On 2015-11-08 20:33:59 +0100, David Brown wrote:
> On 08/11/15 20:11, Florian Weimer wrote:
> >On 11/06/2015 01:32 PM, David Brown wrote:
> >>How about this case:
> >>
> >>int foo(int x) {
> >> if (x > 1290) {
> >> printf("X is wrong here %d, but we don't care\n", x);
> >> }
> >> return x*x*x;
> >>}
> >>
> >>The compiler can eliminate the check and the printf.
> >
> >I don't think the compiler can do that because printf has an externally
> >visible effect, which is sequenced before the undefined behavior, so
> >this program transformation would not be permitted under the as-if rule.
> >
>
> My understanding (and correct me if it's wrong) is that undefined behaviour
> is a feature of a program, rather than just part of it - i.e., if code
> reaches something that causes undefined behaviour, the effect of the whole
> program is undefined. This means that if the compiler knows that it is
> going to hit an integer overflow, it can affect the course of things that
> would have happened without that undefined behaviour.
I agree. However, one may wonder whether the return statement is
always reachable. I mean that since printf does I/O, it might
block forever. But I'm not sure...
--
Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
