C++ LiveInterval::MergeSegmentsInAsValue方法代码示例

//.........这里部分代码省略.........

  for (unsigned i=0; i < NumSlots; ++i) {
    LiveInterval *LI = new LiveInterval(i, 0);
    Intervals.push_back(LI);
    LI->getNextValue(Indexes->getZeroIndex(), VNInfoAllocator);
    SortedSlots.push_back(i);
  }

  // Calculate the liveness of each block.
  calculateLocalLiveness();

  // Propagate the liveness information.
  calculateLiveIntervals(NumSlots);

  // Search for allocas which are used outside of the declared lifetime
  // markers.
  if (ProtectFromEscapedAllocas)
    removeInvalidSlotRanges();

  // Maps old slots to new slots.
  DenseMap<int, int> SlotRemap;
  unsigned RemovedSlots = 0;
  unsigned ReducedSize = 0;

  // Do not bother looking at empty intervals.
  for (unsigned I = 0; I < NumSlots; ++I) {
    if (Intervals[SortedSlots[I]]->empty())
      SortedSlots[I] = -1;
  }

  // This is a simple greedy algorithm for merging allocas. First, sort the
  // slots, placing the largest slots first. Next, perform an n^2 scan and look
  // for disjoint slots. When you find disjoint slots, merge the samller one
  // into the bigger one and update the live interval. Remove the small alloca
  // and continue.

  // Sort the slots according to their size. Place unused slots at the end.
  // Use stable sort to guarantee deterministic code generation.
  std::stable_sort(SortedSlots.begin(), SortedSlots.end(),
                   SlotSizeSorter(MFI));

  bool Changed = true;
  while (Changed) {
    Changed = false;
    for (unsigned I = 0; I < NumSlots; ++I) {
      if (SortedSlots[I] == -1)
        continue;

      for (unsigned J=I+1; J < NumSlots; ++J) {
        if (SortedSlots[J] == -1)
          continue;

        int FirstSlot = SortedSlots[I];
        int SecondSlot = SortedSlots[J];
        LiveInterval *First = Intervals[FirstSlot];
        LiveInterval *Second = Intervals[SecondSlot];
        assert (!First->empty() && !Second->empty() && "Found an empty range");

        // Merge disjoint slots.
        if (!First->overlaps(*Second)) {
          Changed = true;
          First->MergeSegmentsInAsValue(*Second, First->getValNumInfo(0));
          SlotRemap[SecondSlot] = FirstSlot;
          SortedSlots[J] = -1;
          DEBUG(dbgs()<<"Merging #"<<FirstSlot<<" and slots #"<<
                SecondSlot<<" together.\n");
          unsigned MaxAlignment = std::max(MFI->getObjectAlignment(FirstSlot),
                                           MFI->getObjectAlignment(SecondSlot));

          assert(MFI->getObjectSize(FirstSlot) >=
                 MFI->getObjectSize(SecondSlot) &&
                 "Merging a small object into a larger one");

          RemovedSlots+=1;
          ReducedSize += MFI->getObjectSize(SecondSlot);
          MFI->setObjectAlignment(FirstSlot, MaxAlignment);
          MFI->RemoveStackObject(SecondSlot);
        }
      }
    }
  }// While changed.

  // Record statistics.
  StackSpaceSaved += ReducedSize;
  StackSlotMerged += RemovedSlots;
  DEBUG(dbgs()<<"Merge "<<RemovedSlots<<" slots. Saved "<<
        ReducedSize<<" bytes\n");

  // Scan the entire function and update all machine operands that use frame
  // indices to use the remapped frame index.
  expungeSlotMap(SlotRemap, NumSlots);
  remapInstructions(SlotRemap);

  // Release the intervals.
  for (unsigned I = 0; I < NumSlots; ++I) {
    delete Intervals[I];
  }

  return removeAllMarkers();
}