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multithreadingc++11memory-model

Are C++11 compilers allowed to introduce additional loads of atomic variables?

In this answer, bdonlan states that code similar to the following:

int t;
volatile int a, b;

t = x;
a = t;
b = t;

may be transformed by the compiler into:

a = x;
b = x;

My question is, is this still allowed if x is an atomic variable with relaxed loads, as in the following?

atomic<int> x;

int t;
volatile int a, b;

t = x.load(std::memory_order_relaxed);
a = t;
b = t;
assert(a == b);    // Will this hold?

As the title says, are C++11 compilers allowed to introduce additional loads of atomic variables? What about additional stores?

Solution

  • The compiler is allowed to do anything under the as-if rule, but to load the same memory location twice, the compiler would have to show that no other thread could be accessing the same variable. Either way, you are guaranteed that a==b in your example.