llvm-project/compiler-rt/lib/xray/xray_segmented_array.h

//===-- xray_segmented_array.h ---------------------------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file is a part of XRay, a dynamic runtime instrumentation system.
//
// Defines the implementation of a segmented array, with fixed-size chunks
// backing the segments.
//
//===----------------------------------------------------------------------===//
#ifndef XRAY_SEGMENTED_ARRAY_H
#define XRAY_SEGMENTED_ARRAY_H

#include "sanitizer_common/sanitizer_allocator.h"
#include "xray_allocator.h"
#include <type_traits>
#include <utility>

namespace __xray {

namespace {

constexpr size_t gcd(size_t a, size_t b) {
  return (b == 0) ? a : gcd(b, a % b);
}

constexpr size_t lcm(size_t a, size_t b) { return a * b / gcd(a, b); }

} // namespace

/// The Array type provides an interface similar to std::vector<...> but does
/// not shrink in size. Once constructed, elements can be appended but cannot be
/// removed. The implementation is heavily dependent on the contract provided by
/// the Allocator type, in that all memory will be released when the Allocator
/// is destroyed. When an Array is destroyed, it will destroy elements in the
/// backing store but will not free the memory. The parameter N defines how many
/// elements of T there should be in a single block.
///
/// We compute the least common multiple of the size of T and the cache line
/// size, to allow us to maximise the number of T objects we can place in
/// cache-line multiple sized blocks. To get back the number of T's, we divide
/// this least common multiple by the size of T.
template <class T, size_t N = lcm(sizeof(T), kCacheLineSize) / sizeof(T)>
struct Array {
  static constexpr size_t ChunkSize = N;
  static constexpr size_t AllocatorChunkSize = sizeof(T) * ChunkSize;
  using AllocatorType = Allocator<AllocatorChunkSize>;
  static_assert(std::is_trivially_destructible<T>::value,
                "T must be trivially destructible.");

private:
  // TODO: Consider co-locating the chunk information with the data in the
  // Block, as in an intrusive list -- i.e. putting the next and previous
  // pointer values inside the Block storage.
  struct Chunk {
    typename AllocatorType::Block Block;
    static constexpr size_t Size = N;
    Chunk *Prev = nullptr;
    Chunk *Next = nullptr;
  };

  static Chunk SentinelChunk;

  AllocatorType *Alloc;
  Chunk *Head = &SentinelChunk;
  Chunk *Tail = &SentinelChunk;
  size_t Size = 0;
  size_t FreeElements = 0;

  // Here we keep track of chunks in the freelist, to allow us to re-use chunks
  // when elements are trimmed off the end.
  Chunk *Freelist = &SentinelChunk;

  Chunk *NewChunk() {
    // We need to handle the case in which enough elements have been trimmed to
    // allow us to re-use chunks we've allocated before. For this we look into
    // the Freelist, to see whether we need to actually allocate new blocks or
    // just re-use blocks we've already seen before.
    if (Freelist != &SentinelChunk) {
      auto *FreeChunk = Freelist;
      Freelist = FreeChunk->Next;
      FreeChunk->Next = &SentinelChunk;
      return FreeChunk;
    }

    auto Block = Alloc->Allocate();
    if (Block.Data == nullptr)
      return nullptr;
    // TODO: Maybe use a separate managed allocator for Chunk instances?
    auto C = reinterpret_cast<Chunk *>(InternalAlloc(sizeof(Chunk)));
    if (C == nullptr)
      return nullptr;
    C->Block = Block;
    return C;
  }

  static AllocatorType &GetGlobalAllocator() {
    static AllocatorType *const GlobalAllocator = [] {
      AllocatorType *A = reinterpret_cast<AllocatorType *>(
          InternalAlloc(sizeof(AllocatorType)));
      new (A) AllocatorType(2 << 10, 0);
      return A;
    }();

    return *GlobalAllocator;
  }

  Chunk *InitHeadAndTail() {
    DCHECK_EQ(Head, &SentinelChunk);
    DCHECK_EQ(Tail, &SentinelChunk);
    auto Chunk = NewChunk();
    if (Chunk == nullptr)
      return nullptr;
    Chunk->Prev = &SentinelChunk;
    Chunk->Next = &SentinelChunk;
    Head = Chunk;
    Tail = Chunk;
    return Chunk;
  }

  Chunk *AppendNewChunk() {
    auto Chunk = NewChunk();
    if (Chunk == nullptr)
      return nullptr;
    Tail->Next = Chunk;
    Chunk->Prev = Tail;
    Chunk->Next = &SentinelChunk;
    Tail = Chunk;
    return Chunk;
  }

  // This Iterator models a BidirectionalIterator.
  template <class U> class Iterator {
    Chunk *C = nullptr;
    size_t Offset = 0;

  public:
    Iterator(Chunk *IC, size_t Off) : C(IC), Offset(Off) {}

    Iterator &operator++() {
      if (++Offset % N)
        return *this;

      DCHECK_NE(C, &SentinelChunk);

      // At this point, we know that Offset % N == 0, so we must advance the
      // chunk pointer.
      DCHECK_EQ(Offset % N, 0);
      C = C->Next;
      return *this;
    }

    Iterator &operator--() {
      DCHECK_NE(C, &SentinelChunk);
      DCHECK_GT(Offset, 0);

      // We check whether the offset was on a boundary before decrement, to see
      // whether we need to retreat to the previous chunk.
      if ((Offset-- % N) == 0)
        C = C->Prev;
      return *this;
    }

    Iterator operator++(int) {
      Iterator Copy(*this);
      ++(*this);
      return Copy;
    }

    Iterator operator--(int) {
      Iterator Copy(*this);
      --(*this);
      return Copy;
    }

    template <class V, class W>
    friend bool operator==(const Iterator<V> &L, const Iterator<W> &R) {
      return L.C == R.C && L.Offset == R.Offset;
    }

    template <class V, class W>
    friend bool operator!=(const Iterator<V> &L, const Iterator<W> &R) {
      return !(L == R);
    }

    U &operator*() const {
      DCHECK_NE(C, &SentinelChunk);
      return reinterpret_cast<U *>(C->Block.Data)[Offset % N];
    }

    U *operator->() const {
      DCHECK_NE(C, &SentinelChunk);
      return reinterpret_cast<U *>(C->Block.Data) + (Offset % N);
    }
  };

public:
  explicit Array(AllocatorType &A) : Alloc(&A) {}
  Array() : Array(GetGlobalAllocator()) {}

  Array(const Array &) = delete;
  Array(Array &&O) NOEXCEPT : Alloc(O.Alloc),
                              Head(O.Head),
                              Tail(O.Tail),
                              Size(O.Size) {
    O.Head = &SentinelChunk;
    O.Tail = &SentinelChunk;
    O.Size = 0;
  }

  bool empty() const { return Size == 0; }

  AllocatorType &allocator() const {
    DCHECK_NE(Alloc, nullptr);
    return *Alloc;
  }

  size_t size() const { return Size; }

  T *Append(const T &E) {
    if (UNLIKELY(Head == &SentinelChunk))
      if (InitHeadAndTail() == nullptr)
        return nullptr;

    auto Offset = Size % N;
    if (UNLIKELY(Size != 0 && Offset == 0))
      if (AppendNewChunk() == nullptr)
        return nullptr;

    auto Position = reinterpret_cast<T *>(Tail->Block.Data) + Offset;
    *Position = E;
    ++Size;
    FreeElements -= FreeElements ? 1 : 0;
    return Position;
  }

  template <class... Args> T *AppendEmplace(Args &&... args) {
    if (UNLIKELY(Head == &SentinelChunk))
      if (InitHeadAndTail() == nullptr)
        return nullptr;

    auto Offset = Size % N;
    if (UNLIKELY(Size != 0 && Offset == 0))
      if (AppendNewChunk() == nullptr)
        return nullptr;

    auto Position = reinterpret_cast<T *>(Tail->Block.Data) + Offset;
    // In-place construct at Position.
    new (Position) T(std::forward<Args>(args)...);
    ++Size;
    FreeElements -= FreeElements ? 1 : 0;
    return Position;
  }

  T &operator[](size_t Offset) const {
    DCHECK_LE(Offset, Size);
    // We need to traverse the array enough times to find the element at Offset.
    auto C = Head;
    while (Offset >= N) {
      C = C->Next;
      Offset -= N;
      DCHECK_NE(C, &SentinelChunk);
    }
    auto Position = reinterpret_cast<T *>(C->Block.Data) + Offset;
    return *Position;
  }

  T &front() const {
    DCHECK_NE(Head, &SentinelChunk);
    DCHECK_NE(Size, 0u);
    return *reinterpret_cast<T *>(Head->Block.Data);
  }

  T &back() const {
    DCHECK_NE(Tail, &SentinelChunk);
    auto Offset = (Size - 1) % N;
    return *(reinterpret_cast<T *>(Tail->Block.Data) + Offset);
  }

  template <class Predicate> T *find_element(Predicate P) const {
    if (empty())
      return nullptr;

    auto E = end();
    for (auto I = begin(); I != E; ++I)
      if (P(*I))
        return &(*I);

    return nullptr;
  }

  /// Remove N Elements from the end. This leaves the blocks behind, and not
  /// require allocation of new blocks for new elements added after trimming.
  void trim(size_t Elements) {
    DCHECK_LE(Elements, Size);
    Size -= Elements;
    FreeElements += Elements;

    // Here we need to check whether we've cleared enough elements to warrant
    // putting blocks on to the freelist. We determine whether we need to
    // right-size the internal list, by keeping track of the number of "free"
    // elements still in the array.
    auto ChunksToTrim = FreeElements / N;
    for (size_t i = 0; i < ChunksToTrim; ++i, FreeElements -= N) {
      // Put the tail into the Freelist.
      auto *FreeChunk = Tail;
      Tail = Tail->Prev;
      if (Tail == &SentinelChunk)
        Head = Tail;
      else
        Tail->Next = &SentinelChunk;
      FreeChunk->Next = Freelist;
      FreeChunk->Prev = Freelist->Prev;
      Freelist = FreeChunk;
    }
  }

  // Provide iterators.
  Iterator<T> begin() const { return Iterator<T>(Head, 0); }
  Iterator<T> end() const { return Iterator<T>(Tail, Size); }
  Iterator<const T> cbegin() const { return Iterator<const T>(Head, 0); }
  Iterator<const T> cend() const { return Iterator<const T>(Tail, Size); }
};

// We need to have this storage definition out-of-line so that the compiler can
// ensure that storage for the SentinelChunk is defined and has a single
// address.
template <class T, size_t N>
typename Array<T, N>::Chunk Array<T, N>::SentinelChunk;

} // namespace __xray

#endif // XRAY_SEGMENTED_ARRAY_H