Messages in this thread |  | | From | dewar@gnat ... | Subject | RE: [PATCH] C undefined behavior fix | Date | Thu, 10 Jan 2002 07:18:45 -0500 (EST) |
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<<Note that this is not too much of a problem for system programming, as you have a way to be sure they are not combined: just use intermediate variables and set them separately; the nice thing there is that as you use these intermediate variables just once, the compiler will eliminate them. But be careful: the sequence point MUST BE RETAINED, and then the two loads cannot be combined (in case 1 of course). >>
Of course we all understand that sequence points myust be retained, but this is a weak condition compared to the rule that all loads and stores for volatile variables must not be resequenced, and in particular, you seem to agree that two loads *can* be combined if they both appear between two sequence points. I think that's unfortunate, and it is why in Ada we adopted a stricter point of view that avoids the notion of sequence points.
It even seems that if you have two stores between two sequence points then the compiler is free to omit one, and again that seems the wrong decision for the case of volatile variables. If it can omit a store in this way, can it omit a load, i.e. if we have:
x := v - v;
can someone read the sequence point rule to mean that the compiler is free to do only one load here? I hope not, but we have already seen how much confusion there is on this point. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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