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/Support/CFG.h"
  25 #include "llvm/Support/Compiler.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                                   DenseMap<const Value*, Value*> &ValueMap,
  36                                   const char *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(BB->getContext());
  47     if (II->hasName())
  48       NewInst->setName(II->getName()+NameSuffix);
  49     NewBB->getInstList().push_back(NewInst);
  50     ValueMap[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 // ArgMap values.
  73 //
  74 void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
  75                              DenseMap<const Value*, Value*> &ValueMap,
  76                              SmallVectorImpl<ReturnInst*> &Returns,
  77                              const char *NameSuffix, ClonedCodeInfo *CodeInfo) {
  78   assert(NameSuffix && "NameSuffix cannot be null!");
  79
  80 #ifndef NDEBUG
  81   for (Function::const_arg_iterator I = OldFunc->arg_begin(),
  82        E = OldFunc->arg_end(); I != E; ++I)
  83     assert(ValueMap.count(I) && "No mapping from source argument specified!");
  84 #endif
  85
  86   // Clone any attributes.
  87   if (NewFunc->arg_size() == OldFunc->arg_size())
  88     NewFunc->copyAttributesFrom(OldFunc);
  89   else {
  90     //Some arguments were deleted with the ValueMap. Copy arguments one by one
  91     for (Function::const_arg_iterator I = OldFunc->arg_begin(),
  92            E = OldFunc->arg_end(); I != E; ++I)
  93       if (Argument* Anew = dyn_cast<Argument>(ValueMap[I]))
  94         Anew->addAttr( OldFunc->getAttributes()
  95                        .getParamAttributes(I->getArgNo() + 1));
  96     NewFunc->setAttributes(NewFunc->getAttributes()
  97                            .addAttr(0, OldFunc->getAttributes()
  98                                      .getRetAttributes()));
  99     NewFunc->setAttributes(NewFunc->getAttributes()
 100                            .addAttr(~0, OldFunc->getAttributes()
 101                                      .getFnAttributes()));
 102
 103   }
 104
 105   // Loop over all of the basic blocks in the function, cloning them as
 106   // appropriate.  Note that we save BE this way in order to handle cloning of
 107   // recursive functions into themselves.
 108   //
 109   for (Function::const_iterator BI = OldFunc->begin(), BE = OldFunc->end();
 110        BI != BE; ++BI) {
 111     const BasicBlock &BB = *BI;
 112
 113     // Create a new basic block and copy instructions into it!
 114     BasicBlock *CBB = CloneBasicBlock(&BB, ValueMap, NameSuffix, NewFunc,
 115                                       CodeInfo);
 116     ValueMap[&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 ValueMap to do all the hard work.
 124   //
 125   for (Function::iterator BB = cast<BasicBlock>(ValueMap[OldFunc->begin()]),
 126          BE = NewFunc->end(); BB != BE; ++BB)
 127     // Loop over all instructions, fixing each one as we find it...
 128     for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II)
 129       RemapInstruction(II, ValueMap);
 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 ValueMap 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 ValueMap,
 136 /// the arguments are deleted from the resultant function.  The ValueMap 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,
 141                               DenseMap<const Value*, Value*> &ValueMap,
 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 ValueMap.  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 (ValueMap.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 (ValueMap.count(I) == 0) {   // Is this argument preserved?
 165       DestI->setName(I->getName()); // Copy the name over...
 166       ValueMap[I] = DestI++;        // Add mapping to ValueMap
 167     }
 168
 169   SmallVector<ReturnInst*, 8> Returns;  // Ignore returns cloned.
 170   CloneFunctionInto(NewF, F, ValueMap, 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 VISIBILITY_HIDDEN PruningFunctionCloner {
 180     Function *NewFunc;
 181     const Function *OldFunc;
 182     DenseMap<const Value*, Value*> &ValueMap;
 183     SmallVectorImpl<ReturnInst*> &Returns;
 184     const char *NameSuffix;
 185     ClonedCodeInfo *CodeInfo;
 186     const TargetData *TD;
 187     Value *DbgFnStart;
 188   public:
 189     PruningFunctionCloner(Function *newFunc, const Function *oldFunc,
 190                           DenseMap<const Value*, Value*> &valueMap,
 191                           SmallVectorImpl<ReturnInst*> &returns,
 192                           const char *nameSuffix,
 193                           ClonedCodeInfo *codeInfo,
 194                           const TargetData *td)
 195     : NewFunc(newFunc), OldFunc(oldFunc), ValueMap(valueMap), Returns(returns),
 196       NameSuffix(nameSuffix), CodeInfo(codeInfo), TD(td), DbgFnStart(NULL) {
 197     }
 198
 199     /// CloneBlock - The specified block is found to be reachable, clone it and
 200     /// anything that it can reach.
 201     void CloneBlock(const BasicBlock *BB,
 202                     std::vector<const BasicBlock*> &ToClone);
 203
 204   public:
 205     /// ConstantFoldMappedInstruction - Constant fold the specified instruction,
 206     /// mapping its operands through ValueMap if they are available.
 207     Constant *ConstantFoldMappedInstruction(const Instruction *I);
 208   };
 209 }
 210
 211 /// CloneBlock - The specified block is found to be reachable, clone it and
 212 /// anything that it can reach.
 213 void PruningFunctionCloner::CloneBlock(const BasicBlock *BB,
 214                                        std::vector<const BasicBlock*> &ToClone){
 215   Value *&BBEntry = ValueMap[BB];
 216
 217   // Have we already cloned this block?
 218   if (BBEntry) return;
 219
 220   // Nope, clone it now.
 221   BasicBlock *NewBB;
 222   BBEntry = NewBB = BasicBlock::Create(BB->getContext());
 223   if (BB->hasName()) NewBB->setName(BB->getName()+NameSuffix);
 224
 225   bool hasCalls = false, hasDynamicAllocas = false, hasStaticAllocas = false;
 226
 227   // Loop over all instructions, and copy them over, DCE'ing as we go.  This
 228   // loop doesn't include the terminator.
 229   for (BasicBlock::const_iterator II = BB->begin(), IE = --BB->end();
 230        II != IE; ++II) {
 231     // If this instruction constant folds, don't bother cloning the instruction,
 232     // instead, just add the constant to the value map.
 233     if (Constant *C = ConstantFoldMappedInstruction(II)) {
 234       ValueMap[II] = C;
 235       continue;
 236     }
 237
 238     // Do not clone llvm.dbg.region.end. It will be adjusted by the inliner.
 239     if (const DbgFuncStartInst *DFSI = dyn_cast<DbgFuncStartInst>(II)) {
 240       if (DbgFnStart == NULL) {
 241         DISubprogram SP(DFSI->getSubprogram());
 242         if (SP.describes(BB->getParent()))
 243           DbgFnStart = DFSI->getSubprogram();
 244       }
 245     }
 246     if (const DbgRegionEndInst *DREIS = dyn_cast<DbgRegionEndInst>(II)) {
 247       if (DREIS->getContext() == DbgFnStart)
 248         continue;
 249     }
 250
 251     Instruction *NewInst = II->clone(BB->getContext());
 252     if (II->hasName())
 253       NewInst->setName(II->getName()+NameSuffix);
 254     NewBB->getInstList().push_back(NewInst);
 255     ValueMap[II] = NewInst;                // Add instruction map to value.
 256
 257     hasCalls |= (isa<CallInst>(II) && !isa<DbgInfoIntrinsic>(II));
 258     if (const AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
 259       if (isa<ConstantInt>(AI->getArraySize()))
 260         hasStaticAllocas = true;
 261       else
 262         hasDynamicAllocas = true;
 263     }
 264   }
 265
 266   // Finally, clone over the terminator.
 267   const TerminatorInst *OldTI = BB->getTerminator();
 268   bool TerminatorDone = false;
 269   if (const BranchInst *BI = dyn_cast<BranchInst>(OldTI)) {
 270     if (BI->isConditional()) {
 271       // If the condition was a known constant in the callee...
 272       ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
 273       // Or is a known constant in the caller...
 274       if (Cond == 0)
 275         Cond = dyn_cast_or_null<ConstantInt>(ValueMap[BI->getCondition()]);
 276
 277       // Constant fold to uncond branch!
 278       if (Cond) {
 279         BasicBlock *Dest = BI->getSuccessor(!Cond->getZExtValue());
 280         ValueMap[OldTI] = BranchInst::Create(Dest, NewBB);
 281         ToClone.push_back(Dest);
 282         TerminatorDone = true;
 283       }
 284     }
 285   } else if (const SwitchInst *SI = dyn_cast<SwitchInst>(OldTI)) {
 286     // If switching on a value known constant in the caller.
 287     ConstantInt *Cond = dyn_cast<ConstantInt>(SI->getCondition());
 288     if (Cond == 0)  // Or known constant after constant prop in the callee...
 289       Cond = dyn_cast_or_null<ConstantInt>(ValueMap[SI->getCondition()]);
 290     if (Cond) {     // Constant fold to uncond branch!
 291       BasicBlock *Dest = SI->getSuccessor(SI->findCaseValue(Cond));
 292       ValueMap[OldTI] = BranchInst::Create(Dest, NewBB);
 293       ToClone.push_back(Dest);
 294       TerminatorDone = true;
 295     }
 296   }
 297
 298   if (!TerminatorDone) {
 299     Instruction *NewInst = OldTI->clone(BB->getContext());
 300     if (OldTI->hasName())
 301       NewInst->setName(OldTI->getName()+NameSuffix);
 302     NewBB->getInstList().push_back(NewInst);
 303     ValueMap[OldTI] = NewInst;             // Add instruction map to value.
 304
 305     // Recursively clone any reachable successor blocks.
 306     const TerminatorInst *TI = BB->getTerminator();
 307     for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
 308       ToClone.push_back(TI->getSuccessor(i));
 309   }
 310
 311   if (CodeInfo) {
 312     CodeInfo->ContainsCalls          |= hasCalls;
 313     CodeInfo->ContainsUnwinds        |= isa<UnwindInst>(OldTI);
 314     CodeInfo->ContainsDynamicAllocas |= hasDynamicAllocas;
 315     CodeInfo->ContainsDynamicAllocas |= hasStaticAllocas &&
 316       BB != &BB->getParent()->front();
 317   }
 318
 319   if (ReturnInst *RI = dyn_cast<ReturnInst>(NewBB->getTerminator()))
 320     Returns.push_back(RI);
 321 }
 322
 323 /// ConstantFoldMappedInstruction - Constant fold the specified instruction,
 324 /// mapping its operands through ValueMap if they are available.
 325 Constant *PruningFunctionCloner::
 326 ConstantFoldMappedInstruction(const Instruction *I) {
 327   LLVMContext &Context = I->getContext();
 328
 329   SmallVector<Constant*, 8> Ops;
 330   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
 331     if (Constant *Op = dyn_cast_or_null<Constant>(MapValue(I->getOperand(i),
 332                                                            ValueMap,
 333                                                            Context)))
 334       Ops.push_back(Op);
 335     else
 336       return 0;  // All operands not constant!
 337
 338   if (const CmpInst *CI = dyn_cast<CmpInst>(I))
 339     return ConstantFoldCompareInstOperands(CI->getPredicate(),
 340                                            &Ops[0], Ops.size(),
 341                                            Context, TD);
 342
 343   if (const LoadInst *LI = dyn_cast<LoadInst>(I))
 344     if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ops[0]))
 345       if (!LI->isVolatile() && CE->getOpcode() == Instruction::GetElementPtr)
 346         if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getOperand(0)))
 347           if (GV->isConstant() && GV->hasDefinitiveInitializer())
 348             return ConstantFoldLoadThroughGEPConstantExpr(GV->getInitializer(),
 349                                                           CE, Context);
 350
 351   return ConstantFoldInstOperands(I->getOpcode(), I->getType(), &Ops[0],
 352                                   Ops.size(), Context, TD);
 353 }
 354
 355 /// CloneAndPruneFunctionInto - This works exactly like CloneFunctionInto,
 356 /// except that it does some simple constant prop and DCE on the fly.  The
 357 /// effect of this is to copy significantly less code in cases where (for
 358 /// example) a function call with constant arguments is inlined, and those
 359 /// constant arguments cause a significant amount of code in the callee to be
 360 /// dead.  Since this doesn't produce an exact copy of the input, it can't be
 361 /// used for things like CloneFunction or CloneModule.
 362 void llvm::CloneAndPruneFunctionInto(Function *NewFunc, const Function *OldFunc,
 363                                      DenseMap<const Value*, Value*> &ValueMap,
 364                                      SmallVectorImpl<ReturnInst*> &Returns,
 365                                      const char *NameSuffix,
 366                                      ClonedCodeInfo *CodeInfo,
 367                                      const TargetData *TD) {
 368   assert(NameSuffix && "NameSuffix cannot be null!");
 369   LLVMContext &Context = OldFunc->getContext();
 370
 371 #ifndef NDEBUG
 372   for (Function::const_arg_iterator II = OldFunc->arg_begin(),
 373        E = OldFunc->arg_end(); II != E; ++II)
 374     assert(ValueMap.count(II) && "No mapping from source argument specified!");
 375 #endif
 376
 377   PruningFunctionCloner PFC(NewFunc, OldFunc, ValueMap, Returns,
 378                             NameSuffix, CodeInfo, TD);
 379
 380   // Clone the entry block, and anything recursively reachable from it.
 381   std::vector<const BasicBlock*> CloneWorklist;
 382   CloneWorklist.push_back(&OldFunc->getEntryBlock());
 383   while (!CloneWorklist.empty()) {
 384     const BasicBlock *BB = CloneWorklist.back();
 385     CloneWorklist.pop_back();
 386     PFC.CloneBlock(BB, CloneWorklist);
 387   }
 388
 389   // Loop over all of the basic blocks in the old function.  If the block was
 390   // reachable, we have cloned it and the old block is now in the value map:
 391   // insert it into the new function in the right order.  If not, ignore it.
 392   //
 393   // Defer PHI resolution until rest of function is resolved.
 394   SmallVector<const PHINode*, 16> PHIToResolve;
 395   for (Function::const_iterator BI = OldFunc->begin(), BE = OldFunc->end();
 396        BI != BE; ++BI) {
 397     BasicBlock *NewBB = cast_or_null<BasicBlock>(ValueMap[BI]);
 398     if (NewBB == 0) continue;  // Dead block.
 399
 400     // Add the new block to the new function.
 401     NewFunc->getBasicBlockList().push_back(NewBB);
 402
 403     // Loop over all of the instructions in the block, fixing up operand
 404     // references as we go.  This uses ValueMap to do all the hard work.
 405     //
 406     BasicBlock::iterator I = NewBB->begin();
 407
 408     // Handle PHI nodes specially, as we have to remove references to dead
 409     // blocks.
 410     if (PHINode *PN = dyn_cast<PHINode>(I)) {
 411       // Skip over all PHI nodes, remembering them for later.
 412       BasicBlock::const_iterator OldI = BI->begin();
 413       for (; (PN = dyn_cast<PHINode>(I)); ++I, ++OldI)
 414         PHIToResolve.push_back(cast<PHINode>(OldI));
 415     }
 416
 417     // Otherwise, remap the rest of the instructions normally.
 418     for (; I != NewBB->end(); ++I)
 419       RemapInstruction(I, ValueMap);
 420   }
 421
 422   // Defer PHI resolution until rest of function is resolved, PHI resolution
 423   // requires the CFG to be up-to-date.
 424   for (unsigned phino = 0, e = PHIToResolve.size(); phino != e; ) {
 425     const PHINode *OPN = PHIToResolve[phino];
 426     unsigned NumPreds = OPN->getNumIncomingValues();
 427     const BasicBlock *OldBB = OPN->getParent();
 428     BasicBlock *NewBB = cast<BasicBlock>(ValueMap[OldBB]);
 429
 430     // Map operands for blocks that are live and remove operands for blocks
 431     // that are dead.
 432     for (; phino != PHIToResolve.size() &&
 433          PHIToResolve[phino]->getParent() == OldBB; ++phino) {
 434       OPN = PHIToResolve[phino];
 435       PHINode *PN = cast<PHINode>(ValueMap[OPN]);
 436       for (unsigned pred = 0, e = NumPreds; pred != e; ++pred) {
 437         if (BasicBlock *MappedBlock =
 438             cast_or_null<BasicBlock>(ValueMap[PN->getIncomingBlock(pred)])) {
 439           Value *InVal = MapValue(PN->getIncomingValue(pred),
 440                                   ValueMap, Context);
 441           assert(InVal && "Unknown input value?");
 442           PN->setIncomingValue(pred, InVal);
 443           PN->setIncomingBlock(pred, MappedBlock);
 444         } else {
 445           PN->removeIncomingValue(pred, false);
 446           --pred, --e;  // Revisit the next entry.
 447         }
 448       }
 449     }
 450
 451     // The loop above has removed PHI entries for those blocks that are dead
 452     // and has updated others.  However, if a block is live (i.e. copied over)
 453     // but its terminator has been changed to not go to this block, then our
 454     // phi nodes will have invalid entries.  Update the PHI nodes in this
 455     // case.
 456     PHINode *PN = cast<PHINode>(NewBB->begin());
 457     NumPreds = std::distance(pred_begin(NewBB), pred_end(NewBB));
 458     if (NumPreds != PN->getNumIncomingValues()) {
 459       assert(NumPreds < PN->getNumIncomingValues());
 460       // Count how many times each predecessor comes to this block.
 461       std::map<BasicBlock*, unsigned> PredCount;
 462       for (pred_iterator PI = pred_begin(NewBB), E = pred_end(NewBB);
 463            PI != E; ++PI)
 464         --PredCount[*PI];
 465
 466       // Figure out how many entries to remove from each PHI.
 467       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
 468         ++PredCount[PN->getIncomingBlock(i)];
 469
 470       // At this point, the excess predecessor entries are positive in the
 471       // map.  Loop over all of the PHIs and remove excess predecessor
 472       // entries.
 473       BasicBlock::iterator I = NewBB->begin();
 474       for (; (PN = dyn_cast<PHINode>(I)); ++I) {
 475         for (std::map<BasicBlock*, unsigned>::iterator PCI =PredCount.begin(),
 476              E = PredCount.end(); PCI != E; ++PCI) {
 477           BasicBlock *Pred     = PCI->first;
 478           for (unsigned NumToRemove = PCI->second; NumToRemove; --NumToRemove)
 479             PN->removeIncomingValue(Pred, false);
 480         }
 481       }
 482     }
 483
 484     // If the loops above have made these phi nodes have 0 or 1 operand,
 485     // replace them with undef or the input value.  We must do this for
 486     // correctness, because 0-operand phis are not valid.
 487     PN = cast<PHINode>(NewBB->begin());
 488     if (PN->getNumIncomingValues() == 0) {
 489       BasicBlock::iterator I = NewBB->begin();
 490       BasicBlock::const_iterator OldI = OldBB->begin();
 491       while ((PN = dyn_cast<PHINode>(I++))) {
 492         Value *NV = UndefValue::get(PN->getType());
 493         PN->replaceAllUsesWith(NV);
 494         assert(ValueMap[OldI] == PN && "ValueMap mismatch");
 495         ValueMap[OldI] = NV;
 496         PN->eraseFromParent();
 497         ++OldI;
 498       }
 499     }
 500     // NOTE: We cannot eliminate single entry phi nodes here, because of
 501     // ValueMap.  Single entry phi nodes can have multiple ValueMap entries
 502     // pointing at them.  Thus, deleting one would require scanning the ValueMap
 503     // to update any entries in it that would require that.  This would be
 504     // really slow.
 505   }
 506
 507   // Now that the inlined function body has been fully constructed, go through
 508   // and zap unconditional fall-through branches.  This happen all the time when
 509   // specializing code: code specialization turns conditional branches into
 510   // uncond branches, and this code folds them.
 511   Function::iterator I = cast<BasicBlock>(ValueMap[&OldFunc->getEntryBlock()]);
 512   while (I != NewFunc->end()) {
 513     BranchInst *BI = dyn_cast<BranchInst>(I->getTerminator());
 514     if (!BI || BI->isConditional()) { ++I; continue; }
 515
 516     // Note that we can't eliminate uncond branches if the destination has
 517     // single-entry PHI nodes.  Eliminating the single-entry phi nodes would
 518     // require scanning the ValueMap to update any entries that point to the phi
 519     // node.
 520     BasicBlock *Dest = BI->getSuccessor(0);
 521     if (!Dest->getSinglePredecessor() || isa<PHINode>(Dest->begin())) {
 522       ++I; continue;
 523     }
 524
 525     // We know all single-entry PHI nodes in the inlined function have been
 526     // removed, so we just need to splice the blocks.
 527     BI->eraseFromParent();
 528
 529     // Move all the instructions in the succ to the pred.
 530     I->getInstList().splice(I->end(), Dest->getInstList());
 531
 532     // Make all PHI nodes that referred to Dest now refer to I as their source.
 533     Dest->replaceAllUsesWith(I);
 534
 535     // Remove the dest block.
 536     Dest->eraseFromParent();
 537
 538     // Do not increment I, iteratively merge all things this block branches to.
 539   }
 540 }