Re: GNU C keyword volatile, GNU compiler/optimizer and instruction re-ordering

Nick Maclaren <nmm1@cus.cam.ac.uk> wrote:
+ Mike Stump <mrs@kithrup.com> wrote:
+>Nick Maclaren <nmm1@cus.cam.ac.uk> wrote:
+>>Unfortunately, volatile is a complete mess.
+>
+>I think this statement is overly pessimistic. I disagree entirely
+>with this assessment of volatile. I think it is clear what it means,
+>and it is useful. I think the C++ standard clarifies some points for
+>users that might not otherwise know what it means. I think it would
+>be possible to write down exactly what it means and add this to the
+>language standards, but such an undertaking absolutely must be done by
+>people that know what it means. The C++ standard almost got into
+>serious trouble trying to refine and fix volatile. Unfortunately all
+>the right people to do this weren't available to do the right job.
+>Fortunately disaster was averted.


+ Hmm. Well, having been an active member of the BSI C Panel during
+ both the C90 and C99 public comment periods, I can tell you that the
+ working group did not agree with what it should mean, let alone
+ whether the wording made this clear.


+ If you are serious, then I suggest that you write a clear description
+ in standardese and submit it to the working group. Plenty of people
+ on that have tried doing what you say, and so far failed.


The problems get very troublesome. Can a conforming implementation
include a debugger that lets me modify both volatile and non-volatile
variables? What if I only modify volatiles ones? Exactly when is a
register-resident copy of a volatile variable fresh enough? What's
meant by a sequence point? What does it mean to be before or after a
sequence point? I can go on.


That said, I agree with Mike that a clear writeup of these notions iss
both doable and important. (I just know that I find it difficult.) I
would propose that a first project would be to write a very clear
explanation of these concepts in prose that is common to mathematics
and theoretical computer science. When that is in satisfactory shape,
then translate it into standardese.


Tom Payne

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