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From: "miyuki at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/67438] [6 Regression] ~X op ~Y pattern relocation causes loop performance degradation on 32bit x86
Date: Thu, 03 Sep 2015 03:36:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67438

Mikhail Maltsev <miyuki at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |miyuki at gcc dot gnu.org

--- Comment #4 from Mikhail Maltsev <miyuki at gcc dot gnu.org> ---
I looked at gimple dumps. The only difference looks like this. In the "good"
revision after forwprop1:

  <bb 3>:
  _13 = *in_2;
  a_14 = ~_13;
  _17 = MEM[(char *)in_2 + 1B];
  b_18 = ~_17;
  in_20 = &MEM[(void *)in_2 + 3B];
  _21 = MEM[(char *)in_2 + 2B];
  c_22 = ~_21;
  if (a_14 < b_18)
    goto <bb 4>;
  else
    goto <bb 5>;

In the "bad" revision this basic block is simplified:

  <bb 3>:
  _13 = *in_2;
  a_14 = ~_13;
  _17 = MEM[(char *)in_2 + 1B];
  b_18 = ~_17;
  in_20 = &MEM[(void *)in_2 + 3B];
  _21 = MEM[(char *)in_2 + 2B];
  c_22 = ~_21;
  if (_13 > _17)
    goto <bb 4>;
  else
    goto <bb 5>;

Next BB's are:

  <bb 4>: d_23 = MIN_EXPR <a_14, c_22>;
  <bb 5>: d_24 = MIN_EXPR <b_18, c_22>;
  <bb 6>: # d_4 = PHI <d_23(4), d_24(5)>

The condition of "if" is not altered throughout all other passes (it gets
if-converted and vectorized).

Another small difference: VRP adds assertions in bb 4 (a_12 lt_expr b_14, b_14
gt_expr a_12) and bb5 (a_12 ge_expr b_14, b_14 le_expr a_12). For some reason
this does not happen in the "bad" revision.

As I understand, the problem is that if we do not fold the condition, values
_13 and _17 are killed after we calculate a_14 = ~_13 and b_18 = ~_17. But if
we do fold, they are still live (because they are used in the condition), thus,
register pressure increases.