Import LLVM, at r72732.

1 files changed, 533 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/CloneFunction.cpp b/lib/Transforms/Utils/CloneFunction.cpp
new file mode 100644
index 0000000..d0fdefa
--- /dev/null
+++ b/lib/Transforms/Utils/CloneFunction.cpp
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+//===- CloneFunction.cpp - Clone a function into another function ---------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the CloneFunctionInto interface, which is used as the
+// low-level function cloner.  This is used by the CloneFunction and function
+// inliner to do the dirty work of copying the body of a function around.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Instructions.h"
+#include "llvm/IntrinsicInst.h"
+#include "llvm/GlobalVariable.h"
+#include "llvm/Function.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
+#include "llvm/Analysis/ConstantFolding.h"
+#include "llvm/Analysis/DebugInfo.h"
+#include "llvm/ADT/SmallVector.h"
+#include <map>
+using namespace llvm;
+
+// CloneBasicBlock - See comments in Cloning.h
+BasicBlock *llvm::CloneBasicBlock(const BasicBlock *BB,
+                                  DenseMap<const Value*, Value*> &ValueMap,
+                                  const char *NameSuffix, Function *F,
+                                  ClonedCodeInfo *CodeInfo) {
+  BasicBlock *NewBB = BasicBlock::Create("", F);
+  if (BB->hasName()) NewBB->setName(BB->getName()+NameSuffix);
+
+  bool hasCalls = false, hasDynamicAllocas = false, hasStaticAllocas = false;
+  
+  // Loop over all instructions, and copy them over.
+  for (BasicBlock::const_iterator II = BB->begin(), IE = BB->end();
+       II != IE; ++II) {
+    Instruction *NewInst = II->clone();
+    if (II->hasName())
+      NewInst->setName(II->getName()+NameSuffix);
+    NewBB->getInstList().push_back(NewInst);
+    ValueMap[II] = NewInst;                // Add instruction map to value.
+    
+    hasCalls |= (isa<CallInst>(II) && !isa<DbgInfoIntrinsic>(II));
+    if (const AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
+      if (isa<ConstantInt>(AI->getArraySize()))
+        hasStaticAllocas = true;
+      else
+        hasDynamicAllocas = true;
+    }
+  }
+  
+  if (CodeInfo) {
+    CodeInfo->ContainsCalls          |= hasCalls;
+    CodeInfo->ContainsUnwinds        |= isa<UnwindInst>(BB->getTerminator());
+    CodeInfo->ContainsDynamicAllocas |= hasDynamicAllocas;
+    CodeInfo->ContainsDynamicAllocas |= hasStaticAllocas && 
+                                        BB != &BB->getParent()->getEntryBlock();
+  }
+  return NewBB;
+}
+
+// Clone OldFunc into NewFunc, transforming the old arguments into references to
+// ArgMap values.
+//
+void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
+                             DenseMap<const Value*, Value*> &ValueMap,
+                             std::vector<ReturnInst*> &Returns,
+                             const char *NameSuffix, ClonedCodeInfo *CodeInfo) {
+  assert(NameSuffix && "NameSuffix cannot be null!");
+
+#ifndef NDEBUG
+  for (Function::const_arg_iterator I = OldFunc->arg_begin(), 
+       E = OldFunc->arg_end(); I != E; ++I)
+    assert(ValueMap.count(I) && "No mapping from source argument specified!");
+#endif
+
+  // Clone any attributes.
+  if (NewFunc->arg_size() == OldFunc->arg_size())
+    NewFunc->copyAttributesFrom(OldFunc);
+  else {
+    //Some arguments were deleted with the ValueMap. Copy arguments one by one
+    for (Function::const_arg_iterator I = OldFunc->arg_begin(), 
+           E = OldFunc->arg_end(); I != E; ++I)
+      if (Argument* Anew = dyn_cast<Argument>(ValueMap[I]))
+        Anew->addAttr( OldFunc->getAttributes()
+                       .getParamAttributes(I->getArgNo() + 1));
+    NewFunc->setAttributes(NewFunc->getAttributes()
+                           .addAttr(0, OldFunc->getAttributes()
+                                     .getRetAttributes()));
+    NewFunc->setAttributes(NewFunc->getAttributes()
+                           .addAttr(~0, OldFunc->getAttributes()
+                                     .getFnAttributes()));
+
+  }
+
+  // Loop over all of the basic blocks in the function, cloning them as
+  // appropriate.  Note that we save BE this way in order to handle cloning of
+  // recursive functions into themselves.
+  //
+  for (Function::const_iterator BI = OldFunc->begin(), BE = OldFunc->end();
+       BI != BE; ++BI) {
+    const BasicBlock &BB = *BI;
+
+    // Create a new basic block and copy instructions into it!
+    BasicBlock *CBB = CloneBasicBlock(&BB, ValueMap, NameSuffix, NewFunc,
+                                      CodeInfo);
+    ValueMap[&BB] = CBB;                       // Add basic block mapping.
+
+    if (ReturnInst *RI = dyn_cast<ReturnInst>(CBB->getTerminator()))
+      Returns.push_back(RI);
+  }
+
+  // Loop over all of the instructions in the function, fixing up operand
+  // references as we go.  This uses ValueMap to do all the hard work.
+  //
+  for (Function::iterator BB = cast<BasicBlock>(ValueMap[OldFunc->begin()]),
+         BE = NewFunc->end(); BB != BE; ++BB)
+    // Loop over all instructions, fixing each one as we find it...
+    for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II)
+      RemapInstruction(II, ValueMap);
+}
+
+/// CloneFunction - Return a copy of the specified function, but without
+/// embedding the function into another module.  Also, any references specified
+/// in the ValueMap are changed to refer to their mapped value instead of the
+/// original one.  If any of the arguments to the function are in the ValueMap,
+/// the arguments are deleted from the resultant function.  The ValueMap is
+/// updated to include mappings from all of the instructions and basicblocks in
+/// the function from their old to new values.
+///
+Function *llvm::CloneFunction(const Function *F,
+                              DenseMap<const Value*, Value*> &ValueMap,
+                              ClonedCodeInfo *CodeInfo) {
+  std::vector<const Type*> ArgTypes;
+
+  // The user might be deleting arguments to the function by specifying them in
+  // the ValueMap.  If so, we need to not add the arguments to the arg ty vector
+  //
+  for (Function::const_arg_iterator I = F->arg_begin(), E = F->arg_end();
+       I != E; ++I)
+    if (ValueMap.count(I) == 0)  // Haven't mapped the argument to anything yet?
+      ArgTypes.push_back(I->getType());
+
+  // Create a new function type...
+  FunctionType *FTy = FunctionType::get(F->getFunctionType()->getReturnType(),
+                                    ArgTypes, F->getFunctionType()->isVarArg());
+
+  // Create the new function...
+  Function *NewF = Function::Create(FTy, F->getLinkage(), F->getName());
+
+  // Loop over the arguments, copying the names of the mapped arguments over...
+  Function::arg_iterator DestI = NewF->arg_begin();
+  for (Function::const_arg_iterator I = F->arg_begin(), E = F->arg_end();
+       I != E; ++I)
+    if (ValueMap.count(I) == 0) {   // Is this argument preserved?
+      DestI->setName(I->getName()); // Copy the name over...
+      ValueMap[I] = DestI++;        // Add mapping to ValueMap
+    }
+
+  std::vector<ReturnInst*> Returns;  // Ignore returns cloned...
+  CloneFunctionInto(NewF, F, ValueMap, Returns, "", CodeInfo);
+  return NewF;
+}
+
+
+
+namespace {
+  /// PruningFunctionCloner - This class is a private class used to implement
+  /// the CloneAndPruneFunctionInto method.
+  struct VISIBILITY_HIDDEN PruningFunctionCloner {
+    Function *NewFunc;
+    const Function *OldFunc;
+    DenseMap<const Value*, Value*> &ValueMap;
+    std::vector<ReturnInst*> &Returns;
+    const char *NameSuffix;
+    ClonedCodeInfo *CodeInfo;
+    const TargetData *TD;
+    Value *DbgFnStart;
+  public:
+    PruningFunctionCloner(Function *newFunc, const Function *oldFunc,
+                          DenseMap<const Value*, Value*> &valueMap,
+                          std::vector<ReturnInst*> &returns,
+                          const char *nameSuffix, 
+                          ClonedCodeInfo *codeInfo,
+                          const TargetData *td)
+    : NewFunc(newFunc), OldFunc(oldFunc), ValueMap(valueMap), Returns(returns),
+      NameSuffix(nameSuffix), CodeInfo(codeInfo), TD(td), DbgFnStart(NULL) {
+    }
+
+    /// CloneBlock - The specified block is found to be reachable, clone it and
+    /// anything that it can reach.
+    void CloneBlock(const BasicBlock *BB,
+                    std::vector<const BasicBlock*> &ToClone);
+    
+  public:
+    /// ConstantFoldMappedInstruction - Constant fold the specified instruction,
+    /// mapping its operands through ValueMap if they are available.
+    Constant *ConstantFoldMappedInstruction(const Instruction *I);
+  };
+}
+
+/// CloneBlock - The specified block is found to be reachable, clone it and
+/// anything that it can reach.
+void PruningFunctionCloner::CloneBlock(const BasicBlock *BB,
+                                       std::vector<const BasicBlock*> &ToClone){
+  Value *&BBEntry = ValueMap[BB];
+
+  // Have we already cloned this block?
+  if (BBEntry) return;
+  
+  // Nope, clone it now.
+  BasicBlock *NewBB;
+  BBEntry = NewBB = BasicBlock::Create();
+  if (BB->hasName()) NewBB->setName(BB->getName()+NameSuffix);
+
+  bool hasCalls = false, hasDynamicAllocas = false, hasStaticAllocas = false;
+  
+  // Loop over all instructions, and copy them over, DCE'ing as we go.  This
+  // loop doesn't include the terminator.
+  for (BasicBlock::const_iterator II = BB->begin(), IE = --BB->end();
+       II != IE; ++II) {
+    // If this instruction constant folds, don't bother cloning the instruction,
+    // instead, just add the constant to the value map.
+    if (Constant *C = ConstantFoldMappedInstruction(II)) {
+      ValueMap[II] = C;
+      continue;
+    }
+
+    // Do not clone llvm.dbg.region.end. It will be adjusted by the inliner.
+    if (const DbgFuncStartInst *DFSI = dyn_cast<DbgFuncStartInst>(II)) {
+      if (DbgFnStart == NULL) {
+        DISubprogram SP(cast<GlobalVariable>(DFSI->getSubprogram()));
+        if (SP.describes(BB->getParent()))
+          DbgFnStart = DFSI->getSubprogram();
+      }
+    } 
+    if (const DbgRegionEndInst *DREIS = dyn_cast<DbgRegionEndInst>(II)) {
+      if (DREIS->getContext() == DbgFnStart)
+        continue;
+    }
+      
+    Instruction *NewInst = II->clone();
+    if (II->hasName())
+      NewInst->setName(II->getName()+NameSuffix);
+    NewBB->getInstList().push_back(NewInst);
+    ValueMap[II] = NewInst;                // Add instruction map to value.
+    
+    hasCalls |= (isa<CallInst>(II) && !isa<DbgInfoIntrinsic>(II));
+    if (const AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
+      if (isa<ConstantInt>(AI->getArraySize()))
+        hasStaticAllocas = true;
+      else
+        hasDynamicAllocas = true;
+    }
+  }
+  
+  // Finally, clone over the terminator.
+  const TerminatorInst *OldTI = BB->getTerminator();
+  bool TerminatorDone = false;
+  if (const BranchInst *BI = dyn_cast<BranchInst>(OldTI)) {
+    if (BI->isConditional()) {
+      // If the condition was a known constant in the callee...
+      ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
+      // Or is a known constant in the caller...
+      if (Cond == 0)  
+        Cond = dyn_cast_or_null<ConstantInt>(ValueMap[BI->getCondition()]);
+
+      // Constant fold to uncond branch!
+      if (Cond) {
+        BasicBlock *Dest = BI->getSuccessor(!Cond->getZExtValue());
+        ValueMap[OldTI] = BranchInst::Create(Dest, NewBB);
+        ToClone.push_back(Dest);
+        TerminatorDone = true;
+      }
+    }
+  } else if (const SwitchInst *SI = dyn_cast<SwitchInst>(OldTI)) {
+    // If switching on a value known constant in the caller.
+    ConstantInt *Cond = dyn_cast<ConstantInt>(SI->getCondition());
+    if (Cond == 0)  // Or known constant after constant prop in the callee...
+      Cond = dyn_cast_or_null<ConstantInt>(ValueMap[SI->getCondition()]);
+    if (Cond) {     // Constant fold to uncond branch!
+      BasicBlock *Dest = SI->getSuccessor(SI->findCaseValue(Cond));
+      ValueMap[OldTI] = BranchInst::Create(Dest, NewBB);
+      ToClone.push_back(Dest);
+      TerminatorDone = true;
+    }
+  }
+  
+  if (!TerminatorDone) {
+    Instruction *NewInst = OldTI->clone();
+    if (OldTI->hasName())
+      NewInst->setName(OldTI->getName()+NameSuffix);
+    NewBB->getInstList().push_back(NewInst);
+    ValueMap[OldTI] = NewInst;             // Add instruction map to value.
+    
+    // Recursively clone any reachable successor blocks.
+    const TerminatorInst *TI = BB->getTerminator();
+    for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
+      ToClone.push_back(TI->getSuccessor(i));
+  }
+  
+  if (CodeInfo) {
+    CodeInfo->ContainsCalls          |= hasCalls;
+    CodeInfo->ContainsUnwinds        |= isa<UnwindInst>(OldTI);
+    CodeInfo->ContainsDynamicAllocas |= hasDynamicAllocas;
+    CodeInfo->ContainsDynamicAllocas |= hasStaticAllocas && 
+      BB != &BB->getParent()->front();
+  }
+  
+  if (ReturnInst *RI = dyn_cast<ReturnInst>(NewBB->getTerminator()))
+    Returns.push_back(RI);
+}
+
+/// ConstantFoldMappedInstruction - Constant fold the specified instruction,
+/// mapping its operands through ValueMap if they are available.
+Constant *PruningFunctionCloner::
+ConstantFoldMappedInstruction(const Instruction *I) {
+  SmallVector<Constant*, 8> Ops;
+  for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
+    if (Constant *Op = dyn_cast_or_null<Constant>(MapValue(I->getOperand(i),
+                                                           ValueMap)))
+      Ops.push_back(Op);
+    else
+      return 0;  // All operands not constant!
+
+  if (const CmpInst *CI = dyn_cast<CmpInst>(I))
+    return ConstantFoldCompareInstOperands(CI->getPredicate(),
+                                           &Ops[0], Ops.size(), TD);
+
+  if (const LoadInst *LI = dyn_cast<LoadInst>(I))
+    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ops[0]))
+      if (!LI->isVolatile() && CE->getOpcode() == Instruction::GetElementPtr)
+        if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getOperand(0)))
+          if (GV->isConstant() && GV->hasDefinitiveInitializer())
+            return ConstantFoldLoadThroughGEPConstantExpr(GV->getInitializer(),
+                                                          CE);
+
+  return ConstantFoldInstOperands(I->getOpcode(), I->getType(), &Ops[0],
+                                  Ops.size(), TD);
+}
+
+/// CloneAndPruneFunctionInto - This works exactly like CloneFunctionInto,
+/// except that it does some simple constant prop and DCE on the fly.  The
+/// effect of this is to copy significantly less code in cases where (for
+/// example) a function call with constant arguments is inlined, and those
+/// constant arguments cause a significant amount of code in the callee to be
+/// dead.  Since this doesn't produce an exact copy of the input, it can't be
+/// used for things like CloneFunction or CloneModule.
+void llvm::CloneAndPruneFunctionInto(Function *NewFunc, const Function *OldFunc,
+                                     DenseMap<const Value*, Value*> &ValueMap,
+                                     std::vector<ReturnInst*> &Returns,
+                                     const char *NameSuffix, 
+                                     ClonedCodeInfo *CodeInfo,
+                                     const TargetData *TD) {
+  assert(NameSuffix && "NameSuffix cannot be null!");
+  
+#ifndef NDEBUG
+  for (Function::const_arg_iterator II = OldFunc->arg_begin(), 
+       E = OldFunc->arg_end(); II != E; ++II)
+    assert(ValueMap.count(II) && "No mapping from source argument specified!");
+#endif
+
+  PruningFunctionCloner PFC(NewFunc, OldFunc, ValueMap, Returns,
+                            NameSuffix, CodeInfo, TD);
+
+  // Clone the entry block, and anything recursively reachable from it.
+  std::vector<const BasicBlock*> CloneWorklist;
+  CloneWorklist.push_back(&OldFunc->getEntryBlock());
+  while (!CloneWorklist.empty()) {
+    const BasicBlock *BB = CloneWorklist.back();
+    CloneWorklist.pop_back();
+    PFC.CloneBlock(BB, CloneWorklist);
+  }
+  
+  // Loop over all of the basic blocks in the old function.  If the block was
+  // reachable, we have cloned it and the old block is now in the value map:
+  // insert it into the new function in the right order.  If not, ignore it.
+  //
+  // Defer PHI resolution until rest of function is resolved.
+  std::vector<const PHINode*> PHIToResolve;
+  for (Function::const_iterator BI = OldFunc->begin(), BE = OldFunc->end();
+       BI != BE; ++BI) {
+    BasicBlock *NewBB = cast_or_null<BasicBlock>(ValueMap[BI]);
+    if (NewBB == 0) continue;  // Dead block.
+
+    // Add the new block to the new function.
+    NewFunc->getBasicBlockList().push_back(NewBB);
+    
+    // Loop over all of the instructions in the block, fixing up operand
+    // references as we go.  This uses ValueMap to do all the hard work.
+    //
+    BasicBlock::iterator I = NewBB->begin();
+    
+    // Handle PHI nodes specially, as we have to remove references to dead
+    // blocks.
+    if (PHINode *PN = dyn_cast<PHINode>(I)) {
+      // Skip over all PHI nodes, remembering them for later.
+      BasicBlock::const_iterator OldI = BI->begin();
+      for (; (PN = dyn_cast<PHINode>(I)); ++I, ++OldI)
+        PHIToResolve.push_back(cast<PHINode>(OldI));
+    }
+    
+    // Otherwise, remap the rest of the instructions normally.
+    for (; I != NewBB->end(); ++I)
+      RemapInstruction(I, ValueMap);
+  }
+  
+  // Defer PHI resolution until rest of function is resolved, PHI resolution
+  // requires the CFG to be up-to-date.
+  for (unsigned phino = 0, e = PHIToResolve.size(); phino != e; ) {
+    const PHINode *OPN = PHIToResolve[phino];
+    unsigned NumPreds = OPN->getNumIncomingValues();
+    const BasicBlock *OldBB = OPN->getParent();
+    BasicBlock *NewBB = cast<BasicBlock>(ValueMap[OldBB]);
+
+    // Map operands for blocks that are live and remove operands for blocks
+    // that are dead.
+    for (; phino != PHIToResolve.size() &&
+         PHIToResolve[phino]->getParent() == OldBB; ++phino) {
+      OPN = PHIToResolve[phino];
+      PHINode *PN = cast<PHINode>(ValueMap[OPN]);
+      for (unsigned pred = 0, e = NumPreds; pred != e; ++pred) {
+        if (BasicBlock *MappedBlock = 
+            cast_or_null<BasicBlock>(ValueMap[PN->getIncomingBlock(pred)])) {
+          Value *InVal = MapValue(PN->getIncomingValue(pred), ValueMap);
+          assert(InVal && "Unknown input value?");
+          PN->setIncomingValue(pred, InVal);
+          PN->setIncomingBlock(pred, MappedBlock);
+        } else {
+          PN->removeIncomingValue(pred, false);
+          --pred, --e;  // Revisit the next entry.
+        }
+      } 
+    }
+    
+    // The loop above has removed PHI entries for those blocks that are dead
+    // and has updated others.  However, if a block is live (i.e. copied over)
+    // but its terminator has been changed to not go to this block, then our
+    // phi nodes will have invalid entries.  Update the PHI nodes in this
+    // case.
+    PHINode *PN = cast<PHINode>(NewBB->begin());
+    NumPreds = std::distance(pred_begin(NewBB), pred_end(NewBB));
+    if (NumPreds != PN->getNumIncomingValues()) {
+      assert(NumPreds < PN->getNumIncomingValues());
+      // Count how many times each predecessor comes to this block.
+      std::map<BasicBlock*, unsigned> PredCount;
+      for (pred_iterator PI = pred_begin(NewBB), E = pred_end(NewBB);
+           PI != E; ++PI)
+        --PredCount[*PI];
+      
+      // Figure out how many entries to remove from each PHI.
+      for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+        ++PredCount[PN->getIncomingBlock(i)];
+      
+      // At this point, the excess predecessor entries are positive in the
+      // map.  Loop over all of the PHIs and remove excess predecessor
+      // entries.
+      BasicBlock::iterator I = NewBB->begin();
+      for (; (PN = dyn_cast<PHINode>(I)); ++I) {
+        for (std::map<BasicBlock*, unsigned>::iterator PCI =PredCount.begin(),
+             E = PredCount.end(); PCI != E; ++PCI) {
+          BasicBlock *Pred     = PCI->first;
+          for (unsigned NumToRemove = PCI->second; NumToRemove; --NumToRemove)
+            PN->removeIncomingValue(Pred, false);
+        }
+      }
+    }
+    
+    // If the loops above have made these phi nodes have 0 or 1 operand,
+    // replace them with undef or the input value.  We must do this for
+    // correctness, because 0-operand phis are not valid.
+    PN = cast<PHINode>(NewBB->begin());
+    if (PN->getNumIncomingValues() == 0) {
+      BasicBlock::iterator I = NewBB->begin();
+      BasicBlock::const_iterator OldI = OldBB->begin();
+      while ((PN = dyn_cast<PHINode>(I++))) {
+        Value *NV = UndefValue::get(PN->getType());
+        PN->replaceAllUsesWith(NV);
+        assert(ValueMap[OldI] == PN && "ValueMap mismatch");
+        ValueMap[OldI] = NV;
+        PN->eraseFromParent();
+        ++OldI;
+      }
+    }
+    // NOTE: We cannot eliminate single entry phi nodes here, because of
+    // ValueMap.  Single entry phi nodes can have multiple ValueMap entries
+    // pointing at them.  Thus, deleting one would require scanning the ValueMap
+    // to update any entries in it that would require that.  This would be
+    // really slow.
+  }
+  
+  // Now that the inlined function body has been fully constructed, go through
+  // and zap unconditional fall-through branches.  This happen all the time when
+  // specializing code: code specialization turns conditional branches into
+  // uncond branches, and this code folds them.
+  Function::iterator I = cast<BasicBlock>(ValueMap[&OldFunc->getEntryBlock()]);
+  while (I != NewFunc->end()) {
+    BranchInst *BI = dyn_cast<BranchInst>(I->getTerminator());
+    if (!BI || BI->isConditional()) { ++I; continue; }
+    
+    // Note that we can't eliminate uncond branches if the destination has
+    // single-entry PHI nodes.  Eliminating the single-entry phi nodes would
+    // require scanning the ValueMap to update any entries that point to the phi
+    // node.
+    BasicBlock *Dest = BI->getSuccessor(0);
+    if (!Dest->getSinglePredecessor() || isa<PHINode>(Dest->begin())) {
+      ++I; continue;
+    }
+    
+    // We know all single-entry PHI nodes in the inlined function have been
+    // removed, so we just need to splice the blocks.
+    BI->eraseFromParent();
+    
+    // Move all the instructions in the succ to the pred.
+    I->getInstList().splice(I->end(), Dest->getInstList());
+    
+    // Make all PHI nodes that referred to Dest now refer to I as their source.
+    Dest->replaceAllUsesWith(I);
+
+    // Remove the dest block.
+    Dest->eraseFromParent();
+    
+    // Do not increment I, iteratively merge all things this block branches to.
+  }
+}