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      1 //===-- BasicBlockUtils.cpp - BasicBlock Utilities -------------------------==//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This family of functions perform manipulations on basic blocks, and
     11 // instructions contained within basic blocks.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
     16 #include "llvm/Function.h"
     17 #include "llvm/Instructions.h"
     18 #include "llvm/IntrinsicInst.h"
     19 #include "llvm/Constant.h"
     20 #include "llvm/Type.h"
     21 #include "llvm/Analysis/AliasAnalysis.h"
     22 #include "llvm/Analysis/Dominators.h"
     23 #include "llvm/Analysis/LoopInfo.h"
     24 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
     25 #include "llvm/Target/TargetData.h"
     26 #include "llvm/Transforms/Utils/Local.h"
     27 #include "llvm/Transforms/Scalar.h"
     28 #include "llvm/Support/ErrorHandling.h"
     29 #include "llvm/Support/ValueHandle.h"
     30 #include <algorithm>
     31 using namespace llvm;
     32 
     33 /// DeleteDeadBlock - Delete the specified block, which must have no
     34 /// predecessors.
     35 void llvm::DeleteDeadBlock(BasicBlock *BB) {
     36   assert((pred_begin(BB) == pred_end(BB) ||
     37          // Can delete self loop.
     38          BB->getSinglePredecessor() == BB) && "Block is not dead!");
     39   TerminatorInst *BBTerm = BB->getTerminator();
     40 
     41   // Loop through all of our successors and make sure they know that one
     42   // of their predecessors is going away.
     43   for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i)
     44     BBTerm->getSuccessor(i)->removePredecessor(BB);
     45 
     46   // Zap all the instructions in the block.
     47   while (!BB->empty()) {
     48     Instruction &I = BB->back();
     49     // If this instruction is used, replace uses with an arbitrary value.
     50     // Because control flow can't get here, we don't care what we replace the
     51     // value with.  Note that since this block is unreachable, and all values
     52     // contained within it must dominate their uses, that all uses will
     53     // eventually be removed (they are themselves dead).
     54     if (!I.use_empty())
     55       I.replaceAllUsesWith(UndefValue::get(I.getType()));
     56     BB->getInstList().pop_back();
     57   }
     58 
     59   // Zap the block!
     60   BB->eraseFromParent();
     61 }
     62 
     63 /// FoldSingleEntryPHINodes - We know that BB has one predecessor.  If there are
     64 /// any single-entry PHI nodes in it, fold them away.  This handles the case
     65 /// when all entries to the PHI nodes in a block are guaranteed equal, such as
     66 /// when the block has exactly one predecessor.
     67 void llvm::FoldSingleEntryPHINodes(BasicBlock *BB, Pass *P) {
     68   if (!isa<PHINode>(BB->begin())) return;
     69 
     70   AliasAnalysis *AA = 0;
     71   MemoryDependenceAnalysis *MemDep = 0;
     72   if (P) {
     73     AA = P->getAnalysisIfAvailable<AliasAnalysis>();
     74     MemDep = P->getAnalysisIfAvailable<MemoryDependenceAnalysis>();
     75   }
     76 
     77   while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
     78     if (PN->getIncomingValue(0) != PN)
     79       PN->replaceAllUsesWith(PN->getIncomingValue(0));
     80     else
     81       PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
     82 
     83     if (MemDep)
     84       MemDep->removeInstruction(PN);  // Memdep updates AA itself.
     85     else if (AA && isa<PointerType>(PN->getType()))
     86       AA->deleteValue(PN);
     87 
     88     PN->eraseFromParent();
     89   }
     90 }
     91 
     92 
     93 /// DeleteDeadPHIs - Examine each PHI in the given block and delete it if it
     94 /// is dead. Also recursively delete any operands that become dead as
     95 /// a result. This includes tracing the def-use list from the PHI to see if
     96 /// it is ultimately unused or if it reaches an unused cycle.
     97 bool llvm::DeleteDeadPHIs(BasicBlock *BB) {
     98   // Recursively deleting a PHI may cause multiple PHIs to be deleted
     99   // or RAUW'd undef, so use an array of WeakVH for the PHIs to delete.
    100   SmallVector<WeakVH, 8> PHIs;
    101   for (BasicBlock::iterator I = BB->begin();
    102        PHINode *PN = dyn_cast<PHINode>(I); ++I)
    103     PHIs.push_back(PN);
    104 
    105   bool Changed = false;
    106   for (unsigned i = 0, e = PHIs.size(); i != e; ++i)
    107     if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*()))
    108       Changed |= RecursivelyDeleteDeadPHINode(PN);
    109 
    110   return Changed;
    111 }
    112 
    113 /// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor,
    114 /// if possible.  The return value indicates success or failure.
    115 bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, Pass *P) {
    116   // Don't merge away blocks who have their address taken.
    117   if (BB->hasAddressTaken()) return false;
    118 
    119   // Can't merge if there are multiple predecessors, or no predecessors.
    120   BasicBlock *PredBB = BB->getUniquePredecessor();
    121   if (!PredBB) return false;
    122 
    123   // Don't break self-loops.
    124   if (PredBB == BB) return false;
    125   // Don't break invokes.
    126   if (isa<InvokeInst>(PredBB->getTerminator())) return false;
    127 
    128   succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB));
    129   BasicBlock *OnlySucc = BB;
    130   for (; SI != SE; ++SI)
    131     if (*SI != OnlySucc) {
    132       OnlySucc = 0;     // There are multiple distinct successors!
    133       break;
    134     }
    135 
    136   // Can't merge if there are multiple successors.
    137   if (!OnlySucc) return false;
    138 
    139   // Can't merge if there is PHI loop.
    140   for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) {
    141     if (PHINode *PN = dyn_cast<PHINode>(BI)) {
    142       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
    143         if (PN->getIncomingValue(i) == PN)
    144           return false;
    145     } else
    146       break;
    147   }
    148 
    149   // Begin by getting rid of unneeded PHIs.
    150   if (isa<PHINode>(BB->front()))
    151     FoldSingleEntryPHINodes(BB, P);
    152 
    153   // Delete the unconditional branch from the predecessor...
    154   PredBB->getInstList().pop_back();
    155 
    156   // Make all PHI nodes that referred to BB now refer to Pred as their
    157   // source...
    158   BB->replaceAllUsesWith(PredBB);
    159 
    160   // Move all definitions in the successor to the predecessor...
    161   PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
    162 
    163   // Inherit predecessors name if it exists.
    164   if (!PredBB->hasName())
    165     PredBB->takeName(BB);
    166 
    167   // Finally, erase the old block and update dominator info.
    168   if (P) {
    169     if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
    170       if (DomTreeNode *DTN = DT->getNode(BB)) {
    171         DomTreeNode *PredDTN = DT->getNode(PredBB);
    172         SmallVector<DomTreeNode*, 8> Children(DTN->begin(), DTN->end());
    173         for (SmallVector<DomTreeNode*, 8>::iterator DI = Children.begin(),
    174              DE = Children.end(); DI != DE; ++DI)
    175           DT->changeImmediateDominator(*DI, PredDTN);
    176 
    177         DT->eraseNode(BB);
    178       }
    179 
    180       if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>())
    181         LI->removeBlock(BB);
    182 
    183       if (MemoryDependenceAnalysis *MD =
    184             P->getAnalysisIfAvailable<MemoryDependenceAnalysis>())
    185         MD->invalidateCachedPredecessors();
    186     }
    187   }
    188 
    189   BB->eraseFromParent();
    190   return true;
    191 }
    192 
    193 /// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI)
    194 /// with a value, then remove and delete the original instruction.
    195 ///
    196 void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL,
    197                                 BasicBlock::iterator &BI, Value *V) {
    198   Instruction &I = *BI;
    199   // Replaces all of the uses of the instruction with uses of the value
    200   I.replaceAllUsesWith(V);
    201 
    202   // Make sure to propagate a name if there is one already.
    203   if (I.hasName() && !V->hasName())
    204     V->takeName(&I);
    205 
    206   // Delete the unnecessary instruction now...
    207   BI = BIL.erase(BI);
    208 }
    209 
    210 
    211 /// ReplaceInstWithInst - Replace the instruction specified by BI with the
    212 /// instruction specified by I.  The original instruction is deleted and BI is
    213 /// updated to point to the new instruction.
    214 ///
    215 void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL,
    216                                BasicBlock::iterator &BI, Instruction *I) {
    217   assert(I->getParent() == 0 &&
    218          "ReplaceInstWithInst: Instruction already inserted into basic block!");
    219 
    220   // Insert the new instruction into the basic block...
    221   BasicBlock::iterator New = BIL.insert(BI, I);
    222 
    223   // Replace all uses of the old instruction, and delete it.
    224   ReplaceInstWithValue(BIL, BI, I);
    225 
    226   // Move BI back to point to the newly inserted instruction
    227   BI = New;
    228 }
    229 
    230 /// ReplaceInstWithInst - Replace the instruction specified by From with the
    231 /// instruction specified by To.
    232 ///
    233 void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
    234   BasicBlock::iterator BI(From);
    235   ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
    236 }
    237 
    238 /// GetSuccessorNumber - Search for the specified successor of basic block BB
    239 /// and return its position in the terminator instruction's list of
    240 /// successors.  It is an error to call this with a block that is not a
    241 /// successor.
    242 unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) {
    243   TerminatorInst *Term = BB->getTerminator();
    244 #ifndef NDEBUG
    245   unsigned e = Term->getNumSuccessors();
    246 #endif
    247   for (unsigned i = 0; ; ++i) {
    248     assert(i != e && "Didn't find edge?");
    249     if (Term->getSuccessor(i) == Succ)
    250       return i;
    251   }
    252 }
    253 
    254 /// SplitEdge -  Split the edge connecting specified block. Pass P must
    255 /// not be NULL.
    256 BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) {
    257   unsigned SuccNum = GetSuccessorNumber(BB, Succ);
    258 
    259   // If this is a critical edge, let SplitCriticalEdge do it.
    260   TerminatorInst *LatchTerm = BB->getTerminator();
    261   if (SplitCriticalEdge(LatchTerm, SuccNum, P))
    262     return LatchTerm->getSuccessor(SuccNum);
    263 
    264   // If the edge isn't critical, then BB has a single successor or Succ has a
    265   // single pred.  Split the block.
    266   BasicBlock::iterator SplitPoint;
    267   if (BasicBlock *SP = Succ->getSinglePredecessor()) {
    268     // If the successor only has a single pred, split the top of the successor
    269     // block.
    270     assert(SP == BB && "CFG broken");
    271     SP = NULL;
    272     return SplitBlock(Succ, Succ->begin(), P);
    273   }
    274 
    275   // Otherwise, if BB has a single successor, split it at the bottom of the
    276   // block.
    277   assert(BB->getTerminator()->getNumSuccessors() == 1 &&
    278          "Should have a single succ!");
    279   return SplitBlock(BB, BB->getTerminator(), P);
    280 }
    281 
    282 /// SplitBlock - Split the specified block at the specified instruction - every
    283 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
    284 /// to a new block.  The two blocks are joined by an unconditional branch and
    285 /// the loop info is updated.
    286 ///
    287 BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) {
    288   BasicBlock::iterator SplitIt = SplitPt;
    289   while (isa<PHINode>(SplitIt) || isa<LandingPadInst>(SplitIt))
    290     ++SplitIt;
    291   BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
    292 
    293   // The new block lives in whichever loop the old one did. This preserves
    294   // LCSSA as well, because we force the split point to be after any PHI nodes.
    295   if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>())
    296     if (Loop *L = LI->getLoopFor(Old))
    297       L->addBasicBlockToLoop(New, LI->getBase());
    298 
    299   if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
    300     // Old dominates New. New node dominates all other nodes dominated by Old.
    301     if (DomTreeNode *OldNode = DT->getNode(Old)) {
    302       std::vector<DomTreeNode *> Children;
    303       for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end();
    304            I != E; ++I)
    305         Children.push_back(*I);
    306 
    307       DomTreeNode *NewNode = DT->addNewBlock(New,Old);
    308       for (std::vector<DomTreeNode *>::iterator I = Children.begin(),
    309              E = Children.end(); I != E; ++I)
    310         DT->changeImmediateDominator(*I, NewNode);
    311     }
    312   }
    313 
    314   return New;
    315 }
    316 
    317 /// UpdateAnalysisInformation - Update DominatorTree, LoopInfo, and LCCSA
    318 /// analysis information.
    319 static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
    320                                       ArrayRef<BasicBlock *> Preds,
    321                                       Pass *P, bool &HasLoopExit) {
    322   if (!P) return;
    323 
    324   LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
    325   Loop *L = LI ? LI->getLoopFor(OldBB) : 0;
    326 
    327   // If we need to preserve loop analyses, collect some information about how
    328   // this split will affect loops.
    329   bool IsLoopEntry = !!L;
    330   bool SplitMakesNewLoopHeader = false;
    331   if (LI) {
    332     bool PreserveLCSSA = P->mustPreserveAnalysisID(LCSSAID);
    333     for (ArrayRef<BasicBlock*>::iterator
    334            i = Preds.begin(), e = Preds.end(); i != e; ++i) {
    335       BasicBlock *Pred = *i;
    336 
    337       // If we need to preserve LCSSA, determine if any of the preds is a loop
    338       // exit.
    339       if (PreserveLCSSA)
    340         if (Loop *PL = LI->getLoopFor(Pred))
    341           if (!PL->contains(OldBB))
    342             HasLoopExit = true;
    343 
    344       // If we need to preserve LoopInfo, note whether any of the preds crosses
    345       // an interesting loop boundary.
    346       if (!L) continue;
    347       if (L->contains(Pred))
    348         IsLoopEntry = false;
    349       else
    350         SplitMakesNewLoopHeader = true;
    351     }
    352   }
    353 
    354   // Update dominator tree if available.
    355   DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
    356   if (DT)
    357     DT->splitBlock(NewBB);
    358 
    359   if (!L) return;
    360 
    361   if (IsLoopEntry) {
    362     // Add the new block to the nearest enclosing loop (and not an adjacent
    363     // loop). To find this, examine each of the predecessors and determine which
    364     // loops enclose them, and select the most-nested loop which contains the
    365     // loop containing the block being split.
    366     Loop *InnermostPredLoop = 0;
    367     for (ArrayRef<BasicBlock*>::iterator
    368            i = Preds.begin(), e = Preds.end(); i != e; ++i) {
    369       BasicBlock *Pred = *i;
    370       if (Loop *PredLoop = LI->getLoopFor(Pred)) {
    371         // Seek a loop which actually contains the block being split (to avoid
    372         // adjacent loops).
    373         while (PredLoop && !PredLoop->contains(OldBB))
    374           PredLoop = PredLoop->getParentLoop();
    375 
    376         // Select the most-nested of these loops which contains the block.
    377         if (PredLoop && PredLoop->contains(OldBB) &&
    378             (!InnermostPredLoop ||
    379              InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth()))
    380           InnermostPredLoop = PredLoop;
    381       }
    382     }
    383 
    384     if (InnermostPredLoop)
    385       InnermostPredLoop->addBasicBlockToLoop(NewBB, LI->getBase());
    386   } else {
    387     L->addBasicBlockToLoop(NewBB, LI->getBase());
    388     if (SplitMakesNewLoopHeader)
    389       L->moveToHeader(NewBB);
    390   }
    391 }
    392 
    393 /// UpdatePHINodes - Update the PHI nodes in OrigBB to include the values coming
    394 /// from NewBB. This also updates AliasAnalysis, if available.
    395 static void UpdatePHINodes(BasicBlock *OrigBB, BasicBlock *NewBB,
    396                            ArrayRef<BasicBlock*> Preds, BranchInst *BI,
    397                            Pass *P, bool HasLoopExit) {
    398   // Otherwise, create a new PHI node in NewBB for each PHI node in OrigBB.
    399   AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0;
    400   for (BasicBlock::iterator I = OrigBB->begin(); isa<PHINode>(I); ) {
    401     PHINode *PN = cast<PHINode>(I++);
    402 
    403     // Check to see if all of the values coming in are the same.  If so, we
    404     // don't need to create a new PHI node, unless it's needed for LCSSA.
    405     Value *InVal = 0;
    406     if (!HasLoopExit) {
    407       InVal = PN->getIncomingValueForBlock(Preds[0]);
    408       for (unsigned i = 1, e = Preds.size(); i != e; ++i)
    409         if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
    410           InVal = 0;
    411           break;
    412         }
    413     }
    414 
    415     if (InVal) {
    416       // If all incoming values for the new PHI would be the same, just don't
    417       // make a new PHI.  Instead, just remove the incoming values from the old
    418       // PHI.
    419       for (unsigned i = 0, e = Preds.size(); i != e; ++i)
    420         PN->removeIncomingValue(Preds[i], false);
    421     } else {
    422       // If the values coming into the block are not the same, we need a PHI.
    423       // Create the new PHI node, insert it into NewBB at the end of the block
    424       PHINode *NewPHI =
    425         PHINode::Create(PN->getType(), Preds.size(), PN->getName() + ".ph", BI);
    426       if (AA) AA->copyValue(PN, NewPHI);
    427 
    428       // Move all of the PHI values for 'Preds' to the new PHI.
    429       for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
    430         Value *V = PN->removeIncomingValue(Preds[i], false);
    431         NewPHI->addIncoming(V, Preds[i]);
    432       }
    433 
    434       InVal = NewPHI;
    435     }
    436 
    437     // Add an incoming value to the PHI node in the loop for the preheader
    438     // edge.
    439     PN->addIncoming(InVal, NewBB);
    440   }
    441 }
    442 
    443 /// SplitBlockPredecessors - This method transforms BB by introducing a new
    444 /// basic block into the function, and moving some of the predecessors of BB to
    445 /// be predecessors of the new block.  The new predecessors are indicated by the
    446 /// Preds array, which has NumPreds elements in it.  The new block is given a
    447 /// suffix of 'Suffix'.
    448 ///
    449 /// This currently updates the LLVM IR, AliasAnalysis, DominatorTree,
    450 /// LoopInfo, and LCCSA but no other analyses. In particular, it does not
    451 /// preserve LoopSimplify (because it's complicated to handle the case where one
    452 /// of the edges being split is an exit of a loop with other exits).
    453 ///
    454 BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
    455                                          ArrayRef<BasicBlock*> Preds,
    456                                          const char *Suffix, Pass *P) {
    457   // Create new basic block, insert right before the original block.
    458   BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), BB->getName()+Suffix,
    459                                          BB->getParent(), BB);
    460 
    461   // The new block unconditionally branches to the old block.
    462   BranchInst *BI = BranchInst::Create(BB, NewBB);
    463 
    464   // Move the edges from Preds to point to NewBB instead of BB.
    465   for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
    466     // This is slightly more strict than necessary; the minimum requirement
    467     // is that there be no more than one indirectbr branching to BB. And
    468     // all BlockAddress uses would need to be updated.
    469     assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
    470            "Cannot split an edge from an IndirectBrInst");
    471     Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
    472   }
    473 
    474   // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
    475   // node becomes an incoming value for BB's phi node.  However, if the Preds
    476   // list is empty, we need to insert dummy entries into the PHI nodes in BB to
    477   // account for the newly created predecessor.
    478   if (Preds.size() == 0) {
    479     // Insert dummy values as the incoming value.
    480     for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
    481       cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
    482     return NewBB;
    483   }
    484 
    485   // Update DominatorTree, LoopInfo, and LCCSA analysis information.
    486   bool HasLoopExit = false;
    487   UpdateAnalysisInformation(BB, NewBB, Preds, P, HasLoopExit);
    488 
    489   // Update the PHI nodes in BB with the values coming from NewBB.
    490   UpdatePHINodes(BB, NewBB, Preds, BI, P, HasLoopExit);
    491   return NewBB;
    492 }
    493 
    494 /// SplitLandingPadPredecessors - This method transforms the landing pad,
    495 /// OrigBB, by introducing two new basic blocks into the function. One of those
    496 /// new basic blocks gets the predecessors listed in Preds. The other basic
    497 /// block gets the remaining predecessors of OrigBB. The landingpad instruction
    498 /// OrigBB is clone into both of the new basic blocks. The new blocks are given
    499 /// the suffixes 'Suffix1' and 'Suffix2', and are returned in the NewBBs vector.
    500 ///
    501 /// This currently updates the LLVM IR, AliasAnalysis, DominatorTree,
    502 /// DominanceFrontier, LoopInfo, and LCCSA but no other analyses. In particular,
    503 /// it does not preserve LoopSimplify (because it's complicated to handle the
    504 /// case where one of the edges being split is an exit of a loop with other
    505 /// exits).
    506 ///
    507 void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
    508                                        ArrayRef<BasicBlock*> Preds,
    509                                        const char *Suffix1, const char *Suffix2,
    510                                        Pass *P,
    511                                        SmallVectorImpl<BasicBlock*> &NewBBs) {
    512   assert(OrigBB->isLandingPad() && "Trying to split a non-landing pad!");
    513 
    514   // Create a new basic block for OrigBB's predecessors listed in Preds. Insert
    515   // it right before the original block.
    516   BasicBlock *NewBB1 = BasicBlock::Create(OrigBB->getContext(),
    517                                           OrigBB->getName() + Suffix1,
    518                                           OrigBB->getParent(), OrigBB);
    519   NewBBs.push_back(NewBB1);
    520 
    521   // The new block unconditionally branches to the old block.
    522   BranchInst *BI1 = BranchInst::Create(OrigBB, NewBB1);
    523 
    524   // Move the edges from Preds to point to NewBB1 instead of OrigBB.
    525   for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
    526     // This is slightly more strict than necessary; the minimum requirement
    527     // is that there be no more than one indirectbr branching to BB. And
    528     // all BlockAddress uses would need to be updated.
    529     assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
    530            "Cannot split an edge from an IndirectBrInst");
    531     Preds[i]->getTerminator()->replaceUsesOfWith(OrigBB, NewBB1);
    532   }
    533 
    534   // Update DominatorTree, LoopInfo, and LCCSA analysis information.
    535   bool HasLoopExit = false;
    536   UpdateAnalysisInformation(OrigBB, NewBB1, Preds, P, HasLoopExit);
    537 
    538   // Update the PHI nodes in OrigBB with the values coming from NewBB1.
    539   UpdatePHINodes(OrigBB, NewBB1, Preds, BI1, P, HasLoopExit);
    540 
    541   // Move the remaining edges from OrigBB to point to NewBB2.
    542   SmallVector<BasicBlock*, 8> NewBB2Preds;
    543   for (pred_iterator i = pred_begin(OrigBB), e = pred_end(OrigBB);
    544        i != e; ) {
    545     BasicBlock *Pred = *i++;
    546     if (Pred == NewBB1) continue;
    547     assert(!isa<IndirectBrInst>(Pred->getTerminator()) &&
    548            "Cannot split an edge from an IndirectBrInst");
    549     NewBB2Preds.push_back(Pred);
    550     e = pred_end(OrigBB);
    551   }
    552 
    553   BasicBlock *NewBB2 = 0;
    554   if (!NewBB2Preds.empty()) {
    555     // Create another basic block for the rest of OrigBB's predecessors.
    556     NewBB2 = BasicBlock::Create(OrigBB->getContext(),
    557                                 OrigBB->getName() + Suffix2,
    558                                 OrigBB->getParent(), OrigBB);
    559     NewBBs.push_back(NewBB2);
    560 
    561     // The new block unconditionally branches to the old block.
    562     BranchInst *BI2 = BranchInst::Create(OrigBB, NewBB2);
    563 
    564     // Move the remaining edges from OrigBB to point to NewBB2.
    565     for (SmallVectorImpl<BasicBlock*>::iterator
    566            i = NewBB2Preds.begin(), e = NewBB2Preds.end(); i != e; ++i)
    567       (*i)->getTerminator()->replaceUsesOfWith(OrigBB, NewBB2);
    568 
    569     // Update DominatorTree, LoopInfo, and LCCSA analysis information.
    570     HasLoopExit = false;
    571     UpdateAnalysisInformation(OrigBB, NewBB2, NewBB2Preds, P, HasLoopExit);
    572 
    573     // Update the PHI nodes in OrigBB with the values coming from NewBB2.
    574     UpdatePHINodes(OrigBB, NewBB2, NewBB2Preds, BI2, P, HasLoopExit);
    575   }
    576 
    577   LandingPadInst *LPad = OrigBB->getLandingPadInst();
    578   Instruction *Clone1 = LPad->clone();
    579   Clone1->setName(Twine("lpad") + Suffix1);
    580   NewBB1->getInstList().insert(NewBB1->getFirstInsertionPt(), Clone1);
    581 
    582   if (NewBB2) {
    583     Instruction *Clone2 = LPad->clone();
    584     Clone2->setName(Twine("lpad") + Suffix2);
    585     NewBB2->getInstList().insert(NewBB2->getFirstInsertionPt(), Clone2);
    586 
    587     // Create a PHI node for the two cloned landingpad instructions.
    588     PHINode *PN = PHINode::Create(LPad->getType(), 2, "lpad.phi", LPad);
    589     PN->addIncoming(Clone1, NewBB1);
    590     PN->addIncoming(Clone2, NewBB2);
    591     LPad->replaceAllUsesWith(PN);
    592     LPad->eraseFromParent();
    593   } else {
    594     // There is no second clone. Just replace the landing pad with the first
    595     // clone.
    596     LPad->replaceAllUsesWith(Clone1);
    597     LPad->eraseFromParent();
    598   }
    599 }
    600 
    601 /// FindFunctionBackedges - Analyze the specified function to find all of the
    602 /// loop backedges in the function and return them.  This is a relatively cheap
    603 /// (compared to computing dominators and loop info) analysis.
    604 ///
    605 /// The output is added to Result, as pairs of <from,to> edge info.
    606 void llvm::FindFunctionBackedges(const Function &F,
    607      SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
    608   const BasicBlock *BB = &F.getEntryBlock();
    609   if (succ_begin(BB) == succ_end(BB))
    610     return;
    611 
    612   SmallPtrSet<const BasicBlock*, 8> Visited;
    613   SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
    614   SmallPtrSet<const BasicBlock*, 8> InStack;
    615 
    616   Visited.insert(BB);
    617   VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
    618   InStack.insert(BB);
    619   do {
    620     std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
    621     const BasicBlock *ParentBB = Top.first;
    622     succ_const_iterator &I = Top.second;
    623 
    624     bool FoundNew = false;
    625     while (I != succ_end(ParentBB)) {
    626       BB = *I++;
    627       if (Visited.insert(BB)) {
    628         FoundNew = true;
    629         break;
    630       }
    631       // Successor is in VisitStack, it's a back edge.
    632       if (InStack.count(BB))
    633         Result.push_back(std::make_pair(ParentBB, BB));
    634     }
    635 
    636     if (FoundNew) {
    637       // Go down one level if there is a unvisited successor.
    638       InStack.insert(BB);
    639       VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
    640     } else {
    641       // Go up one level.
    642       InStack.erase(VisitStack.pop_back_val().first);
    643     }
    644   } while (!VisitStack.empty());
    645 }
    646 
    647 /// FoldReturnIntoUncondBranch - This method duplicates the specified return
    648 /// instruction into a predecessor which ends in an unconditional branch. If
    649 /// the return instruction returns a value defined by a PHI, propagate the
    650 /// right value into the return. It returns the new return instruction in the
    651 /// predecessor.
    652 ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
    653                                              BasicBlock *Pred) {
    654   Instruction *UncondBranch = Pred->getTerminator();
    655   // Clone the return and add it to the end of the predecessor.
    656   Instruction *NewRet = RI->clone();
    657   Pred->getInstList().push_back(NewRet);
    658 
    659   // If the return instruction returns a value, and if the value was a
    660   // PHI node in "BB", propagate the right value into the return.
    661   for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end();
    662        i != e; ++i)
    663     if (PHINode *PN = dyn_cast<PHINode>(*i))
    664       if (PN->getParent() == BB)
    665         *i = PN->getIncomingValueForBlock(Pred);
    666 
    667   // Update any PHI nodes in the returning block to realize that we no
    668   // longer branch to them.
    669   BB->removePredecessor(Pred);
    670   UncondBranch->eraseFromParent();
    671   return cast<ReturnInst>(NewRet);
    672 }
    673 
    674 /// GetFirstDebugLocInBasicBlock - Return first valid DebugLoc entry in a
    675 /// given basic block.
    676 DebugLoc llvm::GetFirstDebugLocInBasicBlock(const BasicBlock *BB) {
    677   if (const Instruction *I = BB->getFirstNonPHI())
    678     return I->getDebugLoc();
    679   // Scanning entire block may be too expensive, if the first instruction
    680   // does not have valid location info.
    681   return DebugLoc();
    682 }
    683