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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/43491] Unnecessary temporary for global register variable
Date: Thu, 16 Feb 2012 14:55:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43491

--- Comment #7 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-02-16 14:43:51 UTC ---
With tree hoisting we generate

<bb 2>:
  pretmp.5_19 = data_0;
  pretmp.5_20 = data_3;
  i_21 = pretmp.5_19 + pretmp.5_20;
  if (data_3(D) != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 3>:

<bb 4>:
  # v_1 = PHI <v_7(D)(3), 2(2)>
  # i_2 = PHI <i_21(3), 5(2)>
  D.1719_14 = v_1 * i_21;
  D.1718_15 = i_2 * D.1719_14;
  return D.1718_15;

instead of

<bb 2>:
  if (data_3(D) != 0)
    goto <bb 4>;
  else
    goto <bb 3>;

<bb 3>:
  pretmp.5_19 = data_0;
  pretmp.5_21 = data_3;
  i_23 = pretmp.5_19 + pretmp.5_21;
  goto <bb 5>;

<bb 4>:
  data_0.0_4 = data_0;
  data_3.1_5 = data_3;
  i_6 = data_0.0_4 + data_3.1_5;

<bb 5>:
  # v_1 = PHI <v_7(D)(4), 2(3)>
  # i_2 = PHI <i_6(4), 5(3)>
  # i_24 = PHI <i_6(4), i_23(3)>
  D.1719_14 = v_1 * i_24;
  D.1718_15 = i_2 * D.1719_14;
  return D.1718_15;

}

I suppose that's good enough?  See that PRE still inserts loads from
register variables, not sure if you'd want to disallow that as well.