[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 #8 from Paulo J. Matos <paulo@matos-sorge.com> ---
(In reply to Paulo J. Matos from comment #7)
> (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?

Made a mistake. With the attached test, the final gimple before expand for the
loop basic block is:
;;   basic block 5, loop depth 0
;;    pred:       5
;;                4
  # i_26 = PHI <i_1(5), 0(4)>
  # ivtmp.24_18 = PHI <ivtmp.24_12(5), ivtmp.24_29(4)>
  _28 = (void *) ivtmp.24_18;
  _13 = MEM[base: _28, offset: 0B];
  x.4_14 = x;
  _15 = _13 ^ x.4_14;
  MEM[base: _28, offset: 0B] = _15;
  ivtmp.24_12 = ivtmp.24_18 + 4;
  temp_ptr.5_17 = (Sample *) ivtmp.24_12;
  _11 = (unsigned short) i_26;
  _2 = _11 + 1;
  i_1 = (short int) _2;
  _10 = (int) i_1;
  if (_10 < _25)
    goto <bb 5>;
  else
    goto <bb 6>;
;;    succ:       5
;;                6

However, the point is the same. IVOPTS should probably generate an int IV
instead of a short int IV to avoid the sign extend since removing the sign
extend during RTL seems to be quite hard.

What do you think?