17.12.1 Overview

Most of the time we want to write code that is correct on every architecture. To achieve that, we base it on the memory model described in the C11 standard. The compiler might implement some operations in a straightforward manner or emit special instructions to enforce certain guarantees, when the actual hardware architecture is weaker.

Contrary to Intel 64, the C11 memory model is rather weak. It guarantees data dependency ordering, but nothing more, so in the classification mentioned in section 17.4 it corresponds to the second one: weak with dependency ordering. There are other hardware architectures that provide similar weak guarantees, for example ARM.

Because of C weakness, to write portable code wecannot assumethat it will be executed on an usually strong architecture, such as Intel 64, for two reasons.

• When recompiled for other, weaker architecture, the observed program behavior will change because of how the hardware reorderings work.

• When recompiled for the same architecture, compiler reorderings that do not break the weak ordering rules imposed by the standard might occur. That can change the observed program behavior, at least for some execution traces.