[RFC][PATCH][PR40921] Convert x + (-y * z * z) into x - y * z * z

[Richard Biener]

On Tue, Apr 19, 2016 at 1:36 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Tue, Apr 19, 2016 at 1:35 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Mon, Feb 29, 2016 at 11:53 AM, kugan
>> <kugan.vivekanandarajah@linaro.org> wrote:
>>>>
>>>> Err.  I think the way you implement that in reassoc is ad-hoc and not
>>>> related to reassoc at all.
>>>>
>>>> In fact what reassoc is missing is to handle
>>>>
>>>>   -y * z * (-w) * x -> y * x * w * x
>>>>
>>>> thus optimize negates as if they were additional * -1 entries in a
>>>> multiplication chain.  And
>>>> then optimize a single remaining * -1 in the result chain to a negate.
>>>>
>>>> Then match.pd handles x + (-y) -> x - y (independent of -frounding-math
>>>> btw).
>>>>
>>>> So no, this isn't ok as-is, IMHO you want to expand the multiplication ops
>>>> chain
>>>> pulling in the * -1 ops (if single-use, of course).
>>>>
>>>
>>> I agree. Here is the updated patch along what you suggested. Does this look
>>> better ?
>>
>> It looks better but I think you want to do factor_out_negate_expr before the
>> first qsort/optimize_ops_list call to catch -1. * z * (-w) which also means you
>> want to simply append a -1. to the ops list rather than adjusting the result
>> with a negate stmt.
>>
>> You also need to guard all this with ! HONOR_SNANS (type) && (!
>> HONOR_SIGNED_ZEROS (type)
>> || ! COMPLEX_FLOAT_TYPE_P (type)) (see match.pd pattern transforming x
>> * -1. to -x).
>
> And please add at least one testcase.

And it appears to me that you could handle this in linearize_expr_tree
as well, similar
to how we handle MULT_EXPR with acceptable_pow_call there by adding -1. and
op into the ops vec.

Similar for the x + x + x -> 3 * x case we'd want to add a repeat op when seeing
x + 3 * x + x and use ->count in that patch as well.

Best split out the

  if (rhscode == MULT_EXPR
      && TREE_CODE (binrhs) == SSA_NAME
      && acceptable_pow_call (SSA_NAME_DEF_STMT (binrhs), &base, &exponent))
    {
      add_repeat_to_ops_vec (ops, base, exponent);
      gimple_set_visited (SSA_NAME_DEF_STMT (binrhs), true);
    }
  else
    add_to_ops_vec (ops, binrhs);

pattern into a helper that handles the other cases.

Richard.

> Richard.
>
>> Richard.
>>
>>> Thanks,
>>> Kugan