Re: unsigned / double propagation weirdness?

[Richard Guenther <rguenth at tat dot physik dot uni-tuebingen dot de>]

Falk Hueffner wrote:
> Richard Guenther <rguenth@tat.physik.uni-tuebingen.de> writes:
>
>
> > Hi, I'm getting confusing (but consistent along a lot of different
> > compilers) results out of this testcase:
> >
> >
> > int main(int argc, char **argv)
> > {
> >   double dm = 1.0;
> >   unsigned int i = 1;
> >   if (-i*dm != -((double)i)*dm)
> >     abort();
> >   return 0;
> > }
> >
> > From the standard I cannot understand why this should abort.
>
>
> The point is that "-i" is still an unsigned number. "-i" basically
> means "~i + 1", so it's something largish, but not someting negative.
> Another cute instance is:
>
> int a[10];
> int *b = a+1;
> unsigned i = 1;
> return b[-i];
>
> This works on 32 bit platforms and crashes on 64 bit platforms.
>
> A while ago I proposed generally warning about negation of unsigned
> entities, but there were a few objections, and I never got around to
> finishing it.

Warning about such would be indeed very useful.  We even warn about signed/unsigned
comparisons, so there is precedent.

Richard.