[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 #19 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 #16 from Paulo J. Matos <paulo@matos-sorge.com> ---
> (In reply to rguenther@suse.de from comment #15)
> > 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.
> 
> OK, that makes sense. But in GCC 4.8 that doesn't seem to be what happens.
> It seems to be i = (short) ((unsigned short) i + 1)
> Later i is cast to int for comparison.
> 
> Before ivopts this is the end of the loop body:
>   i.7_19 = (unsigned short) i_26;
>   _20 = i.7_19 + 1;
>   i_21 = (short intD.8) _20;
>   _10 = (intD.1) i_21;
>   if (_10 < _25)
>     goto <bb 7>;
>   else
>     goto <bb 6>;
> 
> i is initially a short, then moved to unsigned short. The addition is performed
> and returned to short. Then cast to int for the comparison.
> 
> For GCC 4.5.4 the end of loop body is:
>   iD.2767_18 = iD.2767_26 + 1;
>   D.5046_9 = (intD.0) iD.2767_18;
>   if (D.5046_9 < D.5047_25)
>     goto <bb 5>;
>   else
>     goto <bb 6>;
> 
> Here the addition is made in short int and then there's only one cast to int.

Yes, and thus GCC 4.5 still contains the bug that i++ invokes undefined
behavior when overflowing (which it does not).