You missed my point or perhaps I missed yours.

The generated code in our compiler code is the same  as
(A != B) which is generally implemented as a XOR. I assumed
you were referring a well known common compiler bug where
if ((A == B) != 1) the (A==B) part implemented as a XOR
and is then compared to 1 fails. I walked through the ISO
definitions in my example. BTW this has shown up in different
forms as a WG-14 question at ISO an number of times in the
last few years.

w..

Sergio Masci wrote:

> ----- Original Message -----
> From: Walter Banks <walter@bytecraft.com>
> To: Microcontroller discussion list - Public. <piclist@mit.edu>
> Sent: Friday, October 08, 2004 4:39 PM
> Subject: Re: [PIC] new optimisation in XCSB
>
> >
> >
> > Sergio Masci wrote:
> >
> > > An insteresting question is what does you favourite PIC C compiler make of
> > >
> > >         if ((A == B) == 0)
> > > or
> > >         if ((A == B) != 1)
> > >
> > > xcsb sees this as
> > >
> > >         if  A != B then
> > >
> >
> > C logical operations return 0 or 1. 0 is false 1 is true
> > When logically testing 0 is false non zero is true
> >
> > case of A != B
> >
> >  if ((A == B) == 0)
> >
> > if ( (0) == 0)
> >
> > if (0 == 0)
> >
> > if (1)  The if condition is true
>
> No "(A == B)" can return 0 or 1 so the result is not known at compile time. "(A
> == B)" cannot be reduced to 0 at compile time.
>
> >
> > =====================
> >
> > if ((A == B) != 1)
> >
> > if ((0) != 0)
> > if (0 != 0)
> > if (1)    The if condition is true
>
> Again "(A == B)" cannot be reduced to 0 at compile time, however it is known
> that the result of "(A == B)" can only be 0 or 1 so in this case "!= 1" is the
> same as "== 0"
>
> >
> > =======================
> >
> > I agree that it is the same as
> >  if (A !=B)
> >
> > Walter..
>
> Regards
> Sergio Masci
>
> http://www.xcprod.com/titan/XCSB - optimising PIC compiler
>
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