Re: The annotated annotated annotated C standard part 3



"Clive D. W. Feather" wrote:

In article <aM-dnZzsy5HFZw7anZ2dnUVZ8tGqnZ2d@xxxxxx>, Richard Heathfield
<rjh@xxxxxxxxxxxxxxx> writes
Your writings have created a set of mis-usages and errors in their
turn. For example, we have people raving here that "sequence points
are compilers 101" when in fact they aren't,

If that term means what I think it is then, no, they aren't something
that a beginner compiler-writer might think of.

Here, I think I'd have to agree to differ. They seem to me to be a
perfectly natural way of thinking about compilation.

I'm happy to differ. I'd agree that there's a need to think about things
like "when do I need to flush registers?", but I think the C Standard
took this to a new stage beyond existing language documents.

There are several things at play. Sequence points in the C standard
identified one of the major issues of side-effect and parallel execution.
When you have two assignments to the same variable between
sequence points there are potentially undefined results. The C language
support for auto increment found in the PDP-11 and now many other
processors was an early taste of things to come for processors with
multiple execution streams. It is a testament to C that it can map
effectively onto some diverse architectures.

C was correct in not trying to force compilers to detect multiple writes
between sequence points. I think it was Richard who wrote a small
example whose detection would have been difficult in a compiler.

C is an implementation language, writing ambiguous statements should
produce undefined results. Craftsmen who use the language should
be clear in how they use the language.


Regards

--
Walter Banks
Byte Craft Limited
Tel. (519) 888-6911
Fax (519) 746 6751
http://www.bytecraft.com
walter@xxxxxxxxxxxxx


.

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