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