Re: [PATCH] Factor out division by squares and remove division around comparisons (1/2)

[Jeff Law <law at redhat dot com>]

On 08/17/2017 03:55 AM, Wilco Dijkstra wrote:
> Richard Biener wrote:
>> On Tue, Aug 15, 2017 at 4:11 PM, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
>>> Richard Biener wrote:
>>>>> We also change the association of
>>>>>
>>>>>       x / (y * C) -> (x / C) / y
>>>>>
>>>>> If C is a constant.
>>>>
>>>> Why's that profitable?
>>>
>>> It enables (x * C1) / (y * C2) -> (x * C1/C2) / y for example.
>>> Also 1/y is now available to the reciprocal optimization, see
>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71026 for details.
>>
>> Sure, but on its own it's going to be slower.  So this isn't the
>> correct way to enable those followup transforms.
> 
> How can it be any slower? It's one division and one multiply in both cases.
x / (y * C) -> (x / C) / y

Goes from one division and one multiplication to two divisions.  I'm
guessing that's what Richi is (reasonably) concerned about.

So it may be the case that we need more complex pattern matching here
for when to perform the first transformation to ultimately enable the
second.


Jeff