1 //===-- StructurizeCFG.cpp ------------------------------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 #define DEBUG_TYPE "structurizecfg"
11 #include "llvm/Transforms/Scalar.h"
12 #include "llvm/ADT/MapVector.h"
13 #include "llvm/ADT/SCCIterator.h"
14 #include "llvm/Analysis/RegionInfo.h"
15 #include "llvm/Analysis/RegionIterator.h"
16 #include "llvm/Analysis/RegionPass.h"
17 #include "llvm/IR/Module.h"
18 #include "llvm/Support/PatternMatch.h"
19 #include "llvm/Transforms/Utils/SSAUpdater.h"
22 using namespace llvm::PatternMatch;
26 // Definition of the complex types used in this pass.
28 typedef std::pair<BasicBlock *, Value *> BBValuePair;
30 typedef SmallVector<RegionNode*, 8> RNVector;
31 typedef SmallVector<BasicBlock*, 8> BBVector;
32 typedef SmallVector<BranchInst*, 8> BranchVector;
33 typedef SmallVector<BBValuePair, 2> BBValueVector;
35 typedef SmallPtrSet<BasicBlock *, 8> BBSet;
37 typedef MapVector<PHINode *, BBValueVector> PhiMap;
38 typedef MapVector<BasicBlock *, BBVector> BB2BBVecMap;
40 typedef DenseMap<DomTreeNode *, unsigned> DTN2UnsignedMap;
41 typedef DenseMap<BasicBlock *, PhiMap> BBPhiMap;
42 typedef DenseMap<BasicBlock *, Value *> BBPredicates;
43 typedef DenseMap<BasicBlock *, BBPredicates> PredMap;
44 typedef DenseMap<BasicBlock *, BasicBlock*> BB2BBMap;
46 // The name for newly created blocks.
48 static const char *const FlowBlockName = "Flow";
50 /// @brief Find the nearest common dominator for multiple BasicBlocks
52 /// Helper class for StructurizeCFG
53 /// TODO: Maybe move into common code
54 class NearestCommonDominator {
57 DTN2UnsignedMap IndexMap;
61 bool ExplicitMentioned;
64 /// \brief Start a new query
65 NearestCommonDominator(DominatorTree *DomTree) {
70 /// \brief Add BB to the resulting dominator
71 void addBlock(BasicBlock *BB, bool Remember = true) {
72 DomTreeNode *Node = DT->getNode(BB);
75 unsigned Numbering = 0;
76 for (;Node;Node = Node->getIDom())
77 IndexMap[Node] = ++Numbering;
80 ExplicitMentioned = Remember;
84 for (;Node;Node = Node->getIDom())
85 if (IndexMap.count(Node))
90 assert(Node && "Dominator tree invalid!");
92 unsigned Numbering = IndexMap[Node];
93 if (Numbering > ResultIndex) {
94 Result = Node->getBlock();
95 ResultIndex = Numbering;
96 ExplicitMentioned = Remember && (Result == BB);
97 } else if (Numbering == ResultIndex) {
98 ExplicitMentioned |= Remember;
102 /// \brief Is "Result" one of the BBs added with "Remember" = True?
103 bool wasResultExplicitMentioned() {
104 return ExplicitMentioned;
107 /// \brief Get the query result
108 BasicBlock *getResult() {
113 /// @brief Transforms the control flow graph on one single entry/exit region
116 /// After the transform all "If"/"Then"/"Else" style control flow looks like
128 /// | | 1 = "If" block, calculates the condition
129 /// 4 | 2 = "Then" subregion, runs if the condition is true
130 /// | / 3 = "Flow" blocks, newly inserted flow blocks, rejoins the flow
131 /// |/ 4 = "Else" optional subregion, runs if the condition is false
132 /// 5 5 = "End" block, also rejoins the control flow
135 /// Control flow is expressed as a branch where the true exit goes into the
136 /// "Then"/"Else" region, while the false exit skips the region
137 /// The condition for the optional "Else" region is expressed as a PHI node.
138 /// The incomming values of the PHI node are true for the "If" edge and false
139 /// for the "Then" edge.
141 /// Additionally to that even complicated loops look like this:
148 /// | / 1 = "Entry" block
149 /// |/ 2 = "Loop" optional subregion, with all exits at "Flow" block
150 /// 3 3 = "Flow" block, with back edge to entry block
154 /// The back edge of the "Flow" block is always on the false side of the branch
155 /// while the true side continues the general flow. So the loop condition
156 /// consist of a network of PHI nodes where the true incoming values expresses
157 /// breaks and the false values expresses continue states.
158 class StructurizeCFG : public RegionPass {
160 ConstantInt *BoolTrue;
161 ConstantInt *BoolFalse;
162 UndefValue *BoolUndef;
165 Region *ParentRegion;
172 BBPhiMap DeletedPhis;
173 BB2BBVecMap AddedPhis;
176 BranchVector Conditions;
180 BranchVector LoopConds;
182 RegionNode *PrevNode;
186 void analyzeLoops(RegionNode *N);
188 Value *invert(Value *Condition);
190 Value *buildCondition(BranchInst *Term, unsigned Idx, bool Invert);
192 void gatherPredicates(RegionNode *N);
196 void insertConditions(bool Loops);
198 void delPhiValues(BasicBlock *From, BasicBlock *To);
200 void addPhiValues(BasicBlock *From, BasicBlock *To);
204 void killTerminator(BasicBlock *BB);
206 void changeExit(RegionNode *Node, BasicBlock *NewExit,
207 bool IncludeDominator);
209 BasicBlock *getNextFlow(BasicBlock *Dominator);
211 BasicBlock *needPrefix(bool NeedEmpty);
213 BasicBlock *needPostfix(BasicBlock *Flow, bool ExitUseAllowed);
215 void setPrevNode(BasicBlock *BB);
217 bool dominatesPredicates(BasicBlock *BB, RegionNode *Node);
219 bool isPredictableTrue(RegionNode *Node);
221 void wireFlow(bool ExitUseAllowed, BasicBlock *LoopEnd);
223 void handleLoops(bool ExitUseAllowed, BasicBlock *LoopEnd);
234 initializeRegionInfoPass(*PassRegistry::getPassRegistry());
237 using Pass::doInitialization;
238 virtual bool doInitialization(Region *R, RGPassManager &RGM);
240 virtual bool runOnRegion(Region *R, RGPassManager &RGM);
242 virtual const char *getPassName() const {
243 return "Structurize control flow";
246 void getAnalysisUsage(AnalysisUsage &AU) const {
247 AU.addRequired<DominatorTree>();
248 AU.addPreserved<DominatorTree>();
249 RegionPass::getAnalysisUsage(AU);
253 } // end anonymous namespace
255 char StructurizeCFG::ID = 0;
257 INITIALIZE_PASS_BEGIN(StructurizeCFG, "structurizecfg", "Structurize the CFG",
259 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
260 INITIALIZE_PASS_DEPENDENCY(RegionInfo)
261 INITIALIZE_PASS_END(StructurizeCFG, "structurizecfg", "Structurize the CFG",
264 /// \brief Initialize the types and constants used in the pass
265 bool StructurizeCFG::doInitialization(Region *R, RGPassManager &RGM) {
266 LLVMContext &Context = R->getEntry()->getContext();
268 Boolean = Type::getInt1Ty(Context);
269 BoolTrue = ConstantInt::getTrue(Context);
270 BoolFalse = ConstantInt::getFalse(Context);
271 BoolUndef = UndefValue::get(Boolean);
276 /// \brief Build up the general order of nodes
277 void StructurizeCFG::orderNodes() {
278 scc_iterator<Region *> I = scc_begin(ParentRegion),
279 E = scc_end(ParentRegion);
280 for (Order.clear(); I != E; ++I) {
281 std::vector<RegionNode *> &Nodes = *I;
282 Order.append(Nodes.begin(), Nodes.end());
286 /// \brief Determine the end of the loops
287 void StructurizeCFG::analyzeLoops(RegionNode *N) {
288 if (N->isSubRegion()) {
289 // Test for exit as back edge
290 BasicBlock *Exit = N->getNodeAs<Region>()->getExit();
291 if (Visited.count(Exit))
292 Loops[Exit] = N->getEntry();
295 // Test for sucessors as back edge
296 BasicBlock *BB = N->getNodeAs<BasicBlock>();
297 BranchInst *Term = cast<BranchInst>(BB->getTerminator());
299 for (unsigned i = 0, e = Term->getNumSuccessors(); i != e; ++i) {
300 BasicBlock *Succ = Term->getSuccessor(i);
302 if (Visited.count(Succ))
308 /// \brief Invert the given condition
309 Value *StructurizeCFG::invert(Value *Condition) {
310 // First: Check if it's a constant
311 if (Condition == BoolTrue)
314 if (Condition == BoolFalse)
317 if (Condition == BoolUndef)
320 // Second: If the condition is already inverted, return the original value
321 if (match(Condition, m_Not(m_Value(Condition))))
324 // Third: Check all the users for an invert
325 BasicBlock *Parent = cast<Instruction>(Condition)->getParent();
326 for (Value::use_iterator I = Condition->use_begin(),
327 E = Condition->use_end(); I != E; ++I) {
329 Instruction *User = dyn_cast<Instruction>(*I);
330 if (!User || User->getParent() != Parent)
333 if (match(*I, m_Not(m_Specific(Condition))))
337 // Last option: Create a new instruction
338 return BinaryOperator::CreateNot(Condition, "", Parent->getTerminator());
341 /// \brief Build the condition for one edge
342 Value *StructurizeCFG::buildCondition(BranchInst *Term, unsigned Idx,
344 Value *Cond = Invert ? BoolFalse : BoolTrue;
345 if (Term->isConditional()) {
346 Cond = Term->getCondition();
348 if (Idx != (unsigned)Invert)
354 /// \brief Analyze the predecessors of each block and build up predicates
355 void StructurizeCFG::gatherPredicates(RegionNode *N) {
356 RegionInfo *RI = ParentRegion->getRegionInfo();
357 BasicBlock *BB = N->getEntry();
358 BBPredicates &Pred = Predicates[BB];
359 BBPredicates &LPred = LoopPreds[BB];
361 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
364 // Ignore it if it's a branch from outside into our region entry
365 if (!ParentRegion->contains(*PI))
368 Region *R = RI->getRegionFor(*PI);
369 if (R == ParentRegion) {
371 // It's a top level block in our region
372 BranchInst *Term = cast<BranchInst>((*PI)->getTerminator());
373 for (unsigned i = 0, e = Term->getNumSuccessors(); i != e; ++i) {
374 BasicBlock *Succ = Term->getSuccessor(i);
378 if (Visited.count(*PI)) {
379 // Normal forward edge
380 if (Term->isConditional()) {
381 // Try to treat it like an ELSE block
382 BasicBlock *Other = Term->getSuccessor(!i);
383 if (Visited.count(Other) && !Loops.count(Other) &&
384 !Pred.count(Other) && !Pred.count(*PI)) {
386 Pred[Other] = BoolFalse;
387 Pred[*PI] = BoolTrue;
391 Pred[*PI] = buildCondition(Term, i, false);
395 LPred[*PI] = buildCondition(Term, i, true);
401 // It's an exit from a sub region
402 while(R->getParent() != ParentRegion)
405 // Edge from inside a subregion to its entry, ignore it
409 BasicBlock *Entry = R->getEntry();
410 if (Visited.count(Entry))
411 Pred[Entry] = BoolTrue;
413 LPred[Entry] = BoolFalse;
418 /// \brief Collect various loop and predicate infos
419 void StructurizeCFG::collectInfos() {
427 // Reset the visited nodes
430 for (RNVector::reverse_iterator OI = Order.rbegin(), OE = Order.rend();
433 // Analyze all the conditions leading to a node
434 gatherPredicates(*OI);
436 // Remember that we've seen this node
437 Visited.insert((*OI)->getEntry());
439 // Find the last back edges
444 /// \brief Insert the missing branch conditions
445 void StructurizeCFG::insertConditions(bool Loops) {
446 BranchVector &Conds = Loops ? LoopConds : Conditions;
447 Value *Default = Loops ? BoolTrue : BoolFalse;
448 SSAUpdater PhiInserter;
450 for (BranchVector::iterator I = Conds.begin(),
451 E = Conds.end(); I != E; ++I) {
453 BranchInst *Term = *I;
454 assert(Term->isConditional());
456 BasicBlock *Parent = Term->getParent();
457 BasicBlock *SuccTrue = Term->getSuccessor(0);
458 BasicBlock *SuccFalse = Term->getSuccessor(1);
460 PhiInserter.Initialize(Boolean, "");
461 PhiInserter.AddAvailableValue(&Func->getEntryBlock(), Default);
462 PhiInserter.AddAvailableValue(Loops ? SuccFalse : Parent, Default);
464 BBPredicates &Preds = Loops ? LoopPreds[SuccFalse] : Predicates[SuccTrue];
466 NearestCommonDominator Dominator(DT);
467 Dominator.addBlock(Parent, false);
469 Value *ParentValue = 0;
470 for (BBPredicates::iterator PI = Preds.begin(), PE = Preds.end();
473 if (PI->first == Parent) {
474 ParentValue = PI->second;
477 PhiInserter.AddAvailableValue(PI->first, PI->second);
478 Dominator.addBlock(PI->first);
482 Term->setCondition(ParentValue);
484 if (!Dominator.wasResultExplicitMentioned())
485 PhiInserter.AddAvailableValue(Dominator.getResult(), Default);
487 Term->setCondition(PhiInserter.GetValueInMiddleOfBlock(Parent));
492 /// \brief Remove all PHI values coming from "From" into "To" and remember
493 /// them in DeletedPhis
494 void StructurizeCFG::delPhiValues(BasicBlock *From, BasicBlock *To) {
495 PhiMap &Map = DeletedPhis[To];
496 for (BasicBlock::iterator I = To->begin(), E = To->end();
497 I != E && isa<PHINode>(*I);) {
499 PHINode &Phi = cast<PHINode>(*I++);
500 while (Phi.getBasicBlockIndex(From) != -1) {
501 Value *Deleted = Phi.removeIncomingValue(From, false);
502 Map[&Phi].push_back(std::make_pair(From, Deleted));
507 /// \brief Add a dummy PHI value as soon as we knew the new predecessor
508 void StructurizeCFG::addPhiValues(BasicBlock *From, BasicBlock *To) {
509 for (BasicBlock::iterator I = To->begin(), E = To->end();
510 I != E && isa<PHINode>(*I);) {
512 PHINode &Phi = cast<PHINode>(*I++);
513 Value *Undef = UndefValue::get(Phi.getType());
514 Phi.addIncoming(Undef, From);
516 AddedPhis[To].push_back(From);
519 /// \brief Add the real PHI value as soon as everything is set up
520 void StructurizeCFG::setPhiValues() {
522 for (BB2BBVecMap::iterator AI = AddedPhis.begin(), AE = AddedPhis.end();
525 BasicBlock *To = AI->first;
526 BBVector &From = AI->second;
528 if (!DeletedPhis.count(To))
531 PhiMap &Map = DeletedPhis[To];
532 for (PhiMap::iterator PI = Map.begin(), PE = Map.end();
535 PHINode *Phi = PI->first;
536 Value *Undef = UndefValue::get(Phi->getType());
537 Updater.Initialize(Phi->getType(), "");
538 Updater.AddAvailableValue(&Func->getEntryBlock(), Undef);
539 Updater.AddAvailableValue(To, Undef);
541 NearestCommonDominator Dominator(DT);
542 Dominator.addBlock(To, false);
543 for (BBValueVector::iterator VI = PI->second.begin(),
544 VE = PI->second.end(); VI != VE; ++VI) {
546 Updater.AddAvailableValue(VI->first, VI->second);
547 Dominator.addBlock(VI->first);
550 if (!Dominator.wasResultExplicitMentioned())
551 Updater.AddAvailableValue(Dominator.getResult(), Undef);
553 for (BBVector::iterator FI = From.begin(), FE = From.end();
556 int Idx = Phi->getBasicBlockIndex(*FI);
558 Phi->setIncomingValue(Idx, Updater.GetValueAtEndOfBlock(*FI));
562 DeletedPhis.erase(To);
564 assert(DeletedPhis.empty());
567 /// \brief Remove phi values from all successors and then remove the terminator.
568 void StructurizeCFG::killTerminator(BasicBlock *BB) {
569 TerminatorInst *Term = BB->getTerminator();
573 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
576 delPhiValues(BB, *SI);
579 Term->eraseFromParent();
582 /// \brief Let node exit(s) point to NewExit
583 void StructurizeCFG::changeExit(RegionNode *Node, BasicBlock *NewExit,
584 bool IncludeDominator) {
585 if (Node->isSubRegion()) {
586 Region *SubRegion = Node->getNodeAs<Region>();
587 BasicBlock *OldExit = SubRegion->getExit();
588 BasicBlock *Dominator = 0;
590 // Find all the edges from the sub region to the exit
591 for (pred_iterator I = pred_begin(OldExit), E = pred_end(OldExit);
594 BasicBlock *BB = *I++;
595 if (!SubRegion->contains(BB))
598 // Modify the edges to point to the new exit
599 delPhiValues(BB, OldExit);
600 BB->getTerminator()->replaceUsesOfWith(OldExit, NewExit);
601 addPhiValues(BB, NewExit);
603 // Find the new dominator (if requested)
604 if (IncludeDominator) {
608 Dominator = DT->findNearestCommonDominator(Dominator, BB);
612 // Change the dominator (if requested)
614 DT->changeImmediateDominator(NewExit, Dominator);
616 // Update the region info
617 SubRegion->replaceExit(NewExit);
620 BasicBlock *BB = Node->getNodeAs<BasicBlock>();
622 BranchInst::Create(NewExit, BB);
623 addPhiValues(BB, NewExit);
624 if (IncludeDominator)
625 DT->changeImmediateDominator(NewExit, BB);
629 /// \brief Create a new flow node and update dominator tree and region info
630 BasicBlock *StructurizeCFG::getNextFlow(BasicBlock *Dominator) {
631 LLVMContext &Context = Func->getContext();
632 BasicBlock *Insert = Order.empty() ? ParentRegion->getExit() :
633 Order.back()->getEntry();
634 BasicBlock *Flow = BasicBlock::Create(Context, FlowBlockName,
636 DT->addNewBlock(Flow, Dominator);
637 ParentRegion->getRegionInfo()->setRegionFor(Flow, ParentRegion);
641 /// \brief Create a new or reuse the previous node as flow node
642 BasicBlock *StructurizeCFG::needPrefix(bool NeedEmpty) {
643 BasicBlock *Entry = PrevNode->getEntry();
645 if (!PrevNode->isSubRegion()) {
646 killTerminator(Entry);
647 if (!NeedEmpty || Entry->getFirstInsertionPt() == Entry->end())
652 // create a new flow node
653 BasicBlock *Flow = getNextFlow(Entry);
656 changeExit(PrevNode, Flow, true);
657 PrevNode = ParentRegion->getBBNode(Flow);
661 /// \brief Returns the region exit if possible, otherwise just a new flow node
662 BasicBlock *StructurizeCFG::needPostfix(BasicBlock *Flow,
663 bool ExitUseAllowed) {
664 if (Order.empty() && ExitUseAllowed) {
665 BasicBlock *Exit = ParentRegion->getExit();
666 DT->changeImmediateDominator(Exit, Flow);
667 addPhiValues(Flow, Exit);
670 return getNextFlow(Flow);
673 /// \brief Set the previous node
674 void StructurizeCFG::setPrevNode(BasicBlock *BB) {
675 PrevNode = ParentRegion->contains(BB) ? ParentRegion->getBBNode(BB) : 0;
678 /// \brief Does BB dominate all the predicates of Node ?
679 bool StructurizeCFG::dominatesPredicates(BasicBlock *BB, RegionNode *Node) {
680 BBPredicates &Preds = Predicates[Node->getEntry()];
681 for (BBPredicates::iterator PI = Preds.begin(), PE = Preds.end();
684 if (!DT->dominates(BB, PI->first))
690 /// \brief Can we predict that this node will always be called?
691 bool StructurizeCFG::isPredictableTrue(RegionNode *Node) {
692 BBPredicates &Preds = Predicates[Node->getEntry()];
693 bool Dominated = false;
695 // Regionentry is always true
699 for (BBPredicates::iterator I = Preds.begin(), E = Preds.end();
702 if (I->second != BoolTrue)
705 if (!Dominated && DT->dominates(I->first, PrevNode->getEntry()))
709 // TODO: The dominator check is too strict
713 /// Take one node from the order vector and wire it up
714 void StructurizeCFG::wireFlow(bool ExitUseAllowed,
715 BasicBlock *LoopEnd) {
716 RegionNode *Node = Order.pop_back_val();
717 Visited.insert(Node->getEntry());
719 if (isPredictableTrue(Node)) {
720 // Just a linear flow
722 changeExit(PrevNode, Node->getEntry(), true);
727 // Insert extra prefix node (or reuse last one)
728 BasicBlock *Flow = needPrefix(false);
730 // Insert extra postfix node (or use exit instead)
731 BasicBlock *Entry = Node->getEntry();
732 BasicBlock *Next = needPostfix(Flow, ExitUseAllowed);
734 // let it point to entry and next block
735 Conditions.push_back(BranchInst::Create(Entry, Next, BoolUndef, Flow));
736 addPhiValues(Flow, Entry);
737 DT->changeImmediateDominator(Entry, Flow);
740 while (!Order.empty() && !Visited.count(LoopEnd) &&
741 dominatesPredicates(Entry, Order.back())) {
742 handleLoops(false, LoopEnd);
745 changeExit(PrevNode, Next, false);
750 void StructurizeCFG::handleLoops(bool ExitUseAllowed,
751 BasicBlock *LoopEnd) {
752 RegionNode *Node = Order.back();
753 BasicBlock *LoopStart = Node->getEntry();
755 if (!Loops.count(LoopStart)) {
756 wireFlow(ExitUseAllowed, LoopEnd);
760 if (!isPredictableTrue(Node))
761 LoopStart = needPrefix(true);
763 LoopEnd = Loops[Node->getEntry()];
764 wireFlow(false, LoopEnd);
765 while (!Visited.count(LoopEnd)) {
766 handleLoops(false, LoopEnd);
769 // Create an extra loop end node
770 LoopEnd = needPrefix(false);
771 BasicBlock *Next = needPostfix(LoopEnd, ExitUseAllowed);
772 LoopConds.push_back(BranchInst::Create(Next, LoopStart,
773 BoolUndef, LoopEnd));
774 addPhiValues(LoopEnd, LoopStart);
778 /// After this function control flow looks like it should be, but
779 /// branches and PHI nodes only have undefined conditions.
780 void StructurizeCFG::createFlow() {
781 BasicBlock *Exit = ParentRegion->getExit();
782 bool EntryDominatesExit = DT->dominates(ParentRegion->getEntry(), Exit);
792 while (!Order.empty()) {
793 handleLoops(EntryDominatesExit, 0);
797 changeExit(PrevNode, Exit, EntryDominatesExit);
799 assert(EntryDominatesExit);
802 /// Handle a rare case where the disintegrated nodes instructions
803 /// no longer dominate all their uses. Not sure if this is really nessasary
804 void StructurizeCFG::rebuildSSA() {
806 for (Region::block_iterator I = ParentRegion->block_begin(),
807 E = ParentRegion->block_end();
811 for (BasicBlock::iterator II = BB->begin(), IE = BB->end();
814 bool Initialized = false;
815 for (Use *I = &II->use_begin().getUse(), *Next; I; I = Next) {
819 Instruction *User = cast<Instruction>(I->getUser());
820 if (User->getParent() == BB) {
823 } else if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
824 if (UserPN->getIncomingBlock(*I) == BB)
828 if (DT->dominates(II, User))
832 Value *Undef = UndefValue::get(II->getType());
833 Updater.Initialize(II->getType(), "");
834 Updater.AddAvailableValue(&Func->getEntryBlock(), Undef);
835 Updater.AddAvailableValue(BB, II);
838 Updater.RewriteUseAfterInsertions(*I);
844 /// \brief Run the transformation for each region found
845 bool StructurizeCFG::runOnRegion(Region *R, RGPassManager &RGM) {
846 if (R->isTopLevelRegion())
849 Func = R->getEntry()->getParent();
852 DT = &getAnalysis<DominatorTree>();
857 insertConditions(false);
858 insertConditions(true);
876 /// \brief Create the pass
877 Pass *llvm::createStructurizeCFGPass() {
878 return new StructurizeCFG();