On Sat, 18 Jun 2005, Paul Schlie wrote: > Maybe I didn't phrase my statement well; I fully agree with the cited > paragraph above which specifically says a program containing unspecified > behavior "shall be a correct program and act in accordance with > 5.1.2.3". Which specifies program execution, in terms of an abstract machine > model, which correspondingly requires:
You appear to have confused unspecified behavior (where the possibilities are bounded) and undefined behavior (where the possibilities are unbounded). On *undefined* behavior (such as signed integer overflow), *this International Standard imposes no requirements*. If a program execution involved undefined behavior, *there are no requirements on its execution, even before the undefined behavior occurs in the abstract machine*. Therefore the compiler assumes that you only ever pass it programs which do not execute undefined behavior. If a possible execution might involve undefined behavior, the compiler presumes that the programmer knows more than it can prove and knows that the relevant circumstances cannot arise at execution. For example, a correct program never involves overflow of a signed loop variable, so the compiler presumes that the programmer proved that the loop variable can never overflow at execution and uses this information to optimize the loop: it cannot prove it by itself but using the presumption that the program is correct it can optimize the program better. The traditional form of undefined behavior is for demons to fly out of your nose. We just haven't yet got -fnasal-demons working reliably but it would be conforming for it to be on by default. If you are lucky, it will happen anyway without that option. http://groups.google.com/groups?hl=en&selm=10195%40ksr.com -- Joseph S. Myers http://www.srcf.ucam.org/~jsm28/gcc/ [EMAIL PROTECTED] (personal mail) [EMAIL PROTECTED] (CodeSourcery mail) [EMAIL PROTECTED] (Bugzilla assignments and CCs)
