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/Function.h"
37 #include "llvm/iTerminators.h"
38 #include "llvm/iPHINode.h"
39 #include "llvm/Constant.h"
40 #include "llvm/Analysis/Dominators.h"
41 #include "llvm/Analysis/LoopInfo.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 // Check to see if the values being merged into the new block need PHI
168 // nodes. If so, insert them.
169 for (BasicBlock::iterator I = BB->begin();
170 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
172 // Check to see if all of the values coming in are the same. If so, we
173 // don't need to create a new PHI node.
174 Value *InVal = PN->getIncomingValueForBlock(Preds[0]);
175 for (unsigned i = 1, e = Preds.size(); i != e; ++i)
176 if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
181 // If the values coming into the block are not the same, we need a PHI.
183 // Create the new PHI node, insert it into NewBB at the end of the block
184 PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
186 // Move all of the edges from blocks outside the loop to the new PHI
187 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
188 Value *V = PN->removeIncomingValue(Preds[i]);
189 NewPHI->addIncoming(V, Preds[i]);
193 // Remove all of the edges coming into the PHI nodes from outside of the
195 for (unsigned i = 0, e = Preds.size(); i != e; ++i)
196 PN->removeIncomingValue(Preds[i], false);
199 // Add an incoming value to the PHI node in the loop for the preheader
201 PN->addIncoming(InVal, NewBB);
204 // Now that the PHI nodes are updated, actually move the edges from
205 // Preds to point to NewBB instead of BB.
207 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
208 TerminatorInst *TI = Preds[i]->getTerminator();
209 for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
210 if (TI->getSuccessor(s) == BB)
211 TI->setSuccessor(s, NewBB);
214 } else { // Otherwise the loop is dead...
215 for (BasicBlock::iterator I = BB->begin();
216 PHINode *PN = dyn_cast<PHINode>(I); ++I)
217 // Insert dummy values as the incoming value...
218 PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
223 // ChangeExitBlock - This recursive function is used to change any exit blocks
224 // that use OldExit to use NewExit instead. This is recursive because children
225 // may need to be processed as well.
227 static void ChangeExitBlock(Loop *L, BasicBlock *OldExit, BasicBlock *NewExit) {
228 if (L->hasExitBlock(OldExit)) {
229 L->changeExitBlock(OldExit, NewExit);
230 const std::vector<Loop*> &SubLoops = L->getSubLoops();
231 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
232 ChangeExitBlock(SubLoops[i], OldExit, NewExit);
237 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
238 /// preheader, this method is called to insert one. This method has two phases:
239 /// preheader insertion and analysis updating.
241 void LoopSimplify::InsertPreheaderForLoop(Loop *L) {
242 BasicBlock *Header = L->getHeader();
244 // Compute the set of predecessors of the loop that are not in the loop.
245 std::vector<BasicBlock*> OutsideBlocks;
246 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
248 if (!L->contains(*PI)) // Coming in from outside the loop?
249 OutsideBlocks.push_back(*PI); // Keep track of it...
251 // Split out the loop pre-header
253 SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
255 //===--------------------------------------------------------------------===//
256 // Update analysis results now that we have performed the transformation
259 // We know that we have loop information to update... update it now.
260 if (Loop *Parent = L->getParentLoop())
261 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
263 // If the header for the loop used to be an exit node for another loop, then
264 // we need to update this to know that the loop-preheader is now the exit
265 // node. Note that the only loop that could have our header as an exit node
266 // is a sibling loop, ie, one with the same parent loop, or one if it's
269 const std::vector<Loop*> *ParentSubLoops;
270 if (Loop *Parent = L->getParentLoop())
271 ParentSubLoops = &Parent->getSubLoops();
272 else // Must check top-level loops...
273 ParentSubLoops = &getAnalysis<LoopInfo>().getTopLevelLoops();
275 // Loop over all sibling loops, performing the substitution (recursively to
276 // include child loops)...
277 for (unsigned i = 0, e = ParentSubLoops->size(); i != e; ++i)
278 ChangeExitBlock((*ParentSubLoops)[i], Header, NewBB);
280 DominatorSet &DS = getAnalysis<DominatorSet>(); // Update dominator info
282 // The blocks that dominate NewBB are the blocks that dominate Header,
283 // minus Header, plus NewBB.
284 DominatorSet::DomSetType DomSet = DS.getDominators(Header);
285 DomSet.insert(NewBB); // We dominate ourself
286 DomSet.erase(Header); // Header does not dominate us...
287 DS.addBasicBlock(NewBB, DomSet);
289 // The newly created basic block dominates all nodes dominated by Header.
290 for (Function::iterator I = Header->getParent()->begin(),
291 E = Header->getParent()->end(); I != E; ++I)
292 if (DS.dominates(Header, I))
293 DS.addDominator(I, NewBB);
296 // Update immediate dominator information if we have it...
297 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
298 // Whatever i-dominated the header node now immediately dominates NewBB
299 ID->addNewBlock(NewBB, ID->get(Header));
301 // The preheader now is the immediate dominator for the header node...
302 ID->setImmediateDominator(Header, NewBB);
305 // Update DominatorTree information if it is active.
306 if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
307 // The immediate dominator of the preheader is the immediate dominator of
310 DominatorTree::Node *HeaderNode = DT->getNode(Header);
311 DominatorTree::Node *PHNode = DT->createNewNode(NewBB,
312 HeaderNode->getIDom());
314 // Change the header node so that PNHode is the new immediate dominator
315 DT->changeImmediateDominator(HeaderNode, PHNode);
318 // Update dominance frontier information...
319 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
320 // The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates
321 // everything that Header does, and it strictly dominates Header in
323 assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?");
324 DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second;
325 NewDFSet.erase(Header);
326 DF->addBasicBlock(NewBB, NewDFSet);
328 // Now we must loop over all of the dominance frontiers in the function,
329 // replacing occurrences of Header with NewBB in some cases. If a block
330 // dominates a (now) predecessor of NewBB, but did not strictly dominate
331 // Header, it will have Header in it's DF set, but should now have NewBB in
333 for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
334 // Get all of the dominators of the predecessor...
335 const DominatorSet::DomSetType &PredDoms =
336 DS.getDominators(OutsideBlocks[i]);
337 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
338 PDE = PredDoms.end(); PDI != PDE; ++PDI) {
339 BasicBlock *PredDom = *PDI;
340 // If the loop header is in DF(PredDom), then PredDom didn't dominate
341 // the header but did dominate a predecessor outside of the loop. Now
342 // we change this entry to include the preheader in the DF instead of
344 DominanceFrontier::iterator DFI = DF->find(PredDom);
345 assert(DFI != DF->end() && "No dominance frontier for node?");
346 if (DFI->second.count(Header)) {
347 DF->removeFromFrontier(DFI, Header);
348 DF->addToFrontier(DFI, NewBB);
355 void LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
356 DominatorSet &DS = getAnalysis<DominatorSet>();
357 assert(std::find(L->getExitBlocks().begin(), L->getExitBlocks().end(), Exit)
358 != L->getExitBlocks().end() && "Not a current exit block!");
360 std::vector<BasicBlock*> LoopBlocks;
361 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
363 LoopBlocks.push_back(*I);
365 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
366 BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);
368 // Update Loop Information - we know that the new block will be in the parent
370 if (Loop *Parent = L->getParentLoop())
371 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
373 // Replace any instances of Exit with NewBB in this and any nested loops...
374 for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
375 if (I->hasExitBlock(Exit))
376 I->changeExitBlock(Exit, NewBB); // Update exit block information
378 // Update dominator information (set, immdom, domtree, and domfrontier)
379 UpdateDomInfoForRevectoredPreds(NewBB, LoopBlocks);
382 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
383 /// has more than one backedge in it. If this occurs, revector all of these
384 /// backedges to target a new basic block and have that block branch to the loop
385 /// header. This ensures that loops have exactly one backedge.
387 void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
388 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
390 // Get information about the loop
391 BasicBlock *Preheader = L->getLoopPreheader();
392 BasicBlock *Header = L->getHeader();
393 Function *F = Header->getParent();
395 // Figure out which basic blocks contain back-edges to the loop header.
396 std::vector<BasicBlock*> BackedgeBlocks;
397 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
398 if (*I != Preheader) BackedgeBlocks.push_back(*I);
400 // Create and insert the new backedge block...
401 BasicBlock *BEBlock = new BasicBlock(Header->getName()+".backedge", F);
402 BranchInst *BETerminator = new BranchInst(Header, BEBlock);
404 // Move the new backedge block to right after the last backedge block.
405 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
406 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
408 // Now that the block has been inserted into the function, create PHI nodes in
409 // the backedge block which correspond to any PHI nodes in the header block.
410 for (BasicBlock::iterator I = Header->begin();
411 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
412 PHINode *NewPN = new PHINode(PN->getType(), PN->getName()+".be",
414 NewPN->op_reserve(2*BackedgeBlocks.size());
416 // Loop over the PHI node, moving all entries except the one for the
417 // preheader over to the new PHI node.
418 unsigned PreheaderIdx = ~0U;
419 bool HasUniqueIncomingValue = true;
420 Value *UniqueValue = 0;
421 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
422 BasicBlock *IBB = PN->getIncomingBlock(i);
423 Value *IV = PN->getIncomingValue(i);
424 if (IBB == Preheader) {
427 NewPN->addIncoming(IV, IBB);
428 if (HasUniqueIncomingValue) {
429 if (UniqueValue == 0)
431 else if (UniqueValue != IV)
432 HasUniqueIncomingValue = false;
437 // Delete all of the incoming values from the old PN except the preheader's
438 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
439 if (PreheaderIdx != 0) {
440 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
441 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
443 PN->op_erase(PN->op_begin()+2, PN->op_end());
445 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
446 PN->addIncoming(NewPN, BEBlock);
448 // As an optimization, if all incoming values in the new PhiNode (which is a
449 // subset of the incoming values of the old PHI node) have the same value,
450 // eliminate the PHI Node.
451 if (HasUniqueIncomingValue) {
452 NewPN->replaceAllUsesWith(UniqueValue);
453 BEBlock->getInstList().erase(NewPN);
457 // Now that all of the PHI nodes have been inserted and adjusted, modify the
458 // backedge blocks to just to the BEBlock instead of the header.
459 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
460 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
461 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
462 if (TI->getSuccessor(Op) == Header)
463 TI->setSuccessor(Op, BEBlock);
466 //===--- Update all analyses which we must preserve now -----------------===//
468 // Update Loop Information - we know that this block is now in the current
469 // loop and all parent loops.
470 L->addBasicBlockToLoop(BEBlock, getAnalysis<LoopInfo>());
472 // Replace any instances of Exit with NewBB in this and any nested loops...
473 for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
474 if (I->hasExitBlock(Header))
475 I->changeExitBlock(Header, BEBlock); // Update exit block information
477 // Update dominator information (set, immdom, domtree, and domfrontier)
478 UpdateDomInfoForRevectoredPreds(BEBlock, BackedgeBlocks);
481 /// UpdateDomInfoForRevectoredPreds - This method is used to update the four
482 /// different kinds of dominator information (dominator sets, immediate
483 /// dominators, dominator trees, and dominance frontiers) after a new block has
484 /// been added to the CFG.
486 /// This only supports the case when an existing block (known as "Exit"), had
487 /// some of its predecessors factored into a new basic block. This
488 /// transformation inserts a new basic block ("NewBB"), with a single
489 /// unconditional branch to Exit, and moves some predecessors of "Exit" to now
490 /// branch to NewBB. These predecessors are listed in PredBlocks, even though
491 /// they are the same as pred_begin(NewBB)/pred_end(NewBB).
493 void LoopSimplify::UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB,
494 std::vector<BasicBlock*> &PredBlocks) {
495 assert(succ_begin(NewBB) != succ_end(NewBB) &&
496 ++succ_begin(NewBB) == succ_end(NewBB) &&
497 "NewBB should have a single successor!");
498 DominatorSet &DS = getAnalysis<DominatorSet>();
500 // Update dominator information... The blocks that dominate NewBB are the
501 // intersection of the dominators of predecessors, plus the block itself.
502 // The newly created basic block does not dominate anything except itself.
504 DominatorSet::DomSetType NewBBDomSet = DS.getDominators(PredBlocks[0]);
505 for (unsigned i = 1, e = PredBlocks.size(); i != e; ++i)
506 set_intersect(NewBBDomSet, DS.getDominators(PredBlocks[i]));
507 NewBBDomSet.insert(NewBB); // All blocks dominate themselves...
508 DS.addBasicBlock(NewBB, NewBBDomSet);
510 // Update immediate dominator information if we have it...
511 BasicBlock *NewBBIDom = 0;
512 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
513 // This block does not strictly dominate anything, so it is not an immediate
514 // dominator. To find the immediate dominator of the new exit node, we
515 // trace up the immediate dominators of a predecessor until we find a basic
516 // block that dominates the exit block.
518 BasicBlock *Dom = PredBlocks[0]; // Some random predecessor...
519 while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator...
520 assert(Dom != 0 && "No shared dominator found???");
524 // Set the immediate dominator now...
525 ID->addNewBlock(NewBB, Dom);
526 NewBBIDom = Dom; // Reuse this if calculating DominatorTree info...
529 // Update DominatorTree information if it is active.
530 if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
531 // NewBB doesn't dominate anything, so just create a node and link it into
532 // its immediate dominator. If we don't have ImmediateDominator info
533 // around, calculate the idom as above.
534 DominatorTree::Node *NewBBIDomNode;
536 NewBBIDomNode = DT->getNode(NewBBIDom);
538 NewBBIDomNode = DT->getNode(PredBlocks[0]); // Random pred
539 while (!NewBBDomSet.count(NewBBIDomNode->getBlock())) {
540 NewBBIDomNode = NewBBIDomNode->getIDom();
541 assert(NewBBIDomNode && "No shared dominator found??");
545 // Create the new dominator tree node...
546 DT->createNewNode(NewBB, NewBBIDomNode);
549 // Update dominance frontier information...
550 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
551 // DF(NewBB) is {Exit} because NewBB does not strictly dominate Exit, but it
552 // does dominate itself (and there is an edge (NewBB -> Exit)). Exit is the
553 // single successor of NewBB.
554 DominanceFrontier::DomSetType NewDFSet;
555 BasicBlock *Exit = *succ_begin(NewBB);
556 NewDFSet.insert(Exit);
557 DF->addBasicBlock(NewBB, NewDFSet);
559 // Now we must loop over all of the dominance frontiers in the function,
560 // replacing occurrences of Exit with NewBB in some cases. All blocks that
561 // dominate a block in PredBlocks and contained Exit in their dominance
562 // frontier must be updated to contain NewBB instead. This only occurs if
563 // there is more than one block in PredBlocks.
565 if (PredBlocks.size() > 1) {
566 for (unsigned i = 0, e = PredBlocks.size(); i != e; ++i) {
567 BasicBlock *Pred = PredBlocks[i];
568 // Get all of the dominators of the predecessor...
569 const DominatorSet::DomSetType &PredDoms = DS.getDominators(Pred);
570 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
571 PDE = PredDoms.end(); PDI != PDE; ++PDI) {
572 BasicBlock *PredDom = *PDI;
574 // If the Exit node is in DF(PredDom), then PredDom didn't dominate
575 // Exit but did dominate a predecessor of it. Now we change this
576 // entry to include NewBB in the DF instead of Exit.
577 DominanceFrontier::iterator DFI = DF->find(PredDom);
578 assert(DFI != DF->end() && "No dominance frontier for node?");
579 if (DFI->second.count(Exit)) {
580 DF->removeFromFrontier(DFI, Exit);
581 DF->addToFrontier(DFI, NewBB);