[Bug tree-optimization/71632] [7 Regression] hang at -O3 on x86_64-linux-gnu

["amker at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71632

--- Comment #3 from amker at gcc dot gnu.org ---
The commit if-converts below loop:

  <bb 4>:
  # b.5_38 = PHI <0(3), _10(7)>
  # d_40 = PHI <d_42(3), iftmp.0_22(7)>
  # ivtmp_17 = PHI <5(3), ivtmp_37(7)>
  _6 = (long unsigned int) b.5_38;
  _7 = _6 * 8;
  _8 = _5 + _7;
  _9 = *_8;
  if (_9 > d_40)
    goto <bb 6>;
  else
    goto <bb 5>;

  <bb 5>:

  <bb 6>:
  # iftmp.0_22 = PHI <1.0e+0(4), 0.0(5)>
  _10 = b.5_38 + 1;
  ivtmp_37 = ivtmp_17 - 1;
  if (ivtmp_37 != 0)
    goto <bb 7>;
  else
    goto <bb 8>;

  <bb 7>:
  goto <bb 4>;

into :
  <bb 4>:
  # b.5_38 = PHI <0(3), _10(5)>
  # d_40 = PHI <d_42(3), iftmp.0_22(5)>
  # ivtmp_17 = PHI <5(3), ivtmp_37(5)>
  _6 = (long unsigned int) b.5_38;
  _7 = _6 * 8;
  _8 = _5 + _7;
  _9 = *_8;
  iftmp.0_22 = _9 > d_40 ? 1.0e+0 : 0.0;
  _10 = b.5_38 + 1;
  ivtmp_37 = ivtmp_17 - 1;
  if (ivtmp_37 != 0)
    goto <bb 5>;
  else
    goto <bb 6>;

  <bb 5>:
  goto <bb 4>;

It looks like a correct transform, even the result loop can't be vectorized.

Then the loop is completely unrolled and and long single def-use chain for
"iftmp" is generated.  I suspect this causes exponential behavior in TER?