Function detectJemalloc

3rdparty/libprocess/src/memory_profiler.cpp:145–187

The `detectJemalloc()` function below was taken from the folly library (called `usingJEMalloc()` there), originally distributed by Facebook, Inc under the Apache License. It checks whether jemalloc is used as the current malloc implementation by allocating one byte and checking if the threads allocation counter increases. This requires jemalloc to have been compiled with `--enable-stats`.

// allocating one byte and checking if the threads allocation counter increases.
// This requires jemalloc to have been compiled with `--enable-stats`.
bool detectJemalloc() noexcept {
#ifndef LIBPROCESS_ALLOW_JEMALLOC
  return false;
#else
  static const bool result = [] () noexcept {
    // Some platforms (*cough* OSX *cough*) require weak symbol checks to be
    // in the form if (mallctl != nullptr). Not if (mallctl) or if (!mallctl)
    // (!!). http://goo.gl/xpmctm
    if (mallctl == nullptr || malloc_stats_print == nullptr) {
      return false;
    }

    // "volatile" because gcc optimizes out the reads from *counter, because
    // it "knows" malloc doesn't modify global state...
    volatile uint64_t* counter;
    size_t counterLen = sizeof(uint64_t*);

    if (mallctl("thread.allocatedp", static_cast<void*>(&counter), &counterLen,
                nullptr, 0) != 0) {
      return false;
    }

    if (counterLen != sizeof(uint64_t*)) {
      return false;
    }

    uint64_t origAllocated = *counter;

    // Static because otherwise clever compilers will find out that
    // the ptr is not used and does not escape the scope, so they will
    // just optimize away the malloc.
    static const void* ptr = malloc(1);
    if (!ptr) {
      // wtf, failing to allocate 1 byte
      return false;
    }

    return (origAllocated != *counter);
  }();

  return result;
#endif
}


template<typename T>