basic VRP min/max range overflow question

[Joseph S. Myers]

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/
    jsm@polyomino.org.uk (personal mail)
    joseph@codesourcery.com (CodeSourcery mail)
    jsm28@gcc.gnu.org (Bugzilla assignments and CCs)