB)/pred_end(NewBB).
483 void LoopSimplify::UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB,
484 std::vector<BasicBlock*> &PredBlocks) {
485 assert(succ_begin(NewBB) != succ_end(NewBB) &&
486 ++succ_begin(NewBB) == succ_end(NewBB) &&
487 "NewBB should have a single successor!");
488 DominatorSet &DS = getAnalysis<DominatorSet>();
490 // Update dominator information... The blocks that dominate NewBB are the
491 // intersection of the dominators of predecessors, plus the block itself.
492 // The newly created basic block does not dominate anything except itself.
494 DominatorSet::DomSetType NewBBDomSet = DS.getDominators(PredBlocks[0]);
495 for (unsigned i = 1, e = PredBlocks.size(); i != e; ++i)
496 set_intersect(NewBBDomSet, DS.getDominators(PredBlocks[i]));
497 NewBBDomSet.insert(NewBB); // All blocks dominate themselves...
498 DS.addBasicBlock(NewBB, NewBBDomSet);
500 // Update immediate dominator information if we have it...
501 BasicBlock *NewBBIDom = 0;
502 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
503 // This block does not strictly dominate anything, so it is not an immediate
504 // dominator. To find the immediate dominator of the new exit node, we
505 // trace up the immediate dominators of a predecessor until we find a basic
506 // block that dominates the exit block.
508 BasicBlock *Dom = PredBlocks[0]; // Some random predecessor...
509 while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator...
510 assert(Dom != 0 && "No shared dominator found???");
514 // Set the immediate dominator now...
515 ID->addNewBlock(NewBB, Dom);
516 NewBBIDom = Dom; // Reuse this if calculating DominatorTree info...
519 // Update DominatorTree information if it is active.
520 if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
521 // NewBB doesn't dominate anything, so just create a node and link it into
522 // its immediate dominator. If we don't have ImmediateDominator info
523 // around, calculate the idom as above.
524 DominatorTree::Node *NewBBIDomNode;
526 NewBBIDomNode = DT->getNode(NewBBIDom);
528 NewBBIDomNode = DT->getNode(PredBlocks[0]); // Random pred
529 while (!NewBBDomSet.count(NewBBIDomNode->getBlock())) {
530 NewBBIDomNode = NewBBIDomNode->getIDom();
531 assert(NewBBIDomNode && "No shared dominator found??");
535 // Create the new dominator tree node...
536 DT->createNewNode(NewBB, NewBBIDomNode);
539 // Update dominance frontier information...
540 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
541 // DF(NewBB) is {Exit} because NewBB does not strictly dominate Exit, but it
542 // does dominate itself (and there is an edge (NewBB -> Exit)). Exit is the
543 // single successor of NewBB.
544 DominanceFrontier::DomSetType NewDFSet;
545 BasicBlock *Exit = *succ_begin(NewBB);
546 NewDFSet.insert(Exit);
547 DF->addBasicBlock(NewBB, NewDFSet);
549 // Now we must loop over all of the dominance frontiers in the function,
550 // replacing occurrences of Exit with NewBB in some cases. All blocks that
551 // dominate a block in PredBlocks and contained Exit in their dominance
552 // frontier must be updated to contain NewBB instead. This only occurs if
553 // there is more than one block in PredBlocks.
555 if (PredBlocks.size() > 1) {
556 for (unsigned i = 0, e = PredBlocks.size(); i != e; ++i) {
557 BasicBlock *Pred = PredBlocks[i];
558 // Get all of the dominators of the predecessor...
559 const DominatorSet::DomSetType &PredDoms = DS.getDominators(Pred);
560 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
561 PDE = PredDoms.end(); PDI != PDE; ++PDI) {
562 BasicBlock *PredDom = *PDI;
564 // If the Exit node is in DF(PredDom), then PredDom didn't dominate
565 // Exit but did dominate a predecessor of it. Now we change this
566 // entry to include NewBB in the DF instead of Exit.
567 DominanceFrontier::iterator DFI = DF->find(PredDom);
568 assert(DFI != DF->end() && "No dominance frontier for node?");
569 if (DFI->second.count(Exit)) {
570 DF->removeFromFrontier(DFI, Exit);
571 DF->addToFrontier(DFI, NewBB);