diff --git a/lib/Transforms/IPO/MergeFunctions.cpp b/lib/Transforms/IPO/MergeFunctions.cpp
index 5d838f98aa0..726466ec81d 100644
--- a/lib/Transforms/IPO/MergeFunctions.cpp
+++ b/lib/Transforms/IPO/MergeFunctions.cpp
@@ -69,32 +69,79 @@ using namespace llvm;
 STATISTIC(NumFunctionsMerged, "Number of functions merged");
 
 namespace {
-  /// MergeFunctions finds functions which will generate identical machine code,
-  /// by considering all pointer types to be equivalent. Once identified,
-  /// MergeFunctions will fold them by replacing a call to one to a call to a
-  /// bitcast of the other.
-  ///
-  class MergeFunctions : public ModulePass {
-  public:
-    static char ID;
-    MergeFunctions() : ModulePass(ID) {}
 
-    bool runOnModule(Module &M);
+static unsigned ProfileFunction(const Function *F) {
+  const FunctionType *FTy = F->getFunctionType();
 
-  private:
-    /// MergeTwoFunctions - Merge two equivalent functions. Upon completion, G
-    /// may be deleted, or may be converted into a thunk. In either case, it
-    /// should never be visited again.
-    void MergeTwoFunctions(Function *F, Function *G) const;
-
-    /// WriteThunk - Replace G with a simple tail call to bitcast(F). Also
-    /// replace direct uses of G with bitcast(F).
-    void WriteThunk(Function *F, Function *G) const;
-
-    TargetData *TD;
-  };
+  FoldingSetNodeID ID;
+  ID.AddInteger(F->size());
+  ID.AddInteger(F->getCallingConv());
+  ID.AddBoolean(F->hasGC());
+  ID.AddBoolean(FTy->isVarArg());
+  ID.AddInteger(FTy->getReturnType()->getTypeID());
+  for (unsigned i = 0, e = FTy->getNumParams(); i != e; ++i)
+    ID.AddInteger(FTy->getParamType(i)->getTypeID());
+  return ID.ComputeHash();
 }
 
+class ComparableFunction {
+public:
+  ComparableFunction(Function *Func, TargetData *TD)
+    : Func(Func), Hash(ProfileFunction(Func)), TD(TD) {}
+
+  AssertingVH<Function> const Func;
+  const unsigned Hash;
+  TargetData * const TD;
+};
+
+struct MergeFunctionsEqualityInfo {
+  static ComparableFunction *getEmptyKey() {
+    return reinterpret_cast<ComparableFunction*>(0);
+  }
+  static ComparableFunction *getTombstoneKey() {
+    return reinterpret_cast<ComparableFunction*>(-1);
+  }
+  static unsigned getHashValue(const ComparableFunction *CF) {
+    return CF->Hash;
+  }
+  static bool isEqual(const ComparableFunction *LHS,
+                      const ComparableFunction *RHS);
+};
+
+/// MergeFunctions finds functions which will generate identical machine code,
+/// by considering all pointer types to be equivalent. Once identified,
+/// MergeFunctions will fold them by replacing a call to one to a call to a
+/// bitcast of the other.
+///
+class MergeFunctions : public ModulePass {
+public:
+  static char ID;
+  MergeFunctions() : ModulePass(ID) {}
+
+  bool runOnModule(Module &M);
+
+private:
+  typedef DenseSet<ComparableFunction *, MergeFunctionsEqualityInfo> FnSetType;
+
+
+  /// Insert a ComparableFunction into the FnSet, or merge it away if it's
+  /// equal to one that's already present.
+  bool Insert(FnSetType &FnSet, ComparableFunction *NewF);
+
+  /// MergeTwoFunctions - Merge two equivalent functions. Upon completion, G
+  /// may be deleted, or may be converted into a thunk. In either case, it
+  /// should never be visited again.
+  void MergeTwoFunctions(Function *F, Function *G) const;
+
+  /// WriteThunk - Replace G with a simple tail call to bitcast(F). Also
+  /// replace direct uses of G with bitcast(F). Deletes G.
+  void WriteThunk(Function *F, Function *G) const;
+
+  TargetData *TD;
+};
+
+}  // end anonymous namespace
+
 char MergeFunctions::ID = 0;
 INITIALIZE_PASS(MergeFunctions, "mergefunc", "Merge Functions", false, false);
 
@@ -475,7 +522,7 @@ bool FunctionComparator::Compare() {
 }
 
 /// WriteThunk - Replace G with a simple tail call to bitcast(F). Also replace
-/// direct uses of G with bitcast(F).
+/// direct uses of G with bitcast(F). Deletes G.
 void MergeFunctions::WriteThunk(Function *F, Function *G) const {
   if (!G->mayBeOverridden()) {
     // Redirect direct callers of G to F.
@@ -553,100 +600,138 @@ void MergeFunctions::MergeTwoFunctions(Function *F, Function *G) const {
   ++NumFunctionsMerged;
 }
 
-static unsigned ProfileFunction(const Function *F) {
-  const FunctionType *FTy = F->getFunctionType();
+// Insert - Insert a ComparableFunction into the FnSet, or merge it away if
+// equal to one that's already inserted.
+bool MergeFunctions::Insert(FnSetType &FnSet, ComparableFunction *NewF) {
+  std::pair<FnSetType::iterator, bool> Result = FnSet.insert(NewF);
+  if (Result.second)
+    return false;
 
-  FoldingSetNodeID ID;
-  ID.AddInteger(F->size());
-  ID.AddInteger(F->getCallingConv());
-  ID.AddBoolean(F->hasGC());
-  ID.AddBoolean(FTy->isVarArg());
-  ID.AddInteger(FTy->getReturnType()->getTypeID());
-  for (unsigned i = 0, e = FTy->getNumParams(); i != e; ++i)
-    ID.AddInteger(FTy->getParamType(i)->getTypeID());
-  return ID.ComputeHash();
+  ComparableFunction *OldF = *Result.first;
+  assert(OldF && "Expected a hash collision");
+
+  // Never thunk a strong function to a weak function.
+  assert(!OldF->Func->isWeakForLinker() || NewF->Func->isWeakForLinker());
+
+  DEBUG(dbgs() << "  " << OldF->Func->getName() << " == "
+               << NewF->Func->getName() << '\n');
+
+  Function *DeleteF = NewF->Func;
+  delete NewF;
+  MergeTwoFunctions(OldF->Func, DeleteF);
+  return true;
 }
 
-class ComparableFunction {
-public:
-  ComparableFunction(Function *Func, TargetData *TD)
-    : Func(Func), Hash(ProfileFunction(Func)), TD(TD) {}
+// IsThunk - This method determines whether or not a given Function is a thunk\// like the ones emitted by this pass and therefore not subject to further
+// merging.
+static bool IsThunk(const Function *F) {
+  // The safe direction to fail is to return true. In that case, the function
+  // will be removed from merging analysis. If we failed to including functions
+  // then we may try to merge unmergable thing (ie., identical weak functions)
+  // which will push us into an infinite loop.
 
-  AssertingVH<Function> const Func;
-  const unsigned Hash;
-  TargetData * const TD;
-};
+  if (F->size() != 1)
+    return false;
 
-struct MergeFunctionsEqualityInfo {
-  static ComparableFunction *getEmptyKey() {
-    return reinterpret_cast<ComparableFunction*>(0);
+  const BasicBlock *BB = &F->front();
+  // A thunk is:
+  //   bitcast-inst*
+  //   optional-reg tail call @thunkee(args...*)
+  //   ret void|optional-reg
+  // where the args are in the same order as the arguments.
+
+  // Verify that the sequence of bitcast-inst's are all casts of arguments and
+  // that there aren't any extras (ie. no repeated casts).
+  int LastArgNo = -1;
+  BasicBlock::const_iterator I = BB->begin();
+  while (const BitCastInst *BCI = dyn_cast<BitCastInst>(I)) {
+    const Argument *A = dyn_cast<Argument>(BCI->getOperand(0));
+    if (!A) return false;
+    if ((int)A->getArgNo() >= LastArgNo) return false;
+    LastArgNo = A->getArgNo();
+    ++I;
   }
-  static ComparableFunction *getTombstoneKey() {
-    return reinterpret_cast<ComparableFunction*>(-1);
+
+  // Verify that the call instruction has the same arguments as this function
+  // and that they're all either the incoming argument or a cast of the right
+  // argument.
+  const CallInst *CI = dyn_cast<CallInst>(I++);
+  if (!CI || !CI->isTailCall() ||
+      CI->getNumArgOperands() != F->arg_size()) return false;
+
+  for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i) {
+    const Value *V = CI->getArgOperand(i);
+    const Argument *A = dyn_cast<Argument>(V);
+    if (!A) {
+      const BitCastInst *BCI = dyn_cast<BitCastInst>(V);
+      if (!BCI) return false;
+      A = cast<Argument>(BCI->getOperand(0));
+    }
+    if (A->getArgNo() != i) return false;
   }
-  static unsigned getHashValue(const ComparableFunction *CF) {
-    return CF->Hash;
+
+  // Verify that the terminator is a ret void (if we're void) or a ret of the
+  // call's return, or a ret of a bitcast of the call's return.
+  const Value *RetOp = CI;
+  if (const BitCastInst *BCI = dyn_cast<BitCastInst>(I)) {
+    ++I;
+    if (BCI->getOperand(0) != CI) return false;
+    RetOp = BCI;
   }
-  static bool isEqual(const ComparableFunction *LHS,
-                      const ComparableFunction *RHS) {
-    if (LHS == RHS)
-      return true;
-    if (LHS == getEmptyKey() || LHS == getTombstoneKey() ||
-        RHS == getEmptyKey() || RHS == getTombstoneKey())
-      return false;
-    assert(LHS->TD == RHS->TD && "Comparing functions for different targets");
-    return FunctionComparator(LHS->TD, LHS->Func, RHS->Func).Compare();
-  }
-};
+  const ReturnInst *RI = dyn_cast<ReturnInst>(I);
+  if (!RI) return false;
+  if (RI->getNumOperands() == 0)
+    return CI->getType()->isVoidTy();
+  return RI->getReturnValue() == CI;
+}
 
 bool MergeFunctions::runOnModule(Module &M) {
-  typedef DenseSet<ComparableFunction *, MergeFunctionsEqualityInfo> FnSetType;
-
   bool Changed = false;
   TD = getAnalysisIfAvailable<TargetData>();
 
-  std::vector<Function *> Funcs;
-  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
-    if (!F->isDeclaration() && !F->hasAvailableExternallyLinkage())
-      Funcs.push_back(F);
-  }
-
   bool LocalChanged;
   do {
+    DEBUG(dbgs() << "size: " << M.size() << '\n');
     LocalChanged = false;
-
     FnSetType FnSet;
-    for (unsigned i = 0, e = Funcs.size(); i != e;) {
-      Function *F = Funcs[i];
-      ComparableFunction *NewF = new ComparableFunction(F, TD);
-      std::pair<FnSetType::iterator, bool> Result = FnSet.insert(NewF);
-      if (!Result.second) {
-        ComparableFunction *&OldF = *Result.first;
-        assert(OldF && "Expected a hash collision");
 
-        // NewF will be deleted in favour of OldF unless NewF is strong and
-        // OldF is weak in which case swap them to keep the strong definition.
+    // Insert only strong functions and merge them. Strong function merging
+    // always deletes one of them.
+    for (Module::iterator I = M.begin(), E = M.end(); I != E;) {
+      Function *F = I++;
+      if (!F->isDeclaration() && !F->hasAvailableExternallyLinkage() &&
+          !F->isWeakForLinker() && !IsThunk(F)) {
+        ComparableFunction *CF = new ComparableFunction(F, TD);
+        LocalChanged |= Insert(FnSet, CF);
+      }
+    }
 
-        if (OldF->Func->isWeakForLinker() && !NewF->Func->isWeakForLinker())
-          std::swap(OldF, NewF);
-
-        DEBUG(dbgs() << "  " << OldF->Func->getName() << " == "
-                     << NewF->Func->getName() << '\n');
-
-	Funcs.erase(Funcs.begin() + i);
-	--e;
-
-        Function *DeleteF = NewF->Func;
-        delete NewF;
-        MergeTwoFunctions(OldF->Func, DeleteF);
-	LocalChanged = true;
-        Changed = true;
-      } else {
-	++i;
+    // Insert only weak functions and merge them. By doing these second we
+    // create thunks to the strong function when possible. When two weak
+    // functions are identical, we create a new strong function with two weak
+    // weak thunks to it which are identical but not mergable.
+    for (Module::iterator I = M.begin(), E = M.end(); I != E;) {
+      Function *F = I++;
+      if (!F->isDeclaration() && !F->hasAvailableExternallyLinkage() &&
+          F->isWeakForLinker() && !IsThunk(F)) {
+        ComparableFunction *CF = new ComparableFunction(F, TD);
+        LocalChanged |= Insert(FnSet, CF);
       }
     }
     DeleteContainerPointers(FnSet);
+    Changed |= LocalChanged;
   } while (LocalChanged);
 
   return Changed;
 }
+
+bool MergeFunctionsEqualityInfo::isEqual(const ComparableFunction *LHS,
+                                         const ComparableFunction *RHS) {
+  if (LHS == RHS)
+    return true;
+  if (LHS == getEmptyKey() || LHS == getTombstoneKey() ||
+      RHS == getEmptyKey() || RHS == getTombstoneKey())
+    return false;
+  assert(LHS->TD == RHS->TD && "Comparing functions for different targets");
+  return FunctionComparator(LHS->TD, LHS->Func, RHS->Func).Compare();
+}