Re: GCC optimizes integer overflow: bug or feature?

[Marcin Dalecki <martin at dalecki dot de>]

> But the same applies to floating point numbers. There, the  
> situation is even better, because nowadays I can rely on a float or  
> double being the representation defined in IEEE 754 because there  
> is such overwhelming hardware support.

You better don't. Really! Please just realize for example the impact  
of the (in)famous 80 bit internal (over)precision of a
very common IEEE 754 implementation...

volatile float b = 1.;

if (1. / 3. == b / 3.) {
   printf("HALLO!\n")
} else {
   printf("SURPRISE SURPRISE!\n");
}

or just skim through http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323

> > However it's a quite common mistake to forget how "bad" floats  
> > "model" real numbers.
>
> And it's quite a common mistake to forget how "bad" finite ints  
> "model" integer numbers.

No it isn't. Most people don't think in terms of infinite arithmetics  
when programming.
And I hold up that the difference between finite and infinite is  
actually quite a fundamental
concept. However quite a lot of people expect the floating  
arithmetics rouding to give them
well behaved results.

Marcin Dalecki