Re: Miscompilation of remainder expressions

[David Daney <ddaney at avtrex dot com>]

Roberto Bagnara wrote:
> Robert Dewar wrote:
>
> > Roberto Bagnara wrote:
> >
> > > Reading the thread "Autoconf manual's coverage of signed integer
> > > overflow & portability" I was horrified to discover about GCC's
> > > miscompilation of the remainder expression that causes INT_MIN % -1
> > > to cause a SIGFPE on CPUs of the i386 family.  Are there plans to
> > > fix this bug (which, to me, looks quite serious)?
> > > All the best,
> > >
> > >     Roberto
> > >
> > > P.S. I checked whether this bug affects my code and it does.
> > >       Before yesterday I was completely unsuspecting of such
> > >       a fundamental flaw... I wonder how many know about it.
> >
> >
> > It's truly amazing for real code to be computing remainders
> > in this domain ... seems a bad idea to me, since very few
> > people are comfortably aware of what remainder means for
> > such cases.
>
>
> Everyone knows that dividing a number by -1 or 1 gives
> a 0 remainder.  To the contrary, no one expects a%b to
> raise SIFPE when b != 0.

On the contrary, since the beginning of time SIGFPE has been generated 
on GCC/x86/linux under these conditions.  This is wildly known.

Just because you just found out about it does not mean that 'no one' 
expects it.

David Daney