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Re: [RFC][PATCH][PR40921] Convert x + (-y * z * z) into x - y * z * z
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: kugan <kugan dot vivekanandarajah at linaro dot org>
- Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 19 Apr 2016 13:35:55 +0200
- Subject: Re: [RFC][PATCH][PR40921] Convert x + (-y * z * z) into x - y * z * z
- Authentication-results: sourceware.org; auth=none
- References: <56CFC02F dot 2070801 at linaro dot org> <CAFiYyc3ZVLexGz22eNxhHu1HNptELz-e64oEJ2rdO4Vohr15RQ at mail dot gmail dot com> <56D42341 dot 5090607 at linaro dot org>
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).
Richard.
> Thanks,
> Kugan