Re: How to guarantee memory access orthogonality without atomics?
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On 04/27/2012 09:49 AM, Leonard Plentz wrote: > I'm trying to find the answer to this question about the way that C11 > (or for that matter, C99) does (not) guarantee the > thread-orthogonality of various memory accesses in a cross-platform > manner. > > Suppose I have integers (in the abstract sense, not necessarily > (int)s) X and Y. They are both aligned on P-bit boundaries, such that > the X appears exactly Q bits before Y. (P and Q are generally powers > of 2 and equal, but not necessarily.) > > How do I find the minimum values of P and Q, in a cross-platform > manner, such that accesses to X and Y will never affect one another > due to a race? > > It may seem obvious that if X and Y are anything at least as large as > a byte, then this will simply never happen. But I'm not so sure > because: > > -- It's conceivable that someone could design a very efficient chip in > which it was assumed that no variables belonging to different threads > could ever share a cache line, and presumably C99/C11 could be > implemented on such a chip. I don't think it could. You can't have a data race if the threads don't access the same objects. The only way you'd be be able to implement C on a machine like this would be to allocate objects so that they didn't share cache lines. Andrew.