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

[paulo@matos-sorge.com]

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.