I was testing the relaxed ordering semantic of c++11 memory model on x64, and I was told that on x86/64 only store/load reordering exists, so I wrote the following program to test the relaxed ordering.
Ideally, if reordering exists(which it does), then my program should hit the case of getting "g_a == g_b == 0", but I tested it for a long time, and never get the expected results, could anybody help explain why? thanks.
[update]
sorry for forgetting to mention the compiler I used, the following code won't work when compiled with g++ 4.8.3 on Linux x86/64. thanks to @Mat's reminder, I then try compile it using clang++ 3.4.2, this time I saw the reordering, so it might be a bug in g++.
#include <iostream> #include <thread> #include <atomic> using namespace std; atomic<int> g_a, g_b; atomic<int> g_x, g_y; memory_order order = memory_order_relaxed; void bar1() { register int t = 0; g_x.store(42, order); t = g_y.load(order); g_a = t; } void bar2() { register int t = 0; g_y.store(24, order); t = g_x.load(order); g_b = t; } int main() { for (int i = 0; i < 1000000; ++i) { g_a = 0; g_b = 0; g_x = 0; g_y =0; thread t1(&bar1); thread t2(&bar2); t1.join(); t2.join(); if (g_a.load(order) == 0 && g_b.load(order) == 0) { cout << "g_a == g_b == 0" << endl; } } } 
-O3 -std=c++11 -pthreadusing libstdc++ 4.7.3) on Linux/x86_64. Not seeing this with GCC 4.9.2. Not seeing it with clang++ using libc++.