COFF/ICF.cpp

//===- ICF.cpp ------------------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Implements ICF (Identical COMDAT Folding)
//
//===----------------------------------------------------------------------===//

#include "Chunks.h"
#include <tuple>
#include <unordered_set>
#include <vector>

namespace lld {
namespace coff {
namespace {

struct Hasher {
  size_t operator()(const SectionChunk *C) const { return C->getHash(); }
};

struct Equals {
  bool operator()(const SectionChunk *A, const SectionChunk *B) const {
    return A->equals(B);
  }
};

} // anonymous namespace

// Merge identical COMDAT sections.
// Two sections are considered as identical when their section headers,
// contents and relocations are all the same.
void doICF(const std::vector<Chunk *> &Chunks) {
  std::unordered_set<SectionChunk *, Hasher, Equals> Set;
  bool Redo;
  do {
    Set.clear();
    Redo = false;
    for (Chunk *C : Chunks) {
      auto *SC = dyn_cast<SectionChunk>(C);
      if (!SC || !SC->isCOMDAT() || !SC->isLive())
        continue;
      auto P = Set.insert(SC);
      bool Inserted = P.second;
      if (Inserted)
        continue;
      SectionChunk *Existing = *P.first;
      SC->replaceWith(Existing);
      // By merging sections, two relocations that originally pointed to
      // different locations can now point to the same location.
      // So, repeat the process until a convegence is obtained.
      Redo = true;
    }
  } while (Redo);
}

} // namespace coff
} // namespace lld