问为什么这个测试应用程序在不同的机器上给出了不同的结果？(关于增量后和线程安全)

EN

   unsigned long gCounter = 0;
   const unsigned long WORKS = 1048576;  // pow(2, 20)
   const unsigned long MAX_THREADS = 100;
   const unsigned long WORKER_THREADS = 2;

   unsigned long GetCounter(){
           return gCounter++;
   }

   void *WorkerThread(void *){
           unsigned long items = 0;
           do {
                GetCounter();
                items++;
           } while(items < WORKS);

           printf("Exiting thread: %lu\n", pthread_self());
           return NULL;
    }


    int main(int argc, char* argv[]){
         pthread_t workers[MAX_THREADS];

         //create two threads
         for (int i = 0; i < WORKER_THREADS; i++){
            pthread_create(&workers[i], NULL, WorkerThread, NULL);
         }

         //wait for above threads to exit
         for (int i = 0; i < WORKER_THREADS; i++){
               pthread_join(workers[i], NULL);
         }


         assert( gCounter == (WORKER_THREADS * WORKS));
         return 0;
    }

     Server machine:
     System: Redhat 3.4.2 - 6.FC3  Kernel: LINUX.2.6.10 
     GCC Version: 3.4.2 20041017   
     CPU: AMD Opteron Processor 144 X86_64

     My PC:
     System: ubuntu 3.2.0-40-generic  Kernel: LINUX.3.4
     GCC Version: 4.6.3 (Ubuntu/Linaro )    
     CPU: Intel(R) Pentium(R) Dual  CPU  T2370        

     Build Command (both are the same, turn off optimization)
     g++ -O0 -o test test.cpp -lpthread