Re: unsigned / double propagation weirdness?

[Falk Hueffner <falk dot hueffner at student dot uni-tuebingen dot de>]

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.

-- 
	Falk