Question

“64-bit NoBarrier_Store() not implemented on this platform”
I use tcmalloc on win7 with vs2005.
There is two threads in my app, one do malloc(), the other one do free().The tcmalloc print this when my app start.After debug, i find the following functon can’t work on _WIN32,

// Return a suggested delay in nanoseconds for iteration number "loop"
static int SuggestedDelayNS(int loop) {
  // Weak pseudo-random number generator to get some spread between threads
  // when many are spinning.
  static base::subtle::Atomic64 rand;
  uint64 r = base::subtle::NoBarrier_Load(&rand);
  r = 0x5deece66dLL * r + 0xb;   // numbers from nrand48()
  base::subtle::NoBarrier_Store(&rand, r);

  r <<= 16;   // 48-bit random number now in top 48-bits.
  if (loop < 0 || loop > 32) {   // limit loop to 0..32
    loop = 32;
  }
  // loop>>3 cannot exceed 4 because loop cannot exceed 32.
  // Select top 20..24 bits of lower 48 bits,
  // giving approximately 0ms to 16ms.
  // Mean is exponential in loop for first 32 iterations, then 8ms.
  // The futex path multiplies this by 16, since we expect explicit wakeups
  // almost always on that path.
  return r >> (44 - (loop >> 3));
}

I want to know how to avoid this on win32. thanks very much.

Answer 1

It seems to be using atomic loads and stores without memory barriers. Might make this work a bit faster on some multi-CPU systems.

On an x86, we don’t have those types of operations. Loads and stores are always visible to the others cores in the system. Cache sync is implemented in the hardware, and can’t be controlled by the program.

Perhaps the Atomic library used has Load and Store operations without the NoBarrier prefix? Use those instead.