lib/Transforms/Utils/CloneFunction.cpp

   1 //===- CloneFunction.cpp - Clone a function into another function ---------===//
   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 file implements the CloneFunctionInto interface, which is used as the
  11 // low-level function cloner.  This is used by the CloneFunction and function
  12 // inliner to do the dirty work of copying the body of a function around.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/Transforms/Utils/Cloning.h"
  17 #include "llvm/Constants.h"
  18 #include "llvm/DerivedTypes.h"
  19 #include "llvm/Instructions.h"
  20 #include "llvm/IntrinsicInst.h"
  21 #include "llvm/GlobalVariable.h"
  22 #include "llvm/Function.h"
  23 #include "llvm/LLVMContext.h"
  24 #include "llvm/Metadata.h"
  25 #include "llvm/Support/CFG.h"
  26 #include "llvm/Transforms/Utils/ValueMapper.h"
  27 #include "llvm/Analysis/ConstantFolding.h"
  28 #include "llvm/Analysis/DebugInfo.h"
  29 #include "llvm/ADT/SmallVector.h"
  30 #include <map>
  31 using namespace llvm;
  32
  33 // CloneBasicBlock - See comments in Cloning.h
  34 BasicBlock *llvm::CloneBasicBlock(const BasicBlock *BB,
  35                                   ValueToValueMapTy &VMap,
  36                                   const Twine &NameSuffix, Function *F,
  37                                   ClonedCodeInfo *CodeInfo) {
  38   BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "", F);
  39   if (BB->hasName()) NewBB->setName(BB->getName()+NameSuffix);
  40
  41   bool hasCalls = false, hasDynamicAllocas = false, hasStaticAllocas = false;
  42
  43   // Loop over all instructions, and copy them over.
  44   for (BasicBlock::const_iterator II = BB->begin(), IE = BB->end();
  45        II != IE; ++II) {
  46     Instruction *NewInst = II->clone();
  47     if (II->hasName())
  48       NewInst->setName(II->getName()+NameSuffix);
  49     NewBB->getInstList().push_back(NewInst);
  50     VMap[II] = NewInst;                // Add instruction map to value.
  51
  52     hasCalls |= (isa<CallInst>(II) && !isa<DbgInfoIntrinsic>(II));
  53     if (const AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
  54       if (isa<ConstantInt>(AI->getArraySize()))
  55         hasStaticAllocas = true;
  56       else
  57         hasDynamicAllocas = true;
  58     }
  59   }
  60
  61   if (CodeInfo) {
  62     CodeInfo->ContainsCalls          |= hasCalls;
  63     CodeInfo->ContainsUnwinds        |= isa<UnwindInst>(BB->getTerminator());
  64     CodeInfo->ContainsDynamicAllocas |= hasDynamicAllocas;
  65     CodeInfo->ContainsDynamicAllocas |= hasStaticAllocas &&
  66                                         BB != &BB->getParent()->getEntryBlock();
  67   }
  68   return NewBB;
  69 }
  70
  71 // Clone OldFunc into NewFunc, transforming the old arguments into references to
  72 // VMap values.
  73 //
  74 void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
  75                              ValueToValueMapTy &VMap,
  76                              bool ModuleLevelChanges,
  77                              SmallVectorImpl<ReturnInst*> &Returns,
  78                              const char *NameSuffix, ClonedCodeInfo *CodeInfo) {
  79   assert(NameSuffix && "NameSuffix cannot be null!");
  80
  81 #ifndef NDEBUG
  82   for (Function::const_arg_iterator I = OldFunc->arg_begin(),
  83        E = OldFunc->arg_end(); I != E; ++I)
  84     assert(VMap.count(I) && "No mapping from source argument specified!");
  85 #endif
  86
  87   // Clone any attributes.
  88   if (NewFunc->arg_size() == OldFunc->arg_size())
  89     NewFunc->copyAttributesFrom(OldFunc);
  90   else {
  91     //Some arguments were deleted with the VMap. Copy arguments one by one
  92     for (Function::const_arg_iterator I = OldFunc->arg_begin(),
  93            E = OldFunc->arg_end(); I != E; ++I)
  94       if (Argument* Anew = dyn_cast<Argument>(VMap[I]))
  95         Anew->addAttr( OldFunc->getAttributes()
  96                        .getParamAttributes(I->getArgNo() + 1));
  97     NewFunc->setAttributes(NewFunc->getAttributes()
  98                            .addAttr(0, OldFunc->getAttributes()
  99                                      .getRetAttributes()));
 100     NewFunc->setAttributes(NewFunc->getAttributes()
 101                            .addAttr(~0, OldFunc->getAttributes()
 102                                      .getFnAttributes()));
 103
 104   }
 105
 106   // Loop over all of the basic blocks in the function, cloning them as
 107   // appropriate.  Note that we save BE this way in order to handle cloning of
 108   // recursive functions into themselves.
 109   //
 110   for (Function::const_iterator BI = OldFunc->begin(), BE = OldFunc->end();
 111        BI != BE; ++BI) {
 112     const BasicBlock &BB = *BI;
 113
 114     // Create a new basic block and copy instructions into it!
 115     BasicBlock *CBB = CloneBasicBlock(&BB, VMap, NameSuffix, NewFunc, CodeInfo);
 116     VMap[&BB] = CBB;                       // Add basic block mapping.
 117
 118     if (ReturnInst *RI = dyn_cast<ReturnInst>(CBB->getTerminator()))
 119       Returns.push_back(RI);
 120   }
 121
 122   // Loop over all of the instructions in the function, fixing up operand
 123   // references as we go.  This uses VMap to do all the hard work.
 124   for (Function::iterator BB = cast<BasicBlock>(VMap[OldFunc->begin()]),
 125          BE = NewFunc->end(); BB != BE; ++BB)
 126     // Loop over all instructions, fixing each one as we find it...
 127     for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II)
 128       RemapInstruction(II, VMap,
 129                        ModuleLevelChanges ? RF_None : RF_NoModuleLevelChanges);
 130 }
 131
 132 /// CloneFunction - Return a copy of the specified function, but without
 133 /// embedding the function into another module.  Also, any references specified
 134 /// in the VMap are changed to refer to their mapped value instead of the
 135 /// original one.  If any of the arguments to the function are in the VMap,
 136 /// the arguments are deleted from the resultant function.  The VMap is
 137 /// updated to include mappings from all of the instructions and basicblocks in
 138 /// the function from their old to new values.
 139 ///
 140 Function *llvm::CloneFunction(const Function *F, ValueToValueMapTy &VMap,
 141                               bool ModuleLevelChanges,
 142                               ClonedCodeInfo *CodeInfo) {
 143   std::vector<const Type*> ArgTypes;
 144
 145   // The user might be deleting arguments to the function by specifying them in
 146   // the VMap.  If so, we need to not add the arguments to the arg ty vector
 147   //
 148   for (Function::const_arg_iterator I = F->arg_begin(), E = F->arg_end();
 149        I != E; ++I)
 150     if (VMap.count(I) == 0)  // Haven't mapped the argument to anything yet?
 151       ArgTypes.push_back(I->getType());
 152
 153   // Create a new function type...
 154   FunctionType *FTy = FunctionType::get(F->getFunctionType()->getReturnType(),
 155                                     ArgTypes, F->getFunctionType()->isVarArg());
 156
 157   // Create the new function...
 158   Function *NewF = Function::Create(FTy, F->getLinkage(), F->getName());
 159
 160   // Loop over the arguments, copying the names of the mapped arguments over...
 161   Function::arg_iterator DestI = NewF->arg_begin();
 162   for (Function::const_arg_iterator I = F->arg_begin(), E = F->arg_end();
 163        I != E; ++I)
 164     if (VMap.count(I) == 0) {   // Is this argument preserved?
 165       DestI->setName(I->getName()); // Copy the name over...
 166       VMap[I] = DestI++;        // Add mapping to VMap
 167     }
 168
 169   SmallVector<ReturnInst*, 8> Returns;  // Ignore returns cloned.
 170   CloneFunctionInto(NewF, F, VMap, ModuleLevelChanges, Returns, "", CodeInfo);
 171   return NewF;
 172 }
 173
 174
 175
 176 namespace {
 177   /// PruningFunctionCloner - This class is a private class used to implement
 178   /// the CloneAndPruneFunctionInto method.
 179   struct PruningFunctionCloner {
 180     Function *NewFunc;
 181     const Function *OldFunc;
 182     ValueToValueMapTy &VMap;
 183     bool ModuleLevelChanges;
 184     SmallVectorImpl<ReturnInst*> &Returns;
 185     const char *NameSuffix;
 186     ClonedCodeInfo *CodeInfo;
 187     const TargetData *TD;
 188   public:
 189     PruningFunctionCloner(Function *newFunc, const Function *oldFunc,
 190                           ValueToValueMapTy &valueMap,
 191                           bool moduleLevelChanges,
 192                           SmallVectorImpl<ReturnInst*> &returns,
 193                           const char *nameSuffix,
 194                           ClonedCodeInfo *codeInfo,
 195                           const TargetData *td)
 196     : NewFunc(newFunc), OldFunc(oldFunc),
 197       VMap(valueMap), ModuleLevelChanges(moduleLevelChanges),
 198       Returns(returns), NameSuffix(nameSuffix), CodeInfo(codeInfo), TD(td) {
 199     }
 200
 201     /// CloneBlock - The specified block is found to be reachable, clone it and
 202     /// anything that it can reach.
 203     void CloneBlock(const BasicBlock *BB,
 204                     std::vector<const BasicBlock*> &ToClone);
 205
 206   public:
 207     /// ConstantFoldMappedInstruction - Constant fold the specified instruction,
 208     /// mapping its operands through VMap if they are available.
 209     Constant *ConstantFoldMappedInstruction(const Instruction *I);
 210   };
 211 }
 212
 213 /// CloneBlock - The specified block is found to be reachable, clone it and
 214 /// anything that it can reach.
 215 void PruningFunctionCloner::CloneBlock(const BasicBlock *BB,
 216                                        std::vector<const BasicBlock*> &ToClone){
 217   TrackingVH<Value> &BBEntry = VMap[BB];
 218
 219   // Have we already cloned this block?
 220   if (BBEntry) return;
 221
 222   // Nope, clone it now.
 223   BasicBlock *NewBB;
 224   BBEntry = NewBB = BasicBlock::Create(BB->getContext());
 225   if (BB->hasName()) NewBB->setName(BB->getName()+NameSuffix);
 226
 227   bool hasCalls = false, hasDynamicAllocas = false, hasStaticAllocas = false;
 228
 229   // Loop over all instructions, and copy them over, DCE'ing as we go.  This
 230   // loop doesn't include the terminator.
 231   for (BasicBlock::const_iterator II = BB->begin(), IE = --BB->end();
 232        II != IE; ++II) {
 233     // If this instruction constant folds, don't bother cloning the instruction,
 234     // instead, just add the constant to the value map.
 235     if (Constant *C = ConstantFoldMappedInstruction(II)) {
 236       VMap[II] = C;
 237       continue;
 238     }
 239
 240     Instruction *NewInst = II->clone();
 241     if (II->hasName())
 242       NewInst->setName(II->getName()+NameSuffix);
 243     NewBB->getInstList().push_back(NewInst);
 244     VMap[II] = NewInst;                // Add instruction map to value.
 245
 246     hasCalls |= (isa<CallInst>(II) && !isa<DbgInfoIntrinsic>(II));
 247     if (const AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
 248       if (isa<ConstantInt>(AI->getArraySize()))
 249         hasStaticAllocas = true;
 250       else
 251         hasDynamicAllocas = true;
 252     }
 253   }
 254
 255   // Finally, clone over the terminator.
 256   const TerminatorInst *OldTI = BB->getTerminator();
 257   bool TerminatorDone = false;
 258   if (const BranchInst *BI = dyn_cast<BranchInst>(OldTI)) {
 259     if (BI->isConditional()) {
 260       // If the condition was a known constant in the callee...
 261       ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
 262       // Or is a known constant in the caller...
 263       if (Cond == 0) {
 264         Value *V = VMap[BI->getCondition()];
 265         Cond = dyn_cast_or_null<ConstantInt>(V);
 266       }
 267
 268       // Constant fold to uncond branch!
 269       if (Cond) {
 270         BasicBlock *Dest = BI->getSuccessor(!Cond->getZExtValue());
 271         VMap[OldTI] = BranchInst::Create(Dest, NewBB);
 272         ToClone.push_back(Dest);
 273         TerminatorDone = true;
 274       }
 275     }
 276   } else if (const SwitchInst *SI = dyn_cast<SwitchInst>(OldTI)) {
 277     // If switching on a value known constant in the caller.
 278     ConstantInt *Cond = dyn_cast<ConstantInt>(SI->getCondition());
 279     if (Cond == 0) { // Or known constant after constant prop in the callee...
 280       Value *V = VMap[SI->getCondition()];
 281       Cond = dyn_cast_or_null<ConstantInt>(V);
 282     }
 283     if (Cond) {     // Constant fold to uncond branch!
 284       BasicBlock *Dest = SI->getSuccessor(SI->findCaseValue(Cond));
 285       VMap[OldTI] = BranchInst::Create(Dest, NewBB);
 286       ToClone.push_back(Dest);
 287       TerminatorDone = true;
 288     }
 289   }
 290
 291   if (!TerminatorDone) {
 292     Instruction *NewInst = OldTI->clone();
 293     if (OldTI->hasName())
 294       NewInst->setName(OldTI->getName()+NameSuffix);
 295     NewBB->getInstList().push_back(NewInst);
 296     VMap[OldTI] = NewInst;             // Add instruction map to value.
 297
 298     // Recursively clone any reachable successor blocks.
 299     const TerminatorInst *TI = BB->getTerminator();
 300     for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
 301       ToClone.push_back(TI->getSuccessor(i));
 302   }
 303
 304   if (CodeInfo) {
 305     CodeInfo->ContainsCalls          |= hasCalls;
 306     CodeInfo->ContainsUnwinds        |= isa<UnwindInst>(OldTI);
 307     CodeInfo->ContainsDynamicAllocas |= hasDynamicAllocas;
 308     CodeInfo->ContainsDynamicAllocas |= hasStaticAllocas &&
 309       BB != &BB->getParent()->front();
 310   }
 311
 312   if (ReturnInst *RI = dyn_cast<ReturnInst>(NewBB->getTerminator()))
 313     Returns.push_back(RI);
 314 }
 315
 316 /// ConstantFoldMappedInstruction - Constant fold the specified instruction,
 317 /// mapping its operands through VMap if they are available.
 318 Constant *PruningFunctionCloner::
 319 ConstantFoldMappedInstruction(const Instruction *I) {
 320   SmallVector<Constant*, 8> Ops;
 321   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
 322     if (Constant *Op = dyn_cast_or_null<Constant>(MapValue(I->getOperand(i),
 323                                                            VMap,
 324                   ModuleLevelChanges ? RF_None : RF_NoModuleLevelChanges)))
 325       Ops.push_back(Op);
 326     else
 327       return 0;  // All operands not constant!
 328
 329   if (const CmpInst *CI = dyn_cast<CmpInst>(I))
 330     return ConstantFoldCompareInstOperands(CI->getPredicate(), Ops[0], Ops[1],
 331                                            TD);
 332
 333   if (const LoadInst *LI = dyn_cast<LoadInst>(I))
 334     if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ops[0]))
 335       if (!LI->isVolatile() && CE->getOpcode() == Instruction::GetElementPtr)
 336         if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getOperand(0)))
 337           if (GV->isConstant() && GV->hasDefinitiveInitializer())
 338             return ConstantFoldLoadThroughGEPConstantExpr(GV->getInitializer(),
 339                                                           CE);
 340
 341   return ConstantFoldInstOperands(I->getOpcode(), I->getType(), &Ops[0],
 342                                   Ops.size(), TD);
 343 }
 344
 345 /// CloneAndPruneFunctionInto - This works exactly like CloneFunctionInto,
 346 /// except that it does some simple constant prop and DCE on the fly.  The
 347 /// effect of this is to copy significantly less code in cases where (for
 348 /// example) a function call with constant arguments is inlined, and those
 349 /// constant arguments cause a significant amount of code in the callee to be
 350 /// dead.  Since this doesn't produce an exact copy of the input, it can't be
 351 /// used for things like CloneFunction or CloneModule.
 352 void llvm::CloneAndPruneFunctionInto(Function *NewFunc, const Function *OldFunc,
 353                                      ValueToValueMapTy &VMap,
 354                                      bool ModuleLevelChanges,
 355                                      SmallVectorImpl<ReturnInst*> &Returns,
 356                                      const char *NameSuffix,
 357                                      ClonedCodeInfo *CodeInfo,
 358                                      const TargetData *TD,
 359                                      Instruction *TheCall) {
 360   assert(NameSuffix && "NameSuffix cannot be null!");
 361
 362 #ifndef NDEBUG
 363   for (Function::const_arg_iterator II = OldFunc->arg_begin(),
 364        E = OldFunc->arg_end(); II != E; ++II)
 365     assert(VMap.count(II) && "No mapping from source argument specified!");
 366 #endif
 367
 368   PruningFunctionCloner PFC(NewFunc, OldFunc, VMap, ModuleLevelChanges,
 369                             Returns, NameSuffix, CodeInfo, TD);
 370
 371   // Clone the entry block, and anything recursively reachable from it.
 372   std::vector<const BasicBlock*> CloneWorklist;
 373   CloneWorklist.push_back(&OldFunc->getEntryBlock());
 374   while (!CloneWorklist.empty()) {
 375     const BasicBlock *BB = CloneWorklist.back();
 376     CloneWorklist.pop_back();
 377     PFC.CloneBlock(BB, CloneWorklist);
 378   }
 379
 380   // Loop over all of the basic blocks in the old function.  If the block was
 381   // reachable, we have cloned it and the old block is now in the value map:
 382   // insert it into the new function in the right order.  If not, ignore it.
 383   //
 384   // Defer PHI resolution until rest of function is resolved.
 385   SmallVector<const PHINode*, 16> PHIToResolve;
 386   for (Function::const_iterator BI = OldFunc->begin(), BE = OldFunc->end();
 387        BI != BE; ++BI) {
 388     Value *V = VMap[BI];
 389     BasicBlock *NewBB = cast_or_null<BasicBlock>(V);
 390     if (NewBB == 0) continue;  // Dead block.
 391
 392     // Add the new block to the new function.
 393     NewFunc->getBasicBlockList().push_back(NewBB);
 394
 395     // Loop over all of the instructions in the block, fixing up operand
 396     // references as we go.  This uses VMap to do all the hard work.
 397     //
 398     BasicBlock::iterator I = NewBB->begin();
 399
 400     DebugLoc TheCallDL;
 401     if (TheCall)
 402       TheCallDL = TheCall->getDebugLoc();
 403
 404     // Handle PHI nodes specially, as we have to remove references to dead
 405     // blocks.
 406     if (PHINode *PN = dyn_cast<PHINode>(I)) {
 407       // Skip over all PHI nodes, remembering them for later.
 408       BasicBlock::const_iterator OldI = BI->begin();
 409       for (; (PN = dyn_cast<PHINode>(I)); ++I, ++OldI)
 410         PHIToResolve.push_back(cast<PHINode>(OldI));
 411     }
 412
 413     // Otherwise, remap the rest of the instructions normally.
 414     for (; I != NewBB->end(); ++I)
 415       RemapInstruction(I, VMap,
 416                        ModuleLevelChanges ? RF_None : RF_NoModuleLevelChanges);
 417   }
 418
 419   // Defer PHI resolution until rest of function is resolved, PHI resolution
 420   // requires the CFG to be up-to-date.
 421   for (unsigned phino = 0, e = PHIToResolve.size(); phino != e; ) {
 422     const PHINode *OPN = PHIToResolve[phino];
 423     unsigned NumPreds = OPN->getNumIncomingValues();
 424     const BasicBlock *OldBB = OPN->getParent();
 425     BasicBlock *NewBB = cast<BasicBlock>(VMap[OldBB]);
 426
 427     // Map operands for blocks that are live and remove operands for blocks
 428     // that are dead.
 429     for (; phino != PHIToResolve.size() &&
 430          PHIToResolve[phino]->getParent() == OldBB; ++phino) {
 431       OPN = PHIToResolve[phino];
 432       PHINode *PN = cast<PHINode>(VMap[OPN]);
 433       for (unsigned pred = 0, e = NumPreds; pred != e; ++pred) {
 434         Value *V = VMap[PN->getIncomingBlock(pred)];
 435         if (BasicBlock *MappedBlock = cast_or_null<BasicBlock>(V)) {
 436           Value *InVal = MapValue(PN->getIncomingValue(pred),
 437                                   VMap,
 438                         ModuleLevelChanges ? RF_None : RF_NoModuleLevelChanges);
 439           assert(InVal && "Unknown input value?");
 440           PN->setIncomingValue(pred, InVal);
 441           PN->setIncomingBlock(pred, MappedBlock);
 442         } else {
 443           PN->removeIncomingValue(pred, false);
 444           --pred, --e;  // Revisit the next entry.
 445         }
 446       }
 447     }
 448
 449     // The loop above has removed PHI entries for those blocks that are dead
 450     // and has updated others.  However, if a block is live (i.e. copied over)
 451     // but its terminator has been changed to not go to this block, then our
 452     // phi nodes will have invalid entries.  Update the PHI nodes in this
 453     // case.
 454     PHINode *PN = cast<PHINode>(NewBB->begin());
 455     NumPreds = std::distance(pred_begin(NewBB), pred_end(NewBB));
 456     if (NumPreds != PN->getNumIncomingValues()) {
 457       assert(NumPreds < PN->getNumIncomingValues());
 458       // Count how many times each predecessor comes to this block.
 459       std::map<BasicBlock*, unsigned> PredCount;
 460       for (pred_iterator PI = pred_begin(NewBB), E = pred_end(NewBB);
 461            PI != E; ++PI)
 462         --PredCount[*PI];
 463
 464       // Figure out how many entries to remove from each PHI.
 465       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
 466         ++PredCount[PN->getIncomingBlock(i)];
 467
 468       // At this point, the excess predecessor entries are positive in the
 469       // map.  Loop over all of the PHIs and remove excess predecessor
 470       // entries.
 471       BasicBlock::iterator I = NewBB->begin();
 472       for (; (PN = dyn_cast<PHINode>(I)); ++I) {
 473         for (std::map<BasicBlock*, unsigned>::iterator PCI =PredCount.begin(),
 474              E = PredCount.end(); PCI != E; ++PCI) {
 475           BasicBlock *Pred     = PCI->first;
 476           for (unsigned NumToRemove = PCI->second; NumToRemove; --NumToRemove)
 477             PN->removeIncomingValue(Pred, false);
 478         }
 479       }
 480     }
 481
 482     // If the loops above have made these phi nodes have 0 or 1 operand,
 483     // replace them with undef or the input value.  We must do this for
 484     // correctness, because 0-operand phis are not valid.
 485     PN = cast<PHINode>(NewBB->begin());
 486     if (PN->getNumIncomingValues() == 0) {
 487       BasicBlock::iterator I = NewBB->begin();
 488       BasicBlock::const_iterator OldI = OldBB->begin();
 489       while ((PN = dyn_cast<PHINode>(I++))) {
 490         Value *NV = UndefValue::get(PN->getType());
 491         PN->replaceAllUsesWith(NV);
 492         assert(VMap[OldI] == PN && "VMap mismatch");
 493         VMap[OldI] = NV;
 494         PN->eraseFromParent();
 495         ++OldI;
 496       }
 497     }
 498     // NOTE: We cannot eliminate single entry phi nodes here, because of
 499     // VMap.  Single entry phi nodes can have multiple VMap entries
 500     // pointing at them.  Thus, deleting one would require scanning the VMap
 501     // to update any entries in it that would require that.  This would be
 502     // really slow.
 503   }
 504
 505   // Now that the inlined function body has been fully constructed, go through
 506   // and zap unconditional fall-through branches.  This happen all the time when
 507   // specializing code: code specialization turns conditional branches into
 508   // uncond branches, and this code folds them.
 509   Function::iterator I = cast<BasicBlock>(VMap[&OldFunc->getEntryBlock()]);
 510   while (I != NewFunc->end()) {
 511     BranchInst *BI = dyn_cast<BranchInst>(I->getTerminator());
 512     if (!BI || BI->isConditional()) { ++I; continue; }
 513
 514     // Note that we can't eliminate uncond branches if the destination has
 515     // single-entry PHI nodes.  Eliminating the single-entry phi nodes would
 516     // require scanning the VMap to update any entries that point to the phi
 517     // node.
 518     BasicBlock *Dest = BI->getSuccessor(0);
 519     if (!Dest->getSinglePredecessor() || isa<PHINode>(Dest->begin())) {
 520       ++I; continue;
 521     }
 522
 523     // We know all single-entry PHI nodes in the inlined function have been
 524     // removed, so we just need to splice the blocks.
 525     BI->eraseFromParent();
 526
 527     // Make all PHI nodes that referred to Dest now refer to I as their source.
 528     Dest->replaceAllUsesWith(I);
 529
 530     // Move all the instructions in the succ to the pred.
 531     I->getInstList().splice(I->end(), Dest->getInstList());
 532
 533     // Remove the dest block.
 534     Dest->eraseFromParent();
 535
 536     // Do not increment I, iteratively merge all things this block branches to.
 537   }
 538 }