CFGUpdate.h

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//===- CFGUpdate.h - Encode a CFG Edge Update. ------------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines a CFG Edge Update: Insert or Delete, and two Nodes as the
// Edge ends.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_SUPPORT_CFGUPDATE_H
#define LLVM_SUPPORT_CFGUPDATE_H

#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"

namespace llvm {
namespace cfg {
enum class UpdateKind : unsigned char { Insert, Delete };

template <typename NodePtr> class Update {
  using NodeKindPair = PointerIntPair<NodePtr, 1, UpdateKind>;
  NodePtr From;
  NodeKindPair ToAndKind;

public:
  Update(UpdateKind Kind, NodePtr From, NodePtr To)
      : From(From), ToAndKind(To, Kind) {}

  UpdateKind getKind() const { return ToAndKind.getInt(); }
  NodePtr getFrom() const { return From; }
  NodePtr getTo() const { return ToAndKind.getPointer(); }
  bool operator==(const Update &RHS) const {
    return From == RHS.From && ToAndKind == RHS.ToAndKind;
  }

  void print(raw_ostream &OS) const {
    OS << (getKind() == UpdateKind::Insert ? "Insert " : "Delete ");
    getFrom()->printAsOperand(OS, false);
    OS << " -> ";
    getTo()->printAsOperand(OS, false);
  }

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
  LLVM_DUMP_METHOD void dump() const { print(dbgs()); }
#endif
};

// LegalizeUpdates function simplifies updates assuming a graph structure.
// This function serves double purpose:
// a) It removes redundant updates, which makes it easier to reverse-apply
//    them when traversing CFG.
// b) It optimizes away updates that cancel each other out, as the end result
//    is the same.
template <typename NodePtr>
void LegalizeUpdates(ArrayRef<Update<NodePtr>> AllUpdates,
                     SmallVectorImpl<Update<NodePtr>> &Result,
                     bool InverseGraph) {
  // Count the total number of inserions of each edge.
  // Each insertion adds 1 and deletion subtracts 1. The end number should be
  // one of {-1 (deletion), 0 (NOP), +1 (insertion)}. Otherwise, the sequence
  // of updates contains multiple updates of the same kind and we assert for
  // that case.
  SmallDenseMap<std::pair<NodePtr, NodePtr>, int, 4> Operations;
  Operations.reserve(AllUpdates.size());

  for (const auto &U : AllUpdates) {
    NodePtr From = U.getFrom();
    NodePtr To = U.getTo();
    if (InverseGraph)
      std::swap(From, To); // Reverse edge for postdominators.

    Operations[{From, To}] += (U.getKind() == UpdateKind::Insert ? 1 : -1);
  }

  Result.clear();
  Result.reserve(Operations.size());
  for (auto &Op : Operations) {
    const int NumInsertions = Op.second;
    assert(std::abs(NumInsertions) <= 1 && "Unbalanced operations!");
    if (NumInsertions == 0)
      continue;
    const UpdateKind UK =
        NumInsertions > 0 ? UpdateKind::Insert : UpdateKind::Delete;
    Result.push_back({UK, Op.first.first, Op.first.second});
  }

  // Make the order consistent by not relying on pointer values within the
  // set. Reuse the old Operations map.
  // In the future, we should sort by something else to minimize the amount
  // of work needed to perform the series of updates.
  for (size_t i = 0, e = AllUpdates.size(); i != e; ++i) {
    const auto &U = AllUpdates[i];
    if (!InverseGraph)
      Operations[{U.getFrom(), U.getTo()}] = int(i);
    else
      Operations[{U.getTo(), U.getFrom()}] = int(i);
  }

  llvm::sort(Result,
             [&Operations](const Update<NodePtr> &A, const Update<NodePtr> &B) {
               return Operations[{A.getFrom(), A.getTo()}] >
                      Operations[{B.getFrom(), B.getTo()}];
             });
}

} // end namespace cfg
} // end namespace llvm

#endif // LLVM_SUPPORT_CFGUPDATE_H