http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59999
--- Comment #17 from Paulo J. Matos <pa...@matos-sorge.com> --- (In reply to rguent...@suse.de from comment #15) > On Thu, 6 Feb 2014, pa...@matos-sorge.com wrote: > > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59999 > > > > --- Comment #14 from Paulo J. Matos <pa...@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. I am being thick... indeed I forgot to notice that i++ also invokes undefined behaviour. I guess then GCC sorts that out by casting i into unsigned short for the addition and all the remaining issues then unfold.
