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c++optimizationconcurrencyrestrictrestrict-qualifier

Can I `__restrict__ this` somehow?

I've been watching Mike Acton's talk on Data-oriented design in C++ in CppCon 2014, and he gives this example:

int Foo::Bar(int count)
{
    int value = 0;
    for (int i = 0; i < count; i++) {
        if (m_someDataMemberOfFoo) value++
    }
    return value;
}

And explains how some compilers keep re-reading m_someDataMemberOfFoo in every iteration, perhaps because its value might change due to concurrent access. Regardless of whether it's appropriate for the compiler to do so - can one tell the compiler to specifically ignore any possibility of concurrent access to anything during the execution of some method, so that it may optimize better?

In other words, can I tell it the compiler that this is __restrict__ed?

Solution

    1. __restrict__ is not standardized in C++, so this question can only be answered on a particular platform. For GCC, you can apply __restrict__ to this in the same way as const:

      void T::fn () __restrict__

    2. There is no potential aliasing in your example. C++ specifies undefined behavior for data races.

    3. A new system for C++ restricted pointers is being developed. It will likely be standardized in C++17. Support for this is one of the stated design goals.