C++ SmallDenseMap::find方法代码示例

// This function calculates the number of iterations after which the given Phi
// becomes an invariant. The pre-calculated values are memorized in the map. The
// function (shortcut is I) is calculated according to the following definition:
// Given %x = phi <Inputs from above the loop>, ..., [%y, %back.edge].
//   If %y is a loop invariant, then I(%x) = 1.
//   If %y is a Phi from the loop header, I(%x) = I(%y) + 1.
//   Otherwise, I(%x) is infinite.
// TODO: Actually if %y is an expression that depends only on Phi %z and some
//       loop invariants, we can estimate I(%x) = I(%z) + 1. The example
//       looks like:
//         %x = phi(0, %a),  <-- becomes invariant starting from 3rd iteration.
//         %y = phi(0, 5),
//         %a = %y + 1.
static unsigned calculateIterationsToInvariance(
    PHINode *Phi, Loop *L, BasicBlock *BackEdge,
    SmallDenseMap<PHINode *, unsigned> &IterationsToInvariance) {
  assert(Phi->getParent() == L->getHeader() &&
         "Non-loop Phi should not be checked for turning into invariant.");
  assert(BackEdge == L->getLoopLatch() && "Wrong latch?");
  // If we already know the answer, take it from the map.
  auto I = IterationsToInvariance.find(Phi);
  if (I != IterationsToInvariance.end())
    return I->second;

  // Otherwise we need to analyze the input from the back edge.
  Value *Input = Phi->getIncomingValueForBlock(BackEdge);
  // Place infinity to map to avoid infinite recursion for cycled Phis. Such
  // cycles can never stop on an invariant.
  IterationsToInvariance[Phi] = InfiniteIterationsToInvariance;
  unsigned ToInvariance = InfiniteIterationsToInvariance;

  if (L->isLoopInvariant(Input))
    ToInvariance = 1u;
  else if (PHINode *IncPhi = dyn_cast<PHINode>(Input)) {
    // Only consider Phis in header block.
    if (IncPhi->getParent() != L->getHeader())
      return InfiniteIterationsToInvariance;
    // If the input becomes an invariant after X iterations, then our Phi
    // becomes an invariant after X + 1 iterations.
    unsigned InputToInvariance = calculateIterationsToInvariance(
        IncPhi, L, BackEdge, IterationsToInvariance);
    if (InputToInvariance != InfiniteIterationsToInvariance)
      ToInvariance = InputToInvariance + 1u;
  }

  // If we found that this Phi lies in an invariant chain, update the map.
  if (ToInvariance != InfiniteIterationsToInvariance)
    IterationsToInvariance[Phi] = ToInvariance;
  return ToInvariance;
}

/// Remove dead functions that are not included in DNR (Do Not Remove) list.
bool Inliner::removeDeadFunctions(CallGraph &CG, bool AlwaysInlineOnly) {
  SmallVector<CallGraphNode*, 16> FunctionsToRemove;
  SmallVector<CallGraphNode *, 16> DeadFunctionsInComdats;
  SmallDenseMap<const Comdat *, int, 16> ComdatEntriesAlive;

  auto RemoveCGN = [&](CallGraphNode *CGN) {
    // Remove any call graph edges from the function to its callees.
    CGN->removeAllCalledFunctions();

    // Remove any edges from the external node to the function's call graph
    // node.  These edges might have been made irrelegant due to
    // optimization of the program.
    CG.getExternalCallingNode()->removeAnyCallEdgeTo(CGN);

    // Removing the node for callee from the call graph and delete it.
    FunctionsToRemove.push_back(CGN);
  };

  // Scan for all of the functions, looking for ones that should now be removed
  // from the program.  Insert the dead ones in the FunctionsToRemove set.
  for (CallGraph::iterator I = CG.begin(), E = CG.end(); I != E; ++I) {
    CallGraphNode *CGN = I->second;
    Function *F = CGN->getFunction();
    if (!F || F->isDeclaration())
      continue;

    // Handle the case when this function is called and we only want to care
    // about always-inline functions. This is a bit of a hack to share code
    // between here and the InlineAlways pass.
    if (AlwaysInlineOnly && !F->hasFnAttribute(Attribute::AlwaysInline))
      continue;

    // If the only remaining users of the function are dead constants, remove
    // them.
    F->removeDeadConstantUsers();

    if (!F->isDefTriviallyDead())
      continue;

    // It is unsafe to drop a function with discardable linkage from a COMDAT
    // without also dropping the other members of the COMDAT.
    // The inliner doesn't visit non-function entities which are in COMDAT
    // groups so it is unsafe to do so *unless* the linkage is local.
    if (!F->hasLocalLinkage()) {
      if (const Comdat *C = F->getComdat()) {
        --ComdatEntriesAlive[C];
        DeadFunctionsInComdats.push_back(CGN);
        continue;
      }
    }

    RemoveCGN(CGN);
  }
  if (!DeadFunctionsInComdats.empty()) {
    // Count up all the entities in COMDAT groups
    auto ComdatGroupReferenced = [&](const Comdat *C) {
      auto I = ComdatEntriesAlive.find(C);
      if (I != ComdatEntriesAlive.end())
        ++(I->getSecond());
    };
    for (const Function &F : CG.getModule())
      if (const Comdat *C = F.getComdat())
        ComdatGroupReferenced(C);
    for (const GlobalVariable &GV : CG.getModule().globals())
      if (const Comdat *C = GV.getComdat())
        ComdatGroupReferenced(C);
    for (const GlobalAlias &GA : CG.getModule().aliases())
      if (const Comdat *C = GA.getComdat())
        ComdatGroupReferenced(C);
    for (CallGraphNode *CGN : DeadFunctionsInComdats) {
      Function *F = CGN->getFunction();
      const Comdat *C = F->getComdat();
      int NumAlive = ComdatEntriesAlive[C];
      // We can remove functions in a COMDAT group if the entire group is dead.
      assert(NumAlive >= 0);
      if (NumAlive > 0)
        continue;

      RemoveCGN(CGN);
    }
  }

  if (FunctionsToRemove.empty())
    return false;

  // Now that we know which functions to delete, do so.  We didn't want to do
  // this inline, because that would invalidate our CallGraph::iterator
  // objects. :(
  //
  // Note that it doesn't matter that we are iterating over a non-stable order
  // here to do this, it doesn't matter which order the functions are deleted
  // in.
  array_pod_sort(FunctionsToRemove.begin(), FunctionsToRemove.end());
  FunctionsToRemove.erase(std::unique(FunctionsToRemove.begin(),
                                      FunctionsToRemove.end()),
                          FunctionsToRemove.end());
  for (SmallVectorImpl<CallGraphNode *>::iterator I = FunctionsToRemove.begin(),
                                                  E = FunctionsToRemove.end();
       I != E; ++I) {
//.........这里部分代码省略.........

// Sinks \p I from the loop \p L's preheader to its uses. Returns true if
// sinking is successful.
// \p LoopBlockNumber is used to sort the insertion blocks to ensure
// determinism.
static bool sinkInstruction(Loop &L, Instruction &I,
                            const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
                            const SmallDenseMap<BasicBlock *, int, 16> &LoopBlockNumber,
                            LoopInfo &LI, DominatorTree &DT,
                            BlockFrequencyInfo &BFI) {
  // Compute the set of blocks in loop L which contain a use of I.
  SmallPtrSet<BasicBlock *, 2> BBs;
  for (auto &U : I.uses()) {
    Instruction *UI = cast<Instruction>(U.getUser());
    // We cannot sink I to PHI-uses.
    if (dyn_cast<PHINode>(UI))
      return false;
    // We cannot sink I if it has uses outside of the loop.
    if (!L.contains(LI.getLoopFor(UI->getParent())))
      return false;
    BBs.insert(UI->getParent());
  }

  // findBBsToSinkInto is O(BBs.size() * ColdLoopBBs.size()). We cap the max
  // BBs.size() to avoid expensive computation.
  // FIXME: Handle code size growth for min_size and opt_size.
  if (BBs.size() > MaxNumberOfUseBBsForSinking)
    return false;

  // Find the set of BBs that we should insert a copy of I.
  SmallPtrSet<BasicBlock *, 2> BBsToSinkInto =
      findBBsToSinkInto(L, BBs, ColdLoopBBs, DT, BFI);
  if (BBsToSinkInto.empty())
    return false;

  // Copy the final BBs into a vector and sort them using the total ordering
  // of the loop block numbers as iterating the set doesn't give a useful
  // order. No need to stable sort as the block numbers are a total ordering.
  SmallVector<BasicBlock *, 2> SortedBBsToSinkInto;
  SortedBBsToSinkInto.insert(SortedBBsToSinkInto.begin(), BBsToSinkInto.begin(),
                             BBsToSinkInto.end());
  std::sort(SortedBBsToSinkInto.begin(), SortedBBsToSinkInto.end(),
            [&](BasicBlock *A, BasicBlock *B) {
              return *LoopBlockNumber.find(A) < *LoopBlockNumber.find(B);
            });

  BasicBlock *MoveBB = *SortedBBsToSinkInto.begin();
  // FIXME: Optimize the efficiency for cloned value replacement. The current
  //        implementation is O(SortedBBsToSinkInto.size() * I.num_uses()).
  for (BasicBlock *N : SortedBBsToSinkInto) {
    if (N == MoveBB)
      continue;
    // Clone I and replace its uses.
    Instruction *IC = I.clone();
    IC->setName(I.getName());
    IC->insertBefore(&*N->getFirstInsertionPt());
    // Replaces uses of I with IC in N
    for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE;) {
      Use &U = *UI++;
      auto *I = cast<Instruction>(U.getUser());
      if (I->getParent() == N)
        U.set(IC);
    }
    // Replaces uses of I with IC in blocks dominated by N
    replaceDominatedUsesWith(&I, IC, DT, N);
    DEBUG(dbgs() << "Sinking a clone of " << I << " To: " << N->getName()
                 << '\n');
    NumLoopSunkCloned++;
  }
  DEBUG(dbgs() << "Sinking " << I << " To: " << MoveBB->getName() << '\n');
  NumLoopSunk++;
  I.moveBefore(&*MoveBB->getFirstInsertionPt());

  return true;
}