xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp (revision 6966ac055c3b7a39266fb982493330df7a097997)
1 //===- BasicBlockUtils.cpp - BasicBlock Utilities --------------------------==//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This family of functions perform manipulations on basic blocks, and
10 // instructions contained within basic blocks.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
15 #include "llvm/ADT/ArrayRef.h"
16 #include "llvm/ADT/SmallPtrSet.h"
17 #include "llvm/ADT/SmallVector.h"
18 #include "llvm/ADT/Twine.h"
19 #include "llvm/Analysis/CFG.h"
20 #include "llvm/Analysis/DomTreeUpdater.h"
21 #include "llvm/Analysis/LoopInfo.h"
22 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
23 #include "llvm/Analysis/MemorySSAUpdater.h"
24 #include "llvm/Analysis/PostDominators.h"
25 #include "llvm/IR/BasicBlock.h"
26 #include "llvm/IR/CFG.h"
27 #include "llvm/IR/Constants.h"
28 #include "llvm/IR/DebugInfoMetadata.h"
29 #include "llvm/IR/Dominators.h"
30 #include "llvm/IR/Function.h"
31 #include "llvm/IR/InstrTypes.h"
32 #include "llvm/IR/Instruction.h"
33 #include "llvm/IR/Instructions.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/IR/LLVMContext.h"
36 #include "llvm/IR/Type.h"
37 #include "llvm/IR/User.h"
38 #include "llvm/IR/Value.h"
39 #include "llvm/IR/ValueHandle.h"
40 #include "llvm/Support/Casting.h"
41 #include "llvm/Support/Debug.h"
42 #include "llvm/Support/raw_ostream.h"
43 #include "llvm/Transforms/Utils/Local.h"
44 #include <cassert>
45 #include <cstdint>
46 #include <string>
47 #include <utility>
48 #include <vector>
49 
50 using namespace llvm;
51 
52 #define DEBUG_TYPE "basicblock-utils"
53 
54 void llvm::DetatchDeadBlocks(
55     ArrayRef<BasicBlock *> BBs,
56     SmallVectorImpl<DominatorTree::UpdateType> *Updates,
57     bool KeepOneInputPHIs) {
58   for (auto *BB : BBs) {
59     // Loop through all of our successors and make sure they know that one
60     // of their predecessors is going away.
61     SmallPtrSet<BasicBlock *, 4> UniqueSuccessors;
62     for (BasicBlock *Succ : successors(BB)) {
63       Succ->removePredecessor(BB, KeepOneInputPHIs);
64       if (Updates && UniqueSuccessors.insert(Succ).second)
65         Updates->push_back({DominatorTree::Delete, BB, Succ});
66     }
67 
68     // Zap all the instructions in the block.
69     while (!BB->empty()) {
70       Instruction &I = BB->back();
71       // If this instruction is used, replace uses with an arbitrary value.
72       // Because control flow can't get here, we don't care what we replace the
73       // value with.  Note that since this block is unreachable, and all values
74       // contained within it must dominate their uses, that all uses will
75       // eventually be removed (they are themselves dead).
76       if (!I.use_empty())
77         I.replaceAllUsesWith(UndefValue::get(I.getType()));
78       BB->getInstList().pop_back();
79     }
80     new UnreachableInst(BB->getContext(), BB);
81     assert(BB->getInstList().size() == 1 &&
82            isa<UnreachableInst>(BB->getTerminator()) &&
83            "The successor list of BB isn't empty before "
84            "applying corresponding DTU updates.");
85   }
86 }
87 
88 void llvm::DeleteDeadBlock(BasicBlock *BB, DomTreeUpdater *DTU,
89                            bool KeepOneInputPHIs) {
90   DeleteDeadBlocks({BB}, DTU, KeepOneInputPHIs);
91 }
92 
93 void llvm::DeleteDeadBlocks(ArrayRef <BasicBlock *> BBs, DomTreeUpdater *DTU,
94                             bool KeepOneInputPHIs) {
95 #ifndef NDEBUG
96   // Make sure that all predecessors of each dead block is also dead.
97   SmallPtrSet<BasicBlock *, 4> Dead(BBs.begin(), BBs.end());
98   assert(Dead.size() == BBs.size() && "Duplicating blocks?");
99   for (auto *BB : Dead)
100     for (BasicBlock *Pred : predecessors(BB))
101       assert(Dead.count(Pred) && "All predecessors must be dead!");
102 #endif
103 
104   SmallVector<DominatorTree::UpdateType, 4> Updates;
105   DetatchDeadBlocks(BBs, DTU ? &Updates : nullptr, KeepOneInputPHIs);
106 
107   if (DTU)
108     DTU->applyUpdatesPermissive(Updates);
109 
110   for (BasicBlock *BB : BBs)
111     if (DTU)
112       DTU->deleteBB(BB);
113     else
114       BB->eraseFromParent();
115 }
116 
117 bool llvm::EliminateUnreachableBlocks(Function &F, DomTreeUpdater *DTU,
118                                       bool KeepOneInputPHIs) {
119   df_iterator_default_set<BasicBlock*> Reachable;
120 
121   // Mark all reachable blocks.
122   for (BasicBlock *BB : depth_first_ext(&F, Reachable))
123     (void)BB/* Mark all reachable blocks */;
124 
125   // Collect all dead blocks.
126   std::vector<BasicBlock*> DeadBlocks;
127   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
128     if (!Reachable.count(&*I)) {
129       BasicBlock *BB = &*I;
130       DeadBlocks.push_back(BB);
131     }
132 
133   // Delete the dead blocks.
134   DeleteDeadBlocks(DeadBlocks, DTU, KeepOneInputPHIs);
135 
136   return !DeadBlocks.empty();
137 }
138 
139 void llvm::FoldSingleEntryPHINodes(BasicBlock *BB,
140                                    MemoryDependenceResults *MemDep) {
141   if (!isa<PHINode>(BB->begin())) return;
142 
143   while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
144     if (PN->getIncomingValue(0) != PN)
145       PN->replaceAllUsesWith(PN->getIncomingValue(0));
146     else
147       PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
148 
149     if (MemDep)
150       MemDep->removeInstruction(PN);  // Memdep updates AA itself.
151 
152     PN->eraseFromParent();
153   }
154 }
155 
156 bool llvm::DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI) {
157   // Recursively deleting a PHI may cause multiple PHIs to be deleted
158   // or RAUW'd undef, so use an array of WeakTrackingVH for the PHIs to delete.
159   SmallVector<WeakTrackingVH, 8> PHIs;
160   for (PHINode &PN : BB->phis())
161     PHIs.push_back(&PN);
162 
163   bool Changed = false;
164   for (unsigned i = 0, e = PHIs.size(); i != e; ++i)
165     if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*()))
166       Changed |= RecursivelyDeleteDeadPHINode(PN, TLI);
167 
168   return Changed;
169 }
170 
171 bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
172                                      LoopInfo *LI, MemorySSAUpdater *MSSAU,
173                                      MemoryDependenceResults *MemDep) {
174   if (BB->hasAddressTaken())
175     return false;
176 
177   // Can't merge if there are multiple predecessors, or no predecessors.
178   BasicBlock *PredBB = BB->getUniquePredecessor();
179   if (!PredBB) return false;
180 
181   // Don't break self-loops.
182   if (PredBB == BB) return false;
183   // Don't break unwinding instructions.
184   if (PredBB->getTerminator()->isExceptionalTerminator())
185     return false;
186 
187   // Can't merge if there are multiple distinct successors.
188   if (PredBB->getUniqueSuccessor() != BB)
189     return false;
190 
191   // Can't merge if there is PHI loop.
192   for (PHINode &PN : BB->phis())
193     for (Value *IncValue : PN.incoming_values())
194       if (IncValue == &PN)
195         return false;
196 
197   LLVM_DEBUG(dbgs() << "Merging: " << BB->getName() << " into "
198                     << PredBB->getName() << "\n");
199 
200   // Begin by getting rid of unneeded PHIs.
201   SmallVector<AssertingVH<Value>, 4> IncomingValues;
202   if (isa<PHINode>(BB->front())) {
203     for (PHINode &PN : BB->phis())
204       if (!isa<PHINode>(PN.getIncomingValue(0)) ||
205           cast<PHINode>(PN.getIncomingValue(0))->getParent() != BB)
206         IncomingValues.push_back(PN.getIncomingValue(0));
207     FoldSingleEntryPHINodes(BB, MemDep);
208   }
209 
210   // DTU update: Collect all the edges that exit BB.
211   // These dominator edges will be redirected from Pred.
212   std::vector<DominatorTree::UpdateType> Updates;
213   if (DTU) {
214     Updates.reserve(1 + (2 * succ_size(BB)));
215     // Add insert edges first. Experimentally, for the particular case of two
216     // blocks that can be merged, with a single successor and single predecessor
217     // respectively, it is beneficial to have all insert updates first. Deleting
218     // edges first may lead to unreachable blocks, followed by inserting edges
219     // making the blocks reachable again. Such DT updates lead to high compile
220     // times. We add inserts before deletes here to reduce compile time.
221     for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
222       // This successor of BB may already have PredBB as a predecessor.
223       if (llvm::find(successors(PredBB), *I) == succ_end(PredBB))
224         Updates.push_back({DominatorTree::Insert, PredBB, *I});
225     for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
226       Updates.push_back({DominatorTree::Delete, BB, *I});
227     Updates.push_back({DominatorTree::Delete, PredBB, BB});
228   }
229 
230   if (MSSAU)
231     MSSAU->moveAllAfterMergeBlocks(BB, PredBB, &*(BB->begin()));
232 
233   // Delete the unconditional branch from the predecessor...
234   PredBB->getInstList().pop_back();
235 
236   // Make all PHI nodes that referred to BB now refer to Pred as their
237   // source...
238   BB->replaceAllUsesWith(PredBB);
239 
240   // Move all definitions in the successor to the predecessor...
241   PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
242   new UnreachableInst(BB->getContext(), BB);
243 
244   // Eliminate duplicate dbg.values describing the entry PHI node post-splice.
245   for (auto Incoming : IncomingValues) {
246     if (isa<Instruction>(*Incoming)) {
247       SmallVector<DbgValueInst *, 2> DbgValues;
248       SmallDenseSet<std::pair<DILocalVariable *, DIExpression *>, 2>
249           DbgValueSet;
250       llvm::findDbgValues(DbgValues, Incoming);
251       for (auto &DVI : DbgValues) {
252         auto R = DbgValueSet.insert({DVI->getVariable(), DVI->getExpression()});
253         if (!R.second)
254           DVI->eraseFromParent();
255       }
256     }
257   }
258 
259   // Inherit predecessors name if it exists.
260   if (!PredBB->hasName())
261     PredBB->takeName(BB);
262 
263   if (LI)
264     LI->removeBlock(BB);
265 
266   if (MemDep)
267     MemDep->invalidateCachedPredecessors();
268 
269   // Finally, erase the old block and update dominator info.
270   if (DTU) {
271     assert(BB->getInstList().size() == 1 &&
272            isa<UnreachableInst>(BB->getTerminator()) &&
273            "The successor list of BB isn't empty before "
274            "applying corresponding DTU updates.");
275     DTU->applyUpdatesPermissive(Updates);
276     DTU->deleteBB(BB);
277   }
278 
279   else {
280     BB->eraseFromParent(); // Nuke BB if DTU is nullptr.
281   }
282   return true;
283 }
284 
285 void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL,
286                                 BasicBlock::iterator &BI, Value *V) {
287   Instruction &I = *BI;
288   // Replaces all of the uses of the instruction with uses of the value
289   I.replaceAllUsesWith(V);
290 
291   // Make sure to propagate a name if there is one already.
292   if (I.hasName() && !V->hasName())
293     V->takeName(&I);
294 
295   // Delete the unnecessary instruction now...
296   BI = BIL.erase(BI);
297 }
298 
299 void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL,
300                                BasicBlock::iterator &BI, Instruction *I) {
301   assert(I->getParent() == nullptr &&
302          "ReplaceInstWithInst: Instruction already inserted into basic block!");
303 
304   // Copy debug location to newly added instruction, if it wasn't already set
305   // by the caller.
306   if (!I->getDebugLoc())
307     I->setDebugLoc(BI->getDebugLoc());
308 
309   // Insert the new instruction into the basic block...
310   BasicBlock::iterator New = BIL.insert(BI, I);
311 
312   // Replace all uses of the old instruction, and delete it.
313   ReplaceInstWithValue(BIL, BI, I);
314 
315   // Move BI back to point to the newly inserted instruction
316   BI = New;
317 }
318 
319 void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
320   BasicBlock::iterator BI(From);
321   ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
322 }
323 
324 BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, DominatorTree *DT,
325                             LoopInfo *LI, MemorySSAUpdater *MSSAU) {
326   unsigned SuccNum = GetSuccessorNumber(BB, Succ);
327 
328   // If this is a critical edge, let SplitCriticalEdge do it.
329   Instruction *LatchTerm = BB->getTerminator();
330   if (SplitCriticalEdge(
331           LatchTerm, SuccNum,
332           CriticalEdgeSplittingOptions(DT, LI, MSSAU).setPreserveLCSSA()))
333     return LatchTerm->getSuccessor(SuccNum);
334 
335   // If the edge isn't critical, then BB has a single successor or Succ has a
336   // single pred.  Split the block.
337   if (BasicBlock *SP = Succ->getSinglePredecessor()) {
338     // If the successor only has a single pred, split the top of the successor
339     // block.
340     assert(SP == BB && "CFG broken");
341     SP = nullptr;
342     return SplitBlock(Succ, &Succ->front(), DT, LI, MSSAU);
343   }
344 
345   // Otherwise, if BB has a single successor, split it at the bottom of the
346   // block.
347   assert(BB->getTerminator()->getNumSuccessors() == 1 &&
348          "Should have a single succ!");
349   return SplitBlock(BB, BB->getTerminator(), DT, LI, MSSAU);
350 }
351 
352 unsigned
353 llvm::SplitAllCriticalEdges(Function &F,
354                             const CriticalEdgeSplittingOptions &Options) {
355   unsigned NumBroken = 0;
356   for (BasicBlock &BB : F) {
357     Instruction *TI = BB.getTerminator();
358     if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
359       for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
360         if (SplitCriticalEdge(TI, i, Options))
361           ++NumBroken;
362   }
363   return NumBroken;
364 }
365 
366 BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt,
367                              DominatorTree *DT, LoopInfo *LI,
368                              MemorySSAUpdater *MSSAU) {
369   BasicBlock::iterator SplitIt = SplitPt->getIterator();
370   while (isa<PHINode>(SplitIt) || SplitIt->isEHPad())
371     ++SplitIt;
372   BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
373 
374   // The new block lives in whichever loop the old one did. This preserves
375   // LCSSA as well, because we force the split point to be after any PHI nodes.
376   if (LI)
377     if (Loop *L = LI->getLoopFor(Old))
378       L->addBasicBlockToLoop(New, *LI);
379 
380   if (DT)
381     // Old dominates New. New node dominates all other nodes dominated by Old.
382     if (DomTreeNode *OldNode = DT->getNode(Old)) {
383       std::vector<DomTreeNode *> Children(OldNode->begin(), OldNode->end());
384 
385       DomTreeNode *NewNode = DT->addNewBlock(New, Old);
386       for (DomTreeNode *I : Children)
387         DT->changeImmediateDominator(I, NewNode);
388     }
389 
390   // Move MemoryAccesses still tracked in Old, but part of New now.
391   // Update accesses in successor blocks accordingly.
392   if (MSSAU)
393     MSSAU->moveAllAfterSpliceBlocks(Old, New, &*(New->begin()));
394 
395   return New;
396 }
397 
398 /// Update DominatorTree, LoopInfo, and LCCSA analysis information.
399 static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
400                                       ArrayRef<BasicBlock *> Preds,
401                                       DominatorTree *DT, LoopInfo *LI,
402                                       MemorySSAUpdater *MSSAU,
403                                       bool PreserveLCSSA, bool &HasLoopExit) {
404   // Update dominator tree if available.
405   if (DT) {
406     if (OldBB == DT->getRootNode()->getBlock()) {
407       assert(NewBB == &NewBB->getParent()->getEntryBlock());
408       DT->setNewRoot(NewBB);
409     } else {
410       // Split block expects NewBB to have a non-empty set of predecessors.
411       DT->splitBlock(NewBB);
412     }
413   }
414 
415   // Update MemoryPhis after split if MemorySSA is available
416   if (MSSAU)
417     MSSAU->wireOldPredecessorsToNewImmediatePredecessor(OldBB, NewBB, Preds);
418 
419   // The rest of the logic is only relevant for updating the loop structures.
420   if (!LI)
421     return;
422 
423   assert(DT && "DT should be available to update LoopInfo!");
424   Loop *L = LI->getLoopFor(OldBB);
425 
426   // If we need to preserve loop analyses, collect some information about how
427   // this split will affect loops.
428   bool IsLoopEntry = !!L;
429   bool SplitMakesNewLoopHeader = false;
430   for (BasicBlock *Pred : Preds) {
431     // Preds that are not reachable from entry should not be used to identify if
432     // OldBB is a loop entry or if SplitMakesNewLoopHeader. Unreachable blocks
433     // are not within any loops, so we incorrectly mark SplitMakesNewLoopHeader
434     // as true and make the NewBB the header of some loop. This breaks LI.
435     if (!DT->isReachableFromEntry(Pred))
436       continue;
437     // If we need to preserve LCSSA, determine if any of the preds is a loop
438     // exit.
439     if (PreserveLCSSA)
440       if (Loop *PL = LI->getLoopFor(Pred))
441         if (!PL->contains(OldBB))
442           HasLoopExit = true;
443 
444     // If we need to preserve LoopInfo, note whether any of the preds crosses
445     // an interesting loop boundary.
446     if (!L)
447       continue;
448     if (L->contains(Pred))
449       IsLoopEntry = false;
450     else
451       SplitMakesNewLoopHeader = true;
452   }
453 
454   // Unless we have a loop for OldBB, nothing else to do here.
455   if (!L)
456     return;
457 
458   if (IsLoopEntry) {
459     // Add the new block to the nearest enclosing loop (and not an adjacent
460     // loop). To find this, examine each of the predecessors and determine which
461     // loops enclose them, and select the most-nested loop which contains the
462     // loop containing the block being split.
463     Loop *InnermostPredLoop = nullptr;
464     for (BasicBlock *Pred : Preds) {
465       if (Loop *PredLoop = LI->getLoopFor(Pred)) {
466         // Seek a loop which actually contains the block being split (to avoid
467         // adjacent loops).
468         while (PredLoop && !PredLoop->contains(OldBB))
469           PredLoop = PredLoop->getParentLoop();
470 
471         // Select the most-nested of these loops which contains the block.
472         if (PredLoop && PredLoop->contains(OldBB) &&
473             (!InnermostPredLoop ||
474              InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth()))
475           InnermostPredLoop = PredLoop;
476       }
477     }
478 
479     if (InnermostPredLoop)
480       InnermostPredLoop->addBasicBlockToLoop(NewBB, *LI);
481   } else {
482     L->addBasicBlockToLoop(NewBB, *LI);
483     if (SplitMakesNewLoopHeader)
484       L->moveToHeader(NewBB);
485   }
486 }
487 
488 /// Update the PHI nodes in OrigBB to include the values coming from NewBB.
489 /// This also updates AliasAnalysis, if available.
490 static void UpdatePHINodes(BasicBlock *OrigBB, BasicBlock *NewBB,
491                            ArrayRef<BasicBlock *> Preds, BranchInst *BI,
492                            bool HasLoopExit) {
493   // Otherwise, create a new PHI node in NewBB for each PHI node in OrigBB.
494   SmallPtrSet<BasicBlock *, 16> PredSet(Preds.begin(), Preds.end());
495   for (BasicBlock::iterator I = OrigBB->begin(); isa<PHINode>(I); ) {
496     PHINode *PN = cast<PHINode>(I++);
497 
498     // Check to see if all of the values coming in are the same.  If so, we
499     // don't need to create a new PHI node, unless it's needed for LCSSA.
500     Value *InVal = nullptr;
501     if (!HasLoopExit) {
502       InVal = PN->getIncomingValueForBlock(Preds[0]);
503       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
504         if (!PredSet.count(PN->getIncomingBlock(i)))
505           continue;
506         if (!InVal)
507           InVal = PN->getIncomingValue(i);
508         else if (InVal != PN->getIncomingValue(i)) {
509           InVal = nullptr;
510           break;
511         }
512       }
513     }
514 
515     if (InVal) {
516       // If all incoming values for the new PHI would be the same, just don't
517       // make a new PHI.  Instead, just remove the incoming values from the old
518       // PHI.
519 
520       // NOTE! This loop walks backwards for a reason! First off, this minimizes
521       // the cost of removal if we end up removing a large number of values, and
522       // second off, this ensures that the indices for the incoming values
523       // aren't invalidated when we remove one.
524       for (int64_t i = PN->getNumIncomingValues() - 1; i >= 0; --i)
525         if (PredSet.count(PN->getIncomingBlock(i)))
526           PN->removeIncomingValue(i, false);
527 
528       // Add an incoming value to the PHI node in the loop for the preheader
529       // edge.
530       PN->addIncoming(InVal, NewBB);
531       continue;
532     }
533 
534     // If the values coming into the block are not the same, we need a new
535     // PHI.
536     // Create the new PHI node, insert it into NewBB at the end of the block
537     PHINode *NewPHI =
538         PHINode::Create(PN->getType(), Preds.size(), PN->getName() + ".ph", BI);
539 
540     // NOTE! This loop walks backwards for a reason! First off, this minimizes
541     // the cost of removal if we end up removing a large number of values, and
542     // second off, this ensures that the indices for the incoming values aren't
543     // invalidated when we remove one.
544     for (int64_t i = PN->getNumIncomingValues() - 1; i >= 0; --i) {
545       BasicBlock *IncomingBB = PN->getIncomingBlock(i);
546       if (PredSet.count(IncomingBB)) {
547         Value *V = PN->removeIncomingValue(i, false);
548         NewPHI->addIncoming(V, IncomingBB);
549       }
550     }
551 
552     PN->addIncoming(NewPHI, NewBB);
553   }
554 }
555 
556 BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
557                                          ArrayRef<BasicBlock *> Preds,
558                                          const char *Suffix, DominatorTree *DT,
559                                          LoopInfo *LI, MemorySSAUpdater *MSSAU,
560                                          bool PreserveLCSSA) {
561   // Do not attempt to split that which cannot be split.
562   if (!BB->canSplitPredecessors())
563     return nullptr;
564 
565   // For the landingpads we need to act a bit differently.
566   // Delegate this work to the SplitLandingPadPredecessors.
567   if (BB->isLandingPad()) {
568     SmallVector<BasicBlock*, 2> NewBBs;
569     std::string NewName = std::string(Suffix) + ".split-lp";
570 
571     SplitLandingPadPredecessors(BB, Preds, Suffix, NewName.c_str(), NewBBs, DT,
572                                 LI, MSSAU, PreserveLCSSA);
573     return NewBBs[0];
574   }
575 
576   // Create new basic block, insert right before the original block.
577   BasicBlock *NewBB = BasicBlock::Create(
578       BB->getContext(), BB->getName() + Suffix, BB->getParent(), BB);
579 
580   // The new block unconditionally branches to the old block.
581   BranchInst *BI = BranchInst::Create(BB, NewBB);
582   // Splitting the predecessors of a loop header creates a preheader block.
583   if (LI && LI->isLoopHeader(BB))
584     // Using the loop start line number prevents debuggers stepping into the
585     // loop body for this instruction.
586     BI->setDebugLoc(LI->getLoopFor(BB)->getStartLoc());
587   else
588     BI->setDebugLoc(BB->getFirstNonPHIOrDbg()->getDebugLoc());
589 
590   // Move the edges from Preds to point to NewBB instead of BB.
591   for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
592     // This is slightly more strict than necessary; the minimum requirement
593     // is that there be no more than one indirectbr branching to BB. And
594     // all BlockAddress uses would need to be updated.
595     assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
596            "Cannot split an edge from an IndirectBrInst");
597     assert(!isa<CallBrInst>(Preds[i]->getTerminator()) &&
598            "Cannot split an edge from a CallBrInst");
599     Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
600   }
601 
602   // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
603   // node becomes an incoming value for BB's phi node.  However, if the Preds
604   // list is empty, we need to insert dummy entries into the PHI nodes in BB to
605   // account for the newly created predecessor.
606   if (Preds.empty()) {
607     // Insert dummy values as the incoming value.
608     for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
609       cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
610   }
611 
612   // Update DominatorTree, LoopInfo, and LCCSA analysis information.
613   bool HasLoopExit = false;
614   UpdateAnalysisInformation(BB, NewBB, Preds, DT, LI, MSSAU, PreserveLCSSA,
615                             HasLoopExit);
616 
617   if (!Preds.empty()) {
618     // Update the PHI nodes in BB with the values coming from NewBB.
619     UpdatePHINodes(BB, NewBB, Preds, BI, HasLoopExit);
620   }
621 
622   return NewBB;
623 }
624 
625 void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
626                                        ArrayRef<BasicBlock *> Preds,
627                                        const char *Suffix1, const char *Suffix2,
628                                        SmallVectorImpl<BasicBlock *> &NewBBs,
629                                        DominatorTree *DT, LoopInfo *LI,
630                                        MemorySSAUpdater *MSSAU,
631                                        bool PreserveLCSSA) {
632   assert(OrigBB->isLandingPad() && "Trying to split a non-landing pad!");
633 
634   // Create a new basic block for OrigBB's predecessors listed in Preds. Insert
635   // it right before the original block.
636   BasicBlock *NewBB1 = BasicBlock::Create(OrigBB->getContext(),
637                                           OrigBB->getName() + Suffix1,
638                                           OrigBB->getParent(), OrigBB);
639   NewBBs.push_back(NewBB1);
640 
641   // The new block unconditionally branches to the old block.
642   BranchInst *BI1 = BranchInst::Create(OrigBB, NewBB1);
643   BI1->setDebugLoc(OrigBB->getFirstNonPHI()->getDebugLoc());
644 
645   // Move the edges from Preds to point to NewBB1 instead of OrigBB.
646   for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
647     // This is slightly more strict than necessary; the minimum requirement
648     // is that there be no more than one indirectbr branching to BB. And
649     // all BlockAddress uses would need to be updated.
650     assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
651            "Cannot split an edge from an IndirectBrInst");
652     Preds[i]->getTerminator()->replaceUsesOfWith(OrigBB, NewBB1);
653   }
654 
655   bool HasLoopExit = false;
656   UpdateAnalysisInformation(OrigBB, NewBB1, Preds, DT, LI, MSSAU, PreserveLCSSA,
657                             HasLoopExit);
658 
659   // Update the PHI nodes in OrigBB with the values coming from NewBB1.
660   UpdatePHINodes(OrigBB, NewBB1, Preds, BI1, HasLoopExit);
661 
662   // Move the remaining edges from OrigBB to point to NewBB2.
663   SmallVector<BasicBlock*, 8> NewBB2Preds;
664   for (pred_iterator i = pred_begin(OrigBB), e = pred_end(OrigBB);
665        i != e; ) {
666     BasicBlock *Pred = *i++;
667     if (Pred == NewBB1) continue;
668     assert(!isa<IndirectBrInst>(Pred->getTerminator()) &&
669            "Cannot split an edge from an IndirectBrInst");
670     NewBB2Preds.push_back(Pred);
671     e = pred_end(OrigBB);
672   }
673 
674   BasicBlock *NewBB2 = nullptr;
675   if (!NewBB2Preds.empty()) {
676     // Create another basic block for the rest of OrigBB's predecessors.
677     NewBB2 = BasicBlock::Create(OrigBB->getContext(),
678                                 OrigBB->getName() + Suffix2,
679                                 OrigBB->getParent(), OrigBB);
680     NewBBs.push_back(NewBB2);
681 
682     // The new block unconditionally branches to the old block.
683     BranchInst *BI2 = BranchInst::Create(OrigBB, NewBB2);
684     BI2->setDebugLoc(OrigBB->getFirstNonPHI()->getDebugLoc());
685 
686     // Move the remaining edges from OrigBB to point to NewBB2.
687     for (BasicBlock *NewBB2Pred : NewBB2Preds)
688       NewBB2Pred->getTerminator()->replaceUsesOfWith(OrigBB, NewBB2);
689 
690     // Update DominatorTree, LoopInfo, and LCCSA analysis information.
691     HasLoopExit = false;
692     UpdateAnalysisInformation(OrigBB, NewBB2, NewBB2Preds, DT, LI, MSSAU,
693                               PreserveLCSSA, HasLoopExit);
694 
695     // Update the PHI nodes in OrigBB with the values coming from NewBB2.
696     UpdatePHINodes(OrigBB, NewBB2, NewBB2Preds, BI2, HasLoopExit);
697   }
698 
699   LandingPadInst *LPad = OrigBB->getLandingPadInst();
700   Instruction *Clone1 = LPad->clone();
701   Clone1->setName(Twine("lpad") + Suffix1);
702   NewBB1->getInstList().insert(NewBB1->getFirstInsertionPt(), Clone1);
703 
704   if (NewBB2) {
705     Instruction *Clone2 = LPad->clone();
706     Clone2->setName(Twine("lpad") + Suffix2);
707     NewBB2->getInstList().insert(NewBB2->getFirstInsertionPt(), Clone2);
708 
709     // Create a PHI node for the two cloned landingpad instructions only
710     // if the original landingpad instruction has some uses.
711     if (!LPad->use_empty()) {
712       assert(!LPad->getType()->isTokenTy() &&
713              "Split cannot be applied if LPad is token type. Otherwise an "
714              "invalid PHINode of token type would be created.");
715       PHINode *PN = PHINode::Create(LPad->getType(), 2, "lpad.phi", LPad);
716       PN->addIncoming(Clone1, NewBB1);
717       PN->addIncoming(Clone2, NewBB2);
718       LPad->replaceAllUsesWith(PN);
719     }
720     LPad->eraseFromParent();
721   } else {
722     // There is no second clone. Just replace the landing pad with the first
723     // clone.
724     LPad->replaceAllUsesWith(Clone1);
725     LPad->eraseFromParent();
726   }
727 }
728 
729 ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
730                                              BasicBlock *Pred,
731                                              DomTreeUpdater *DTU) {
732   Instruction *UncondBranch = Pred->getTerminator();
733   // Clone the return and add it to the end of the predecessor.
734   Instruction *NewRet = RI->clone();
735   Pred->getInstList().push_back(NewRet);
736 
737   // If the return instruction returns a value, and if the value was a
738   // PHI node in "BB", propagate the right value into the return.
739   for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end();
740        i != e; ++i) {
741     Value *V = *i;
742     Instruction *NewBC = nullptr;
743     if (BitCastInst *BCI = dyn_cast<BitCastInst>(V)) {
744       // Return value might be bitcasted. Clone and insert it before the
745       // return instruction.
746       V = BCI->getOperand(0);
747       NewBC = BCI->clone();
748       Pred->getInstList().insert(NewRet->getIterator(), NewBC);
749       *i = NewBC;
750     }
751     if (PHINode *PN = dyn_cast<PHINode>(V)) {
752       if (PN->getParent() == BB) {
753         if (NewBC)
754           NewBC->setOperand(0, PN->getIncomingValueForBlock(Pred));
755         else
756           *i = PN->getIncomingValueForBlock(Pred);
757       }
758     }
759   }
760 
761   // Update any PHI nodes in the returning block to realize that we no
762   // longer branch to them.
763   BB->removePredecessor(Pred);
764   UncondBranch->eraseFromParent();
765 
766   if (DTU)
767     DTU->applyUpdates({{DominatorTree::Delete, Pred, BB}});
768 
769   return cast<ReturnInst>(NewRet);
770 }
771 
772 Instruction *llvm::SplitBlockAndInsertIfThen(Value *Cond,
773                                              Instruction *SplitBefore,
774                                              bool Unreachable,
775                                              MDNode *BranchWeights,
776                                              DominatorTree *DT, LoopInfo *LI,
777                                              BasicBlock *ThenBlock) {
778   BasicBlock *Head = SplitBefore->getParent();
779   BasicBlock *Tail = Head->splitBasicBlock(SplitBefore->getIterator());
780   Instruction *HeadOldTerm = Head->getTerminator();
781   LLVMContext &C = Head->getContext();
782   Instruction *CheckTerm;
783   bool CreateThenBlock = (ThenBlock == nullptr);
784   if (CreateThenBlock) {
785     ThenBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
786     if (Unreachable)
787       CheckTerm = new UnreachableInst(C, ThenBlock);
788     else
789       CheckTerm = BranchInst::Create(Tail, ThenBlock);
790     CheckTerm->setDebugLoc(SplitBefore->getDebugLoc());
791   } else
792     CheckTerm = ThenBlock->getTerminator();
793   BranchInst *HeadNewTerm =
794     BranchInst::Create(/*ifTrue*/ThenBlock, /*ifFalse*/Tail, Cond);
795   HeadNewTerm->setMetadata(LLVMContext::MD_prof, BranchWeights);
796   ReplaceInstWithInst(HeadOldTerm, HeadNewTerm);
797 
798   if (DT) {
799     if (DomTreeNode *OldNode = DT->getNode(Head)) {
800       std::vector<DomTreeNode *> Children(OldNode->begin(), OldNode->end());
801 
802       DomTreeNode *NewNode = DT->addNewBlock(Tail, Head);
803       for (DomTreeNode *Child : Children)
804         DT->changeImmediateDominator(Child, NewNode);
805 
806       // Head dominates ThenBlock.
807       if (CreateThenBlock)
808         DT->addNewBlock(ThenBlock, Head);
809       else
810         DT->changeImmediateDominator(ThenBlock, Head);
811     }
812   }
813 
814   if (LI) {
815     if (Loop *L = LI->getLoopFor(Head)) {
816       L->addBasicBlockToLoop(ThenBlock, *LI);
817       L->addBasicBlockToLoop(Tail, *LI);
818     }
819   }
820 
821   return CheckTerm;
822 }
823 
824 void llvm::SplitBlockAndInsertIfThenElse(Value *Cond, Instruction *SplitBefore,
825                                          Instruction **ThenTerm,
826                                          Instruction **ElseTerm,
827                                          MDNode *BranchWeights) {
828   BasicBlock *Head = SplitBefore->getParent();
829   BasicBlock *Tail = Head->splitBasicBlock(SplitBefore->getIterator());
830   Instruction *HeadOldTerm = Head->getTerminator();
831   LLVMContext &C = Head->getContext();
832   BasicBlock *ThenBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
833   BasicBlock *ElseBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
834   *ThenTerm = BranchInst::Create(Tail, ThenBlock);
835   (*ThenTerm)->setDebugLoc(SplitBefore->getDebugLoc());
836   *ElseTerm = BranchInst::Create(Tail, ElseBlock);
837   (*ElseTerm)->setDebugLoc(SplitBefore->getDebugLoc());
838   BranchInst *HeadNewTerm =
839     BranchInst::Create(/*ifTrue*/ThenBlock, /*ifFalse*/ElseBlock, Cond);
840   HeadNewTerm->setMetadata(LLVMContext::MD_prof, BranchWeights);
841   ReplaceInstWithInst(HeadOldTerm, HeadNewTerm);
842 }
843 
844 Value *llvm::GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
845                              BasicBlock *&IfFalse) {
846   PHINode *SomePHI = dyn_cast<PHINode>(BB->begin());
847   BasicBlock *Pred1 = nullptr;
848   BasicBlock *Pred2 = nullptr;
849 
850   if (SomePHI) {
851     if (SomePHI->getNumIncomingValues() != 2)
852       return nullptr;
853     Pred1 = SomePHI->getIncomingBlock(0);
854     Pred2 = SomePHI->getIncomingBlock(1);
855   } else {
856     pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
857     if (PI == PE) // No predecessor
858       return nullptr;
859     Pred1 = *PI++;
860     if (PI == PE) // Only one predecessor
861       return nullptr;
862     Pred2 = *PI++;
863     if (PI != PE) // More than two predecessors
864       return nullptr;
865   }
866 
867   // We can only handle branches.  Other control flow will be lowered to
868   // branches if possible anyway.
869   BranchInst *Pred1Br = dyn_cast<BranchInst>(Pred1->getTerminator());
870   BranchInst *Pred2Br = dyn_cast<BranchInst>(Pred2->getTerminator());
871   if (!Pred1Br || !Pred2Br)
872     return nullptr;
873 
874   // Eliminate code duplication by ensuring that Pred1Br is conditional if
875   // either are.
876   if (Pred2Br->isConditional()) {
877     // If both branches are conditional, we don't have an "if statement".  In
878     // reality, we could transform this case, but since the condition will be
879     // required anyway, we stand no chance of eliminating it, so the xform is
880     // probably not profitable.
881     if (Pred1Br->isConditional())
882       return nullptr;
883 
884     std::swap(Pred1, Pred2);
885     std::swap(Pred1Br, Pred2Br);
886   }
887 
888   if (Pred1Br->isConditional()) {
889     // The only thing we have to watch out for here is to make sure that Pred2
890     // doesn't have incoming edges from other blocks.  If it does, the condition
891     // doesn't dominate BB.
892     if (!Pred2->getSinglePredecessor())
893       return nullptr;
894 
895     // If we found a conditional branch predecessor, make sure that it branches
896     // to BB and Pred2Br.  If it doesn't, this isn't an "if statement".
897     if (Pred1Br->getSuccessor(0) == BB &&
898         Pred1Br->getSuccessor(1) == Pred2) {
899       IfTrue = Pred1;
900       IfFalse = Pred2;
901     } else if (Pred1Br->getSuccessor(0) == Pred2 &&
902                Pred1Br->getSuccessor(1) == BB) {
903       IfTrue = Pred2;
904       IfFalse = Pred1;
905     } else {
906       // We know that one arm of the conditional goes to BB, so the other must
907       // go somewhere unrelated, and this must not be an "if statement".
908       return nullptr;
909     }
910 
911     return Pred1Br->getCondition();
912   }
913 
914   // Ok, if we got here, both predecessors end with an unconditional branch to
915   // BB.  Don't panic!  If both blocks only have a single (identical)
916   // predecessor, and THAT is a conditional branch, then we're all ok!
917   BasicBlock *CommonPred = Pred1->getSinglePredecessor();
918   if (CommonPred == nullptr || CommonPred != Pred2->getSinglePredecessor())
919     return nullptr;
920 
921   // Otherwise, if this is a conditional branch, then we can use it!
922   BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator());
923   if (!BI) return nullptr;
924 
925   assert(BI->isConditional() && "Two successors but not conditional?");
926   if (BI->getSuccessor(0) == Pred1) {
927     IfTrue = Pred1;
928     IfFalse = Pred2;
929   } else {
930     IfTrue = Pred2;
931     IfFalse = Pred1;
932   }
933   return BI->getCondition();
934 }
935