tools/llvm-diff/DifferenceEngine.cpp - fp2-dev/platform/external/llvm - Gitiles

 //===-- DifferenceEngine.cpp - Structural function/module comparison ------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This header defines the implementation of the LLVM difference
 // engine, which structurally compares global values within a module.
 //
 //===----------------------------------------------------------------------===//

 #include "DifferenceEngine.h"

 #include "llvm/Constants.h"
 #include "llvm/Function.h"
 #include "llvm/Instructions.h"
 #include "llvm/Module.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/Support/CallSite.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/type_traits.h"

 #include <utility>

 using namespace llvm;

 namespace {

 /// A priority queue, implemented as a heap.
 template <class T, class Sorter, unsigned InlineCapacity>
 class PriorityQueue {
   Sorter Precedes;
   llvm::SmallVector<T, InlineCapacity> Storage;

 public:
   PriorityQueue(const Sorter &Precedes) : Precedes(Precedes) {}

   /// Checks whether the heap is empty.
   bool empty() const { return Storage.empty(); }

   /// Insert a new value on the heap.
   void insert(const T &V) {
     unsigned Index = Storage.size();
     Storage.push_back(V);
     if (Index == 0) return;

     T *data = Storage.data();
     while (true) {
       unsigned Target = (Index + 1) / 2 - 1;
       if (!Precedes(data[Index], data[Target])) return;
       std::swap(data[Index], data[Target]);
       if (Target == 0) return;
       Index = Target;
     }
   }

   /// Remove the minimum value in the heap.  Only valid on a non-empty heap.
   T remove_min() {
     assert(!empty());
     T tmp = Storage[0];

     unsigned NewSize = Storage.size() - 1;
     if (NewSize) {
       // Move the slot at the end to the beginning.
       if (isPodLike<T>::value)
         Storage[0] = Storage[NewSize];
       else
         std::swap(Storage[0], Storage[NewSize]);

       // Bubble the root up as necessary.
       unsigned Index = 0;
       while (true) {
         // With a 1-based index, the children would be Index*2 and Index*2+1.
         unsigned R = (Index + 1) * 2;
         unsigned L = R - 1;

         // If R is out of bounds, we're done after this in any case.
         if (R >= NewSize) {
           // If L is also out of bounds, we're done immediately.
           if (L >= NewSize) break;

           // Otherwise, test whether we should swap L and Index.
           if (Precedes(Storage[L], Storage[Index]))
             std::swap(Storage[L], Storage[Index]);
           break;
         }

         // Otherwise, we need to compare with the smaller of L and R.
         // Prefer R because it's closer to the end of the array.
         unsigned IndexToTest = (Precedes(Storage[L], Storage[R]) ? L : R);

         // If Index is >= the min of L and R, then heap ordering is restored.
         if (!Precedes(Storage[IndexToTest], Storage[Index]))
           break;

         // Otherwise, keep bubbling up.
         std::swap(Storage[IndexToTest], Storage[Index]);
         Index = IndexToTest;
       }
     }
     Storage.pop_back();

     return tmp;
   }
 };

 /// A function-scope difference engine.
 class FunctionDifferenceEngine {
   DifferenceEngine &Engine;

   /// The current mapping from old local values to new local values.
   DenseMap<Value*, Value*> Values;

   /// The current mapping from old blocks to new blocks.
   DenseMap<BasicBlock*, BasicBlock*> Blocks;

   DenseSet<std::pair<Value*, Value*> > TentativeValues;

   unsigned getUnprocPredCount(BasicBlock *Block) const {
     unsigned Count = 0;
     for (pred_iterator I = pred_begin(Block), E = pred_end(Block); I != E; ++I)
       if (!Blocks.count(*I)) Count++;
     return Count;
   }

   typedef std::pair<BasicBlock*, BasicBlock*> BlockPair;

   /// A type which sorts a priority queue by the number of unprocessed
   /// predecessor blocks it has remaining.
   ///
   /// This is actually really expensive to calculate.
   struct QueueSorter {
     const FunctionDifferenceEngine &fde;
     explicit QueueSorter(const FunctionDifferenceEngine &fde) : fde(fde) {}

     bool operator()(const BlockPair &Old, const BlockPair &New) {
       return fde.getUnprocPredCount(Old.first)
            < fde.getUnprocPredCount(New.first);
     }
   };

   /// A queue of unified blocks to process.
   PriorityQueue<BlockPair, QueueSorter, 20> Queue;

   /// Try to unify the given two blocks.  Enqueues them for processing
   /// if they haven't already been processed.
   ///
   /// Returns true if there was a problem unifying them.
   bool tryUnify(BasicBlock *L, BasicBlock *R) {
     BasicBlock *&Ref = Blocks[L];

     if (Ref) {
       if (Ref == R) return false;

       Engine.logf("successor %l cannot be equivalent to %r; "
                   "it's already equivalent to %r")
         << L << R << Ref;
       return true;
     }

     Ref = R;
     Queue.insert(BlockPair(L, R));
     return false;
   }

   /// Unifies two instructions, given that they're known not to have
   /// structural differences.
   void unify(Instruction *L, Instruction *R) {
     DifferenceEngine::Context C(Engine, L, R);

     bool Result = diff(L, R, true, true);
     assert(!Result && "structural differences second time around?");
     (void) Result;
     if (!L->use_empty())
       Values[L] = R;
   }

   void processQueue() {
     while (!Queue.empty()) {
       BlockPair Pair = Queue.remove_min();
       diff(Pair.first, Pair.second);
     }
   }

   void diff(BasicBlock *L, BasicBlock *R) {
     DifferenceEngine::Context C(Engine, L, R);

     BasicBlock::iterator LI = L->begin(), LE = L->end();
     BasicBlock::iterator RI = R->begin();

     llvm::SmallVector<std::pair<Instruction*,Instruction*>, 20> TentativePairs;

     do {
       assert(LI != LE && RI != R->end());
       Instruction *LeftI = &*LI, *RightI = &*RI;

       // If the instructions differ, start the more sophisticated diff
       // algorithm at the start of the block.
       if (diff(LeftI, RightI, false, false)) {
         TentativeValues.clear();
         return runBlockDiff(L->begin(), R->begin());
       }

       // Otherwise, tentatively unify them.
       if (!LeftI->use_empty())
         TentativeValues.insert(std::make_pair(LeftI, RightI));

       ++LI, ++RI;
     } while (LI != LE); // This is sufficient: we can't get equality of
                         // terminators if there are residual instructions.

     // Unify everything in the block, non-tentatively this time.
     TentativeValues.clear();
     for (LI = L->begin(), RI = R->begin(); LI != LE; ++LI, ++RI)
       unify(&*LI, &*RI);
   }

   bool matchForBlockDiff(Instruction *L, Instruction *R);
   void runBlockDiff(BasicBlock::iterator LI, BasicBlock::iterator RI);

   bool diffCallSites(CallSite L, CallSite R, bool Complain) {
     // FIXME: call attributes
     if (!equivalentAsOperands(L.getCalledValue(), R.getCalledValue())) {
       if (Complain) Engine.log("called functions differ");
       return true;
     }
     if (L.arg_size() != R.arg_size()) {
       if (Complain) Engine.log("argument counts differ");
       return true;
     }
     for (unsigned I = 0, E = L.arg_size(); I != E; ++I)
       if (!equivalentAsOperands(L.getArgument(I), R.getArgument(I))) {
         if (Complain)
           Engine.logf("arguments %l and %r differ")
             << L.getArgument(I) << R.getArgument(I);
         return true;
       }
     return false;
   }

   bool diff(Instruction *L, Instruction *R, bool Complain, bool TryUnify) {
     // FIXME: metadata (if Complain is set)

     // Different opcodes always imply different operations.
     if (L->getOpcode() != R->getOpcode()) {
       if (Complain) Engine.log("different instruction types");
       return true;
     }

     if (isa<CmpInst>(L)) {
       if (cast<CmpInst>(L)->getPredicate()
             != cast<CmpInst>(R)->getPredicate()) {
         if (Complain) Engine.log("different predicates");
         return true;
       }
     } else if (isa<CallInst>(L)) {
       return diffCallSites(CallSite(L), CallSite(R), Complain);
     } else if (isa<PHINode>(L)) {
       // FIXME: implement.

       // This is really weird;  type uniquing is broken?
       if (L->getType() != R->getType()) {
         if (!L->getType()->isPointerTy() || !R->getType()->isPointerTy()) {
           if (Complain) Engine.log("different phi types");
           return true;
         }
       }
       return false;

     // Terminators.
     } else if (isa<InvokeInst>(L)) {
       InvokeInst *LI = cast<InvokeInst>(L);
       InvokeInst *RI = cast<InvokeInst>(R);
       if (diffCallSites(CallSite(LI), CallSite(RI), Complain))
         return true;

       if (TryUnify) {
         tryUnify(LI->getNormalDest(), RI->getNormalDest());
         tryUnify(LI->getUnwindDest(), RI->getUnwindDest());
       }
       return false;

     } else if (isa<BranchInst>(L)) {
       BranchInst *LI = cast<BranchInst>(L);
       BranchInst *RI = cast<BranchInst>(R);
       if (LI->isConditional() != RI->isConditional()) {
         if (Complain) Engine.log("branch conditionality differs");
         return true;
       }

       if (LI->isConditional()) {
         if (!equivalentAsOperands(LI->getCondition(), RI->getCondition())) {
           if (Complain) Engine.log("branch conditions differ");
           return true;
         }
         if (TryUnify) tryUnify(LI->getSuccessor(1), RI->getSuccessor(1));
       }
       if (TryUnify) tryUnify(LI->getSuccessor(0), RI->getSuccessor(0));
       return false;

     } else if (isa<SwitchInst>(L)) {
       SwitchInst *LI = cast<SwitchInst>(L);
       SwitchInst *RI = cast<SwitchInst>(R);
       if (!equivalentAsOperands(LI->getCondition(), RI->getCondition())) {
         if (Complain) Engine.log("switch conditions differ");
         return true;
       }
       if (TryUnify) tryUnify(LI->getDefaultDest(), RI->getDefaultDest());

       bool Difference = false;

       DenseMap<ConstantInt*,BasicBlock*> LCases;

       for (SwitchInst::CaseIt I = LI->case_begin(), E = LI->case_end();
            I != E; ++I)
         LCases[I.getCaseValue()] = I.getCaseSuccessor();

       for (SwitchInst::CaseIt I = RI->case_begin(), E = RI->case_end();
            I != E; ++I) {
         ConstantInt *CaseValue = I.getCaseValue();
         BasicBlock *LCase = LCases[CaseValue];
         if (LCase) {
           if (TryUnify) tryUnify(LCase, I.getCaseSuccessor());
           LCases.erase(CaseValue);
         } else if (Complain || !Difference) {
           if (Complain)
             Engine.logf("right switch has extra case %r") << CaseValue;
           Difference = true;
         }
       }
       if (!Difference)
         for (DenseMap<ConstantInt*,BasicBlock*>::iterator
                I = LCases.begin(), E = LCases.end(); I != E; ++I) {
           if (Complain)
             Engine.logf("left switch has extra case %l") << I->first;
           Difference = true;
         }
       return Difference;
     } else if (isa<UnreachableInst>(L)) {
       return false;
     }

     if (L->getNumOperands() != R->getNumOperands()) {
       if (Complain) Engine.log("instructions have different operand counts");
       return true;
     }

     for (unsigned I = 0, E = L->getNumOperands(); I != E; ++I) {
       Value *LO = L->getOperand(I), *RO = R->getOperand(I);
       if (!equivalentAsOperands(LO, RO)) {
         if (Complain) Engine.logf("operands %l and %r differ") << LO << RO;
         return true;
       }
     }

     return false;
   }

   bool equivalentAsOperands(Constant *L, Constant *R) {
     // Use equality as a preliminary filter.
     if (L == R)
       return true;

     if (L->getValueID() != R->getValueID())
       return false;

     // Ask the engine about global values.
     if (isa<GlobalValue>(L))
       return Engine.equivalentAsOperands(cast<GlobalValue>(L),
                                          cast<GlobalValue>(R));

     // Compare constant expressions structurally.
     if (isa<ConstantExpr>(L))
       return equivalentAsOperands(cast<ConstantExpr>(L),
                                   cast<ConstantExpr>(R));

     // Nulls of the "same type" don't always actually have the same
     // type; I don't know why.  Just white-list them.
     if (isa<ConstantPointerNull>(L))
       return true;

     // Block addresses only match if we've already encountered the
     // block.  FIXME: tentative matches?
     if (isa<BlockAddress>(L))
       return Blocks[cast<BlockAddress>(L)->getBasicBlock()]
                  == cast<BlockAddress>(R)->getBasicBlock();

     return false;
   }

   bool equivalentAsOperands(ConstantExpr *L, ConstantExpr *R) {
     if (L == R)
       return true;
     if (L->getOpcode() != R->getOpcode())
       return false;

     switch (L->getOpcode()) {
     case Instruction::ICmp:
     case Instruction::FCmp:
       if (L->getPredicate() != R->getPredicate())
         return false;
       break;

     case Instruction::GetElementPtr:
       // FIXME: inbounds?
       break;

     default:
       break;
     }

     if (L->getNumOperands() != R->getNumOperands())
       return false;

     for (unsigned I = 0, E = L->getNumOperands(); I != E; ++I)
       if (!equivalentAsOperands(L->getOperand(I), R->getOperand(I)))
         return false;

     return true;
   }

   bool equivalentAsOperands(Value *L, Value *R) {
     // Fall out if the values have different kind.
     // This possibly shouldn't take priority over oracles.
     if (L->getValueID() != R->getValueID())
       return false;

     // Value subtypes:  Argument, Constant, Instruction, BasicBlock,
     //                  InlineAsm, MDNode, MDString, PseudoSourceValue

     if (isa<Constant>(L))
       return equivalentAsOperands(cast<Constant>(L), cast<Constant>(R));

     if (isa<Instruction>(L))
       return Values[L] == R || TentativeValues.count(std::make_pair(L, R));

     if (isa<Argument>(L))
       return Values[L] == R;

     if (isa<BasicBlock>(L))
       return Blocks[cast<BasicBlock>(L)] != R;

     // Pretend everything else is identical.
     return true;
   }

   // Avoid a gcc warning about accessing 'this' in an initializer.
   FunctionDifferenceEngine *this_() { return this; }

 public:
   FunctionDifferenceEngine(DifferenceEngine &Engine) :
     Engine(Engine), Queue(QueueSorter(*this_())) {}

   void diff(Function *L, Function *R) {
     if (L->arg_size() != R->arg_size())
       Engine.log("different argument counts");

     // Map the arguments.
     for (Function::arg_iterator
            LI = L->arg_begin(), LE = L->arg_end(),
            RI = R->arg_begin(), RE = R->arg_end();
          LI != LE && RI != RE; ++LI, ++RI)
       Values[&*LI] = &*RI;

     tryUnify(&*L->begin(), &*R->begin());
     processQueue();
   }
 };

 struct DiffEntry {
   DiffEntry() : Cost(0) {}

   unsigned Cost;
   llvm::SmallVector<char, 8> Path; // actually of DifferenceEngine::DiffChange
 };

 bool FunctionDifferenceEngine::matchForBlockDiff(Instruction *L,
                                                  Instruction *R) {
   return !diff(L, R, false, false);
 }

 void FunctionDifferenceEngine::runBlockDiff(BasicBlock::iterator LStart,
                                             BasicBlock::iterator RStart) {
   BasicBlock::iterator LE = LStart->getParent()->end();
   BasicBlock::iterator RE = RStart->getParent()->end();

   unsigned NL = std::distance(LStart, LE);

   SmallVector<DiffEntry, 20> Paths1(NL+1);
   SmallVector<DiffEntry, 20> Paths2(NL+1);

   DiffEntry *Cur = Paths1.data();
   DiffEntry *Next = Paths2.data();

   const unsigned LeftCost = 2;
   const unsigned RightCost = 2;
   const unsigned MatchCost = 0;

   assert(TentativeValues.empty());

   // Initialize the first column.
   for (unsigned I = 0; I != NL+1; ++I) {
     Cur[I].Cost = I * LeftCost;
     for (unsigned J = 0; J != I; ++J)
       Cur[I].Path.push_back(DC_left);
   }

   for (BasicBlock::iterator RI = RStart; RI != RE; ++RI) {
     // Initialize the first row.
     Next[0] = Cur[0];
     Next[0].Cost += RightCost;
     Next[0].Path.push_back(DC_right);

     unsigned Index = 1;
     for (BasicBlock::iterator LI = LStart; LI != LE; ++LI, ++Index) {
       if (matchForBlockDiff(&*LI, &*RI)) {
         Next[Index] = Cur[Index-1];
         Next[Index].Cost += MatchCost;
         Next[Index].Path.push_back(DC_match);
         TentativeValues.insert(std::make_pair(&*LI, &*RI));
       } else if (Next[Index-1].Cost <= Cur[Index].Cost) {
         Next[Index] = Next[Index-1];
         Next[Index].Cost += LeftCost;
         Next[Index].Path.push_back(DC_left);
       } else {
         Next[Index] = Cur[Index];
         Next[Index].Cost += RightCost;
         Next[Index].Path.push_back(DC_right);
       }
     }

     std::swap(Cur, Next);
   }

   // We don't need the tentative values anymore; everything from here
   // on out should be non-tentative.
   TentativeValues.clear();

   SmallVectorImpl<char> &Path = Cur[NL].Path;
   BasicBlock::iterator LI = LStart, RI = RStart;

   DiffLogBuilder Diff(Engine.getConsumer());

   // Drop trailing matches.
   while (Path.back() == DC_match)
     Path.pop_back();

   // Skip leading matches.
   SmallVectorImpl<char>::iterator
     PI = Path.begin(), PE = Path.end();
   while (PI != PE && *PI == DC_match) {
     unify(&*LI, &*RI);
     ++PI, ++LI, ++RI;
   }

   for (; PI != PE; ++PI) {
     switch (static_cast<DiffChange>(*PI)) {
     case DC_match:
       assert(LI != LE && RI != RE);
       {
         Instruction *L = &*LI, *R = &*RI;
         unify(L, R);
         Diff.addMatch(L, R);
       }
       ++LI; ++RI;
       break;

     case DC_left:
       assert(LI != LE);
       Diff.addLeft(&*LI);
       ++LI;
       break;

     case DC_right:
       assert(RI != RE);
       Diff.addRight(&*RI);
       ++RI;
       break;
     }
   }

   // Finishing unifying and complaining about the tails of the block,
   // which should be matches all the way through.
   while (LI != LE) {
     assert(RI != RE);
     unify(&*LI, &*RI);
     ++LI, ++RI;
   }

   // If the terminators have different kinds, but one is an invoke and the
   // other is an unconditional branch immediately following a call, unify
   // the results and the destinations.
   TerminatorInst *LTerm = LStart->getParent()->getTerminator();
   TerminatorInst *RTerm = RStart->getParent()->getTerminator();
   if (isa<BranchInst>(LTerm) && isa<InvokeInst>(RTerm)) {
     if (cast<BranchInst>(LTerm)->isConditional()) return;
     BasicBlock::iterator I = LTerm;
     if (I == LStart->getParent()->begin()) return;
     --I;
     if (!isa<CallInst>(*I)) return;
     CallInst *LCall = cast<CallInst>(&*I);
     InvokeInst *RInvoke = cast<InvokeInst>(RTerm);
     if (!equivalentAsOperands(LCall->getCalledValue(), RInvoke->getCalledValue()))
       return;
     if (!LCall->use_empty())
       Values[LCall] = RInvoke;
     tryUnify(LTerm->getSuccessor(0), RInvoke->getNormalDest());
   } else if (isa<InvokeInst>(LTerm) && isa<BranchInst>(RTerm)) {
     if (cast<BranchInst>(RTerm)->isConditional()) return;
     BasicBlock::iterator I = RTerm;
     if (I == RStart->getParent()->begin()) return;
     --I;
     if (!isa<CallInst>(*I)) return;
     CallInst *RCall = cast<CallInst>(I);
     InvokeInst *LInvoke = cast<InvokeInst>(LTerm);
     if (!equivalentAsOperands(LInvoke->getCalledValue(), RCall->getCalledValue()))
       return;
     if (!LInvoke->use_empty())
       Values[LInvoke] = RCall;
     tryUnify(LInvoke->getNormalDest(), RTerm->getSuccessor(0));
   }
 }

 }

 void DifferenceEngine::Oracle::anchor() { }

 void DifferenceEngine::diff(Function *L, Function *R) {
   Context C(*this, L, R);

   // FIXME: types
   // FIXME: attributes and CC
   // FIXME: parameter attributes

   // If both are declarations, we're done.
   if (L->empty() && R->empty())
     return;
   else if (L->empty())
     log("left function is declaration, right function is definition");
   else if (R->empty())
     log("right function is declaration, left function is definition");
   else
     FunctionDifferenceEngine(*this).diff(L, R);
 }

 void DifferenceEngine::diff(Module *L, Module *R) {
   StringSet<> LNames;
   SmallVector<std::pair<Function*,Function*>, 20> Queue;

   for (Module::iterator I = L->begin(), E = L->end(); I != E; ++I) {
     Function *LFn = &*I;
     LNames.insert(LFn->getName());

     if (Function *RFn = R->getFunction(LFn->getName()))
       Queue.push_back(std::make_pair(LFn, RFn));
     else
       logf("function %l exists only in left module") << LFn;
   }

   for (Module::iterator I = R->begin(), E = R->end(); I != E; ++I) {
     Function *RFn = &*I;
     if (!LNames.count(RFn->getName()))
       logf("function %r exists only in right module") << RFn;
   }

   for (SmallVectorImpl<std::pair<Function*,Function*> >::iterator
          I = Queue.begin(), E = Queue.end(); I != E; ++I)
     diff(I->first, I->second);
 }

 bool DifferenceEngine::equivalentAsOperands(GlobalValue *L, GlobalValue *R) {
   if (globalValueOracle) return (*globalValueOracle)(L, R);
   return L->getName() == R->getName();
 }
	//===-- DifferenceEngine.cpp - Structural function/module comparison ------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This header defines the implementation of the LLVM difference
	// engine, which structurally compares global values within a module.
	//
	//===----------------------------------------------------------------------===//

	#include "DifferenceEngine.h"

	#include "llvm/Constants.h"
	#include "llvm/Function.h"
	#include "llvm/Instructions.h"
	#include "llvm/Module.h"
	#include "llvm/ADT/DenseMap.h"
	#include "llvm/ADT/DenseSet.h"
	#include "llvm/ADT/SmallVector.h"
	#include "llvm/ADT/StringRef.h"
	#include "llvm/ADT/StringSet.h"
	#include "llvm/Support/CallSite.h"
	#include "llvm/Support/CFG.h"
	#include "llvm/Support/ErrorHandling.h"
	#include "llvm/Support/raw_ostream.h"
	#include "llvm/Support/type_traits.h"

	#include <utility>

	using namespace llvm;

	namespace {

	/// A priority queue, implemented as a heap.
	template <class T, class Sorter, unsigned InlineCapacity>
	class PriorityQueue {
	Sorter Precedes;
	llvm::SmallVector<T, InlineCapacity> Storage;

	public:
	PriorityQueue(const Sorter &Precedes) : Precedes(Precedes) {}

	/// Checks whether the heap is empty.
	bool empty() const { return Storage.empty(); }

	/// Insert a new value on the heap.
	void insert(const T &V) {
	unsigned Index = Storage.size();
	Storage.push_back(V);
	if (Index == 0) return;

	T *data = Storage.data();
	while (true) {
	unsigned Target = (Index + 1) / 2 - 1;
	if (!Precedes(data[Index], data[Target])) return;
	std::swap(data[Index], data[Target]);
	if (Target == 0) return;
	Index = Target;
	}
	}

	/// Remove the minimum value in the heap. Only valid on a non-empty heap.
	T remove_min() {
	assert(!empty());
	T tmp = Storage[0];

	unsigned NewSize = Storage.size() - 1;
	if (NewSize) {
	// Move the slot at the end to the beginning.
	if (isPodLike<T>::value)
	Storage[0] = Storage[NewSize];
	else
	std::swap(Storage[0], Storage[NewSize]);

	// Bubble the root up as necessary.
	unsigned Index = 0;
	while (true) {
	// With a 1-based index, the children would be Index2 and Index2+1.
	unsigned R = (Index + 1) * 2;
	unsigned L = R - 1;

	// If R is out of bounds, we're done after this in any case.
	if (R >= NewSize) {
	// If L is also out of bounds, we're done immediately.
	if (L >= NewSize) break;

	// Otherwise, test whether we should swap L and Index.
	if (Precedes(Storage[L], Storage[Index]))
	std::swap(Storage[L], Storage[Index]);
	break;
	}

	// Otherwise, we need to compare with the smaller of L and R.
	// Prefer R because it's closer to the end of the array.
	unsigned IndexToTest = (Precedes(Storage[L], Storage[R]) ? L : R);

	// If Index is >= the min of L and R, then heap ordering is restored.
	if (!Precedes(Storage[IndexToTest], Storage[Index]))
	break;

	// Otherwise, keep bubbling up.
	std::swap(Storage[IndexToTest], Storage[Index]);
	Index = IndexToTest;
	}
	}
	Storage.pop_back();

	return tmp;
	}
	};

	/// A function-scope difference engine.
	class FunctionDifferenceEngine {
	DifferenceEngine &Engine;

	/// The current mapping from old local values to new local values.
	DenseMap<Value, Value> Values;

	/// The current mapping from old blocks to new blocks.
	DenseMap<BasicBlock, BasicBlock> Blocks;

	DenseSet<std::pair<Value, Value> > TentativeValues;

	unsigned getUnprocPredCount(BasicBlock *Block) const {
	unsigned Count = 0;
	for (pred_iterator I = pred_begin(Block), E = pred_end(Block); I != E; ++I)
	if (!Blocks.count(*I)) Count++;
	return Count;
	}

	typedef std::pair<BasicBlock, BasicBlock> BlockPair;

	/// A type which sorts a priority queue by the number of unprocessed
	/// predecessor blocks it has remaining.
	///
	/// This is actually really expensive to calculate.
	struct QueueSorter {
	const FunctionDifferenceEngine &fde;
	explicit QueueSorter(const FunctionDifferenceEngine &fde) : fde(fde) {}

	bool operator()(const BlockPair &Old, const BlockPair &New) {
	return fde.getUnprocPredCount(Old.first)
	< fde.getUnprocPredCount(New.first);
	}
	};

	/// A queue of unified blocks to process.
	PriorityQueue<BlockPair, QueueSorter, 20> Queue;

	/// Try to unify the given two blocks. Enqueues them for processing
	/// if they haven't already been processed.
	///
	/// Returns true if there was a problem unifying them.
	bool tryUnify(BasicBlock L, BasicBlock R) {
	BasicBlock *&Ref = Blocks[L];

	if (Ref) {
	if (Ref == R) return false;

	Engine.logf("successor %l cannot be equivalent to %r; "
	"it's already equivalent to %r")
	<< L << R << Ref;
	return true;
	}

	Ref = R;
	Queue.insert(BlockPair(L, R));
	return false;
	}

	/// Unifies two instructions, given that they're known not to have
	/// structural differences.
	void unify(Instruction L, Instruction R) {
	DifferenceEngine::Context C(Engine, L, R);

	bool Result = diff(L, R, true, true);
	assert(!Result && "structural differences second time around?");
	(void) Result;
	if (!L->use_empty())
	Values[L] = R;
	}

	void processQueue() {
	while (!Queue.empty()) {
	BlockPair Pair = Queue.remove_min();
	diff(Pair.first, Pair.second);
	}
	}

	void diff(BasicBlock L, BasicBlock R) {
	DifferenceEngine::Context C(Engine, L, R);

	BasicBlock::iterator LI = L->begin(), LE = L->end();
	BasicBlock::iterator RI = R->begin();

	llvm::SmallVector<std::pair<Instruction,Instruction>, 20> TentativePairs;

	do {
	assert(LI != LE && RI != R->end());
	Instruction LeftI = &LI, RightI = &RI;

	// If the instructions differ, start the more sophisticated diff
	// algorithm at the start of the block.
	if (diff(LeftI, RightI, false, false)) {
	TentativeValues.clear();
	return runBlockDiff(L->begin(), R->begin());
	}

	// Otherwise, tentatively unify them.
	if (!LeftI->use_empty())
	TentativeValues.insert(std::make_pair(LeftI, RightI));

	++LI, ++RI;
	} while (LI != LE); // This is sufficient: we can't get equality of
	// terminators if there are residual instructions.

	// Unify everything in the block, non-tentatively this time.
	TentativeValues.clear();
	for (LI = L->begin(), RI = R->begin(); LI != LE; ++LI, ++RI)
	unify(&LI, &RI);
	}

	bool matchForBlockDiff(Instruction L, Instruction R);
	void runBlockDiff(BasicBlock::iterator LI, BasicBlock::iterator RI);

	bool diffCallSites(CallSite L, CallSite R, bool Complain) {
	// FIXME: call attributes
	if (!equivalentAsOperands(L.getCalledValue(), R.getCalledValue())) {
	if (Complain) Engine.log("called functions differ");
	return true;
	}
	if (L.arg_size() != R.arg_size()) {
	if (Complain) Engine.log("argument counts differ");
	return true;
	}
	for (unsigned I = 0, E = L.arg_size(); I != E; ++I)
	if (!equivalentAsOperands(L.getArgument(I), R.getArgument(I))) {
	if (Complain)
	Engine.logf("arguments %l and %r differ")
	<< L.getArgument(I) << R.getArgument(I);
	return true;
	}
	return false;
	}

	bool diff(Instruction L, Instruction R, bool Complain, bool TryUnify) {
	// FIXME: metadata (if Complain is set)

	// Different opcodes always imply different operations.
	if (L->getOpcode() != R->getOpcode()) {
	if (Complain) Engine.log("different instruction types");
	return true;
	}

	if (isa<CmpInst>(L)) {
	if (cast<CmpInst>(L)->getPredicate()
	!= cast<CmpInst>(R)->getPredicate()) {
	if (Complain) Engine.log("different predicates");
	return true;
	}
	} else if (isa<CallInst>(L)) {
	return diffCallSites(CallSite(L), CallSite(R), Complain);
	} else if (isa<PHINode>(L)) {
	// FIXME: implement.

	// This is really weird; type uniquing is broken?
	if (L->getType() != R->getType()) {
	if (!L->getType()->isPointerTy() \|\| !R->getType()->isPointerTy()) {
	if (Complain) Engine.log("different phi types");
	return true;
	}
	}
	return false;

	// Terminators.
	} else if (isa<InvokeInst>(L)) {
	InvokeInst *LI = cast<InvokeInst>(L);
	InvokeInst *RI = cast<InvokeInst>(R);
	if (diffCallSites(CallSite(LI), CallSite(RI), Complain))
	return true;

	if (TryUnify) {
	tryUnify(LI->getNormalDest(), RI->getNormalDest());
	tryUnify(LI->getUnwindDest(), RI->getUnwindDest());
	}
	return false;

	} else if (isa<BranchInst>(L)) {
	BranchInst *LI = cast<BranchInst>(L);
	BranchInst *RI = cast<BranchInst>(R);
	if (LI->isConditional() != RI->isConditional()) {
	if (Complain) Engine.log("branch conditionality differs");
	return true;
	}

	if (LI->isConditional()) {
	if (!equivalentAsOperands(LI->getCondition(), RI->getCondition())) {
	if (Complain) Engine.log("branch conditions differ");
	return true;
	}
	if (TryUnify) tryUnify(LI->getSuccessor(1), RI->getSuccessor(1));
	}
	if (TryUnify) tryUnify(LI->getSuccessor(0), RI->getSuccessor(0));
	return false;

	} else if (isa<SwitchInst>(L)) {
	SwitchInst *LI = cast<SwitchInst>(L);
	SwitchInst *RI = cast<SwitchInst>(R);
	if (!equivalentAsOperands(LI->getCondition(), RI->getCondition())) {
	if (Complain) Engine.log("switch conditions differ");
	return true;
	}
	if (TryUnify) tryUnify(LI->getDefaultDest(), RI->getDefaultDest());

	bool Difference = false;

	DenseMap<ConstantInt,BasicBlock> LCases;

	for (SwitchInst::CaseIt I = LI->case_begin(), E = LI->case_end();
	I != E; ++I)
	LCases[I.getCaseValue()] = I.getCaseSuccessor();

	for (SwitchInst::CaseIt I = RI->case_begin(), E = RI->case_end();
	I != E; ++I) {
	ConstantInt *CaseValue = I.getCaseValue();
	BasicBlock *LCase = LCases[CaseValue];
	if (LCase) {
	if (TryUnify) tryUnify(LCase, I.getCaseSuccessor());
	LCases.erase(CaseValue);
	} else if (Complain \|\| !Difference) {
	if (Complain)
	Engine.logf("right switch has extra case %r") << CaseValue;
	Difference = true;
	}
	}
	if (!Difference)
	for (DenseMap<ConstantInt,BasicBlock>::iterator
	I = LCases.begin(), E = LCases.end(); I != E; ++I) {
	if (Complain)
	Engine.logf("left switch has extra case %l") << I->first;
	Difference = true;
	}
	return Difference;
	} else if (isa<UnreachableInst>(L)) {
	return false;
	}

	if (L->getNumOperands() != R->getNumOperands()) {
	if (Complain) Engine.log("instructions have different operand counts");
	return true;
	}

	for (unsigned I = 0, E = L->getNumOperands(); I != E; ++I) {
	Value LO = L->getOperand(I), RO = R->getOperand(I);
	if (!equivalentAsOperands(LO, RO)) {
	if (Complain) Engine.logf("operands %l and %r differ") << LO << RO;
	return true;
	}
	}

	return false;
	}

	bool equivalentAsOperands(Constant L, Constant R) {
	// Use equality as a preliminary filter.
	if (L == R)
	return true;

	if (L->getValueID() != R->getValueID())
	return false;

	// Ask the engine about global values.
	if (isa<GlobalValue>(L))
	return Engine.equivalentAsOperands(cast<GlobalValue>(L),
	cast<GlobalValue>(R));

	// Compare constant expressions structurally.
	if (isa<ConstantExpr>(L))
	return equivalentAsOperands(cast<ConstantExpr>(L),
	cast<ConstantExpr>(R));

	// Nulls of the "same type" don't always actually have the same
	// type; I don't know why. Just white-list them.
	if (isa<ConstantPointerNull>(L))
	return true;

	// Block addresses only match if we've already encountered the
	// block. FIXME: tentative matches?
	if (isa<BlockAddress>(L))
	return Blocks[cast<BlockAddress>(L)->getBasicBlock()]
	== cast<BlockAddress>(R)->getBasicBlock();

	return false;
	}

	bool equivalentAsOperands(ConstantExpr L, ConstantExpr R) {
	if (L == R)
	return true;
	if (L->getOpcode() != R->getOpcode())
	return false;

	switch (L->getOpcode()) {
	case Instruction::ICmp:
	case Instruction::FCmp:
	if (L->getPredicate() != R->getPredicate())
	return false;
	break;

	case Instruction::GetElementPtr:
	// FIXME: inbounds?
	break;

	default:
	break;
	}

	if (L->getNumOperands() != R->getNumOperands())
	return false;

	for (unsigned I = 0, E = L->getNumOperands(); I != E; ++I)
	if (!equivalentAsOperands(L->getOperand(I), R->getOperand(I)))
	return false;

	return true;
	}

	bool equivalentAsOperands(Value L, Value R) {
	// Fall out if the values have different kind.
	// This possibly shouldn't take priority over oracles.
	if (L->getValueID() != R->getValueID())
	return false;

	// Value subtypes: Argument, Constant, Instruction, BasicBlock,
	// InlineAsm, MDNode, MDString, PseudoSourceValue

	if (isa<Constant>(L))
	return equivalentAsOperands(cast<Constant>(L), cast<Constant>(R));

	if (isa<Instruction>(L))
	return Values[L] == R \|\| TentativeValues.count(std::make_pair(L, R));

	if (isa<Argument>(L))
	return Values[L] == R;

	if (isa<BasicBlock>(L))
	return Blocks[cast<BasicBlock>(L)] != R;

	// Pretend everything else is identical.
	return true;
	}

	// Avoid a gcc warning about accessing 'this' in an initializer.
	FunctionDifferenceEngine *this_() { return this; }

	public:
	FunctionDifferenceEngine(DifferenceEngine &Engine) :
	Engine(Engine), Queue(QueueSorter(*this_())) {}

	void diff(Function L, Function R) {
	if (L->arg_size() != R->arg_size())
	Engine.log("different argument counts");

	// Map the arguments.
	for (Function::arg_iterator
	LI = L->arg_begin(), LE = L->arg_end(),
	RI = R->arg_begin(), RE = R->arg_end();
	LI != LE && RI != RE; ++LI, ++RI)
	Values[&LI] = &RI;

	tryUnify(&L->begin(), &R->begin());
	processQueue();
	}
	};

	struct DiffEntry {
	DiffEntry() : Cost(0) {}

	unsigned Cost;
	llvm::SmallVector<char, 8> Path; // actually of DifferenceEngine::DiffChange
	};

	bool FunctionDifferenceEngine::matchForBlockDiff(Instruction *L,
	Instruction *R) {
	return !diff(L, R, false, false);
	}

	void FunctionDifferenceEngine::runBlockDiff(BasicBlock::iterator LStart,
	BasicBlock::iterator RStart) {
	BasicBlock::iterator LE = LStart->getParent()->end();
	BasicBlock::iterator RE = RStart->getParent()->end();

	unsigned NL = std::distance(LStart, LE);

	SmallVector<DiffEntry, 20> Paths1(NL+1);
	SmallVector<DiffEntry, 20> Paths2(NL+1);

	DiffEntry *Cur = Paths1.data();
	DiffEntry *Next = Paths2.data();

	const unsigned LeftCost = 2;
	const unsigned RightCost = 2;
	const unsigned MatchCost = 0;

	assert(TentativeValues.empty());

	// Initialize the first column.
	for (unsigned I = 0; I != NL+1; ++I) {
	Cur[I].Cost = I * LeftCost;
	for (unsigned J = 0; J != I; ++J)
	Cur[I].Path.push_back(DC_left);
	}

	for (BasicBlock::iterator RI = RStart; RI != RE; ++RI) {
	// Initialize the first row.
	Next[0] = Cur[0];
	Next[0].Cost += RightCost;
	Next[0].Path.push_back(DC_right);

	unsigned Index = 1;
	for (BasicBlock::iterator LI = LStart; LI != LE; ++LI, ++Index) {
	if (matchForBlockDiff(&LI, &RI)) {
	Next[Index] = Cur[Index-1];
	Next[Index].Cost += MatchCost;
	Next[Index].Path.push_back(DC_match);
	TentativeValues.insert(std::make_pair(&LI, &RI));
	} else if (Next[Index-1].Cost <= Cur[Index].Cost) {
	Next[Index] = Next[Index-1];
	Next[Index].Cost += LeftCost;
	Next[Index].Path.push_back(DC_left);
	} else {
	Next[Index] = Cur[Index];
	Next[Index].Cost += RightCost;
	Next[Index].Path.push_back(DC_right);
	}
	}

	std::swap(Cur, Next);
	}

	// We don't need the tentative values anymore; everything from here
	// on out should be non-tentative.
	TentativeValues.clear();

	SmallVectorImpl<char> &Path = Cur[NL].Path;
	BasicBlock::iterator LI = LStart, RI = RStart;

	DiffLogBuilder Diff(Engine.getConsumer());

	// Drop trailing matches.
	while (Path.back() == DC_match)
	Path.pop_back();

	// Skip leading matches.
	SmallVectorImpl<char>::iterator
	PI = Path.begin(), PE = Path.end();
	while (PI != PE && *PI == DC_match) {
	unify(&LI, &RI);
	++PI, ++LI, ++RI;
	}

	for (; PI != PE; ++PI) {
	switch (static_cast<DiffChange>(*PI)) {
	case DC_match:
	assert(LI != LE && RI != RE);
	{
	Instruction L = &LI, R = &RI;
	unify(L, R);
	Diff.addMatch(L, R);
	}
	++LI; ++RI;
	break;

	case DC_left:
	assert(LI != LE);
	Diff.addLeft(&*LI);
	++LI;
	break;

	case DC_right:
	assert(RI != RE);
	Diff.addRight(&*RI);
	++RI;
	break;
	}
	}

	// Finishing unifying and complaining about the tails of the block,
	// which should be matches all the way through.
	while (LI != LE) {
	assert(RI != RE);
	unify(&LI, &RI);
	++LI, ++RI;
	}

	// If the terminators have different kinds, but one is an invoke and the
	// other is an unconditional branch immediately following a call, unify
	// the results and the destinations.
	TerminatorInst *LTerm = LStart->getParent()->getTerminator();
	TerminatorInst *RTerm = RStart->getParent()->getTerminator();
	if (isa<BranchInst>(LTerm) && isa<InvokeInst>(RTerm)) {
	if (cast<BranchInst>(LTerm)->isConditional()) return;
	BasicBlock::iterator I = LTerm;
	if (I == LStart->getParent()->begin()) return;
	--I;
	if (!isa<CallInst>(*I)) return;
	CallInst LCall = cast<CallInst>(&I);
	InvokeInst *RInvoke = cast<InvokeInst>(RTerm);
	if (!equivalentAsOperands(LCall->getCalledValue(), RInvoke->getCalledValue()))
	return;
	if (!LCall->use_empty())
	Values[LCall] = RInvoke;
	tryUnify(LTerm->getSuccessor(0), RInvoke->getNormalDest());
	} else if (isa<InvokeInst>(LTerm) && isa<BranchInst>(RTerm)) {
	if (cast<BranchInst>(RTerm)->isConditional()) return;
	BasicBlock::iterator I = RTerm;
	if (I == RStart->getParent()->begin()) return;
	--I;
	if (!isa<CallInst>(*I)) return;
	CallInst *RCall = cast<CallInst>(I);
	InvokeInst *LInvoke = cast<InvokeInst>(LTerm);
	if (!equivalentAsOperands(LInvoke->getCalledValue(), RCall->getCalledValue()))
	return;
	if (!LInvoke->use_empty())
	Values[LInvoke] = RCall;
	tryUnify(LInvoke->getNormalDest(), RTerm->getSuccessor(0));
	}
	}

	}

	void DifferenceEngine::Oracle::anchor() { }

	void DifferenceEngine::diff(Function L, Function R) {
	Context C(*this, L, R);

	// FIXME: types
	// FIXME: attributes and CC
	// FIXME: parameter attributes

	// If both are declarations, we're done.
	if (L->empty() && R->empty())
	return;
	else if (L->empty())
	log("left function is declaration, right function is definition");
	else if (R->empty())
	log("right function is declaration, left function is definition");
	else
	FunctionDifferenceEngine(*this).diff(L, R);
	}

	void DifferenceEngine::diff(Module L, Module R) {
	StringSet<> LNames;
	SmallVector<std::pair<Function,Function>, 20> Queue;

	for (Module::iterator I = L->begin(), E = L->end(); I != E; ++I) {
	Function LFn = &I;
	LNames.insert(LFn->getName());

	if (Function *RFn = R->getFunction(LFn->getName()))
	Queue.push_back(std::make_pair(LFn, RFn));
	else
	logf("function %l exists only in left module") << LFn;
	}

	for (Module::iterator I = R->begin(), E = R->end(); I != E; ++I) {
	Function RFn = &I;
	if (!LNames.count(RFn->getName()))
	logf("function %r exists only in right module") << RFn;
	}

	for (SmallVectorImpl<std::pair<Function,Function> >::iterator
	I = Queue.begin(), E = Queue.end(); I != E; ++I)
	diff(I->first, I->second);
	}

	bool DifferenceEngine::equivalentAsOperands(GlobalValue L, GlobalValue R) {
	if (globalValueOracle) return (*globalValueOracle)(L, R);
	return L->getName() == R->getName();
	}