[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 #7 from Paulo J. Matos <paulo@matos-sorge.com> ---
(In reply to Richard Biener from comment #5)
> Apart from expand there is the redundant-extension-elimination, ree.c.

In expand we get the following gimple for the loop:
;;   basic block 4, loop depth 0
;;    pred:       2
;;                4
  # i_15 = PHI <0(2), i_12(4)>
  # _18 = PHI <0(2), _4(4)>
  _6 = arr[_18];
  _7 = _6 + 1;
  arr[_18] = _7;
  _17 = (unsigned short) i_15;
  _13 = _17 + 1;
  i_12 = (short int) _13;
  _4 = (int) i_12;
  if (_4 < limit_5(D))
    goto <bb 4>;
  else
    goto <bb 3>;
;;    succ:       4
;;                3


Where _13 is an unsigned short and what we want to eliminate is this sign
extend:
  _4 = (int) i_12;

This doesn't seem trivial in the expand phase because to eliminate the sign
expand, you promote i_12 to int and have then to promote a bunch of other
variables, whose insn have been already emitted when you get here. Shouldn't
this be ivopts noticing that if it generates an int IV, it saves a sign extend
and therefore is better?