Date: Fri, 25 Nov 1994 21:41:56 -0500 (CDT)
From: Aaron Ucko <UCKO AT VAX1 DOT ROCKHURST DOT EDU>
Subject: Re: -1441992/4 = 1073381326
To: OKRA AT max DOT tiac DOT net
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu
Organization: Rockhurst College; Kansas City, MO

>  This program:
> 
>int main() {
> 
>  long x = -1441992;
>  unsigned y = 4;
   ^^^^^^^^
> 
>  printf("%d / %d = %d\n",x,y,x/y);
>}
> 
>  Spits this out:
>
>-1441992 / 4 = 1073381326
>
>  The division is done at compile time if I compile with optimizations on. 
>A more complex program where the division can't be done at compile time
>gets the same result doing the division at run time.  Can anyone tell me
>why?  As far as I know, even in the strange world of CPU math, overflows
>can't account for this result.  The only thing I can think of is that the
>compiler is using DIV instead of IDIV (however, in this case, dividing by
>4, it shouldn't use any DIV).  The funny math only shows up when x is
>negative (never tried y negative...).  It doesn't matter whether I'm using
>the C or the C++ compiler.

Yes it should.  Since y is unsigned and x is `only' a long, x gets cast to
an unsigned before the division. (although actually it's not quite so
simple; the compiler chooses `unsigned long' as the data type large enough
to hold both numbers, so they both get cast to that, so a DIV is used.  Why
not just make y an ordinary signed integer?

--- Aaron Ucko (ucko AT vax1 DOT rockhurst DOT edu; finger for PGP public key) -=- httyp!
-=*=-Just because you're paranoid doesn't mean they aren't out to get you.-=*=-
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