Re: How to guarantee memory access orthogonality without atomics?
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Thank you for commenting! So let me understand you: X is 4 bytes wide. It's based at address 27. Y is also 4 bytes wide. It's based at address 31. Cache lines happen to be 32 bytes long. Processor 0 can write to X any time, and processor 1 can write to Y anytime. Writing to Y causes an implicit read-modify-write to the line containing X. But this doesn't matter because it's assumed under C99/C11 that the processor will enforce the orthogonality of X and Y, provided that their sizes are at least a multiple of whole bytes based at whole-byte boundaries. Is that correct? "You can't have a data race if the threads don't access the same objects." -- If you or anyone else has a spec subsection you can cite for this, that would be most helpful. This is, in fact, the guarantee I'm seeking, to the extent that objects and base addresses are byte-granular. On Fri, Apr 27, 2012 at 4:03 PM, Andrew Haley <aph@xxxxxxxxxx> wrote: > > 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.