Re: basic VRP min/max range overflow question

[Paul Schlie <schlie at comcast dot net>]

> From: Robert Dewar <dewar@adacore.com>
> Paul Schlie wrote:
> 
>> As in my mind, the semantics of foo() dictate that it print the value of
>> the storage location which was allocated to the variable "a", where unless
>> "a" is initialized with an explicit value, may be arbitrary. So I've got no
>> problem with arbitrary results or behavior, I just simply believe they are
>> implicitly constrained to the remaining rules of the language, i.e. all
>> side-effects must be expressed upon reaching a sequence point which
>> logically bounds the effects of the evaluation of any expression.
> 
> This cannot be formalized, and is not what the standard says. The fact that
> you believe it is interesting (though I don't think you can write a formal
> description of what you believe in C standard terms), but we operate by what
> the standard formally says, not by what one person informally believes.
>
>> (where if an undefined behavior it did delete the program being executed it
>> wouldn't resume execution beyond the next sequence point, but if it does, it
>> must continue to abide by the languages rules regardless of the resulting
>> side effects from the preceding behaviors)
> 
> Sequence points simply do not have this semantics. If they did, then nearly
> all useful optimizations would be prohibited. You are essentially positing
> a model in which the state at every sequence point is not only defined by
> the standard, but must be reflected in the implementation with no use of
> as-if semantics. I don't see this as meaningful, and I don't think this can
> be formalized. I am quite *sure* that it is undesirable.

- You may be correct, although it's not obviously the case? (As requiring
  all undefined behavior be encapsulated between sequence points already
  seems implied to me, as I don't see any explicit counter examples
  requiring otherwise. Nor do any optimizations which are obviously more
  useful than potentially counterproductive seem to require the violation of
  C's sequence point semantics?)