Re: [patch tree-ssa-reassoc.c]: Better reassoication for comparision and boolean-logic

[Richard Guenther <richard.guenther@gmail.com>]

On Thu, May 19, 2011 at 2:56 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Thu, May 19, 2011 at 2:48 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
>> Hello,
>>
>> This patch improves reassociation folding for comparision. It expands
>> expressions within binary-AND/OR expression like (X | Y) == 0 to (X ==
>> 0 && Y == 0)
>> and (X | Y) != 0 to (X != 0 || Y != 0).  This is necessary to allow
>> better reassociation
>> on weak pre-folded logical expressions.  This unfolding gets undone
>> anyway later by pass,
>> so no disadvantage gets introduced.
>> Also while going through BB-list, it tries to do some little
>> type-sinking for SSA sequences
>> like "D1 = (type) bool1; D2 = (type) bool2; D3 = D1 & D2;' to 'D1 =
>> bool1 & bool2; D2 = (type) D1;'.
>> This folding has the advantage to see better through intermediate
>> results with none-boolean type.
>> The function eliminate_redundant_comparison () got reworded so, that
>> doesn't break in all cases.
>> It now continues to find duplicates and tries to find inverse variant
>> (folded to constant). By this
>> change we don't combine possible weak optimizations too fast, before
>> we can find and handle
>> inverse or duplicates.
>
> sinking casting belongs not here but instead to tree-ssa-forwprop.
> I'm not sure that a != 0 | b != 0 is the better canonical variant than
> a | b != 0 though.

For example w/o your patch I see on the first testcase:

<bb 2>:
  D.2689_3 = a_2(D) != 0;
  D.2690_5 = b_4(D) != 0;
  D.2691_6 = D.2690_5 & D.2689_3;
  if (D.2691_6 != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 3>:
  D.2693_7 = b_4(D) | a_2(D);
  D.2694_8 = D.2693_7 == 0;
  D.2695_10 = c_9(D) != 0;
  D.2696_11 = D.2695_10 & D.2694_8;
  D.2685_16 = (int) D.2696_11;

canonicalizing to the latter (which is also smaller) would be

<bb 2>:
  D.2691_5 = a_2(D) & b_4(D);
  D.2691_6 = D.2691_5 != 0;
  if (D.2691_6 != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 3>:
  D.2693_7 = b_4(D) | a_2(D);
  D.2694_8 = ~D.2693_7;
  D.2696_11 = c_9(D) & D.2694_8;
  D.2696_11 = D.2696_11 != 0;
  D.2685_16 = (int) D.2696_11;

that looks re-associatable as good as the other.

Richard.