Fast vector experiments

fastvector.cc

// Essentially a std::vector<uint8_t>, but uses mmap+mremap to manipulate the
// Linux page table. Not portable. Memory is not freed when the class is
// destructed; use FastVector::free.

// In some basic benchmarks, this is not faster than realloc (with glibc),
// although in principle it should provide better guarantees that data movement
// is avoided (regardless of alloc size).

#include <sys/mman.h>

class FastVector {
 private:
  void* addr_ = nullptr;
  std::size_t size_ = 0;
  int errno_ = 0;

 public:
  FastVector(std::size_t minSize) {
    size_ = roundUpTo(minSize, PAGE_SIZE);

    addr_ = mmap(NULL, size_, PROT_READ | PROT_WRITE,
      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

    if (addr_ == MAP_FAILED)
      errno_ = errno;
  }

  std::ptrdiff_t resize(std::size_t newSize, bool roundUp = true) {
    if (roundUp)
      newSize = roundUpTo(newSize, PAGE_SIZE);

    std::size_t oldSize = size_;

    addr_ = mremap(addr_, oldSize, newSize, MREMAP_MAYMOVE);

    if (addr_ == MAP_FAILED) {
      errno_ = errno;
      return -1;
    }

    size_ = newSize;

    return newSize - oldSize;
  }

  inline std::uint8_t* operator*() {
    return static_cast<std::uint8_t*>(addr_);
  }

  inline std::size_t size() { return size_; }

  inline int getErrno() { return errno_; }

  static int free(void* addr, std::size_t size) {
    return munmap(addr, size);
  }
};