4.2.2 Ordering

The ordering of visibility of side effects performed in other threads is undefined, unless explicit synchronization is used. Implementations of the library may guarantee that the visibility of side-effects performed by another thread is ordered according to the control flow of the other thread (strong ordering), but multi-processor implementations might not be strongly ordered. Portable code should not assume strong ordering, and should use explicit synchronization where the order of side effects is important. There is currently no library introspection facility to determine if the implementation is strongly or weakly ordered.

Because of the possibility of weak ordering, the compiler is free to assume that the effects of other threads may be ignored between explicit synchronization points, and it may perform any optimizations which preserve the semantics of a single-thread model regardless of their effects on other threads -- for example, common sub-expression elimination, or changing the order of evaluation.

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Common Dylan and Functional Extensions - 31 Mar 00