Re: [Bug c/35753] apparent integer math bug

[Andrew Pinski <pinskia at gmail dot com>]

Sent from my iPhone

On Mar 29, 2008, at 12:16, "regehr at cs dot utah dot edu" <gcc-bugzilla@gcc.gnu.org 
> wrote:

> ------- Comment #3 from regehr at cs dot utah dot edu  2008-03-29  
> 19:16 -------
> Subject: Re:  apparent integer math bug
>
> > This multiplication overflows so the resulting behavior is undefined.
>
> Right-- but always in the past it has been (or seemed) fine to assume
> two's complement overflow behavior for signed arithmetic under gcc on
> two's complement platforms.
>
> I have seen the "strict overflow" options and those seem quite
> reasonable.  However, changing overflow behavior for multiply seems  
> risky!
>
> Is it still reasonable at least to rely on two's complement behavior  
> for
> signed addition and subtraction?
It never was reasonable to depend on this behaviour, just GCC got  
better at optimizing these cases. We have had the option -fwrapv since  
at least 3.3 so that people can use that option if the want full  
wrapping behavior with signed types. Note the C standard is over 18  
years old now so I had hoped people would still not making the mistake  
of thinking signed interger being wrapping types.



> Thanks,
>
> John Regehr
>
>
> --
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35753