[Bug rtl-optimization/59999] [4.9 Regression] Sign extension in loop regression blocks generation of zero overhead loop

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59999

--- Comment #15 from rguenther at suse dot de <rguenther at suse dot de> ---
On Thu, 6 Feb 2014, paulo@matos-sorge.com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59999
> 
> --- Comment #14 from Paulo J. Matos <paulo@matos-sorge.com> ---
> Something like this which looks much simpler hits the same problem:
> extern int arr[];
> 
> void
> foo32 (int limit)
> {
>   short i;
>   for (i = 0; (int)i < limit; i++)
>     arr[i] += 1;
> }

Exactly the same problem.  C integral type promotion rules make
that i = (short)((int)i + 1) again.  Note that (int)i + 1
does not overflow, (short) ((int)i + 1) invokes implementation-defined
behavior which in our case is modulo-2 reduction.

Nothing guarantees that (short)i + 1 does not overflow.