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Re: [PATCH] Factor out division by squares and remove division around comparisons (1/2)
On Thu, Aug 17, 2017 at 11:55 AM, Wilco Dijkstra <Wilco.Dijkstra@arm.com> 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 / C) / y is two divisions. If you restrict it to the case where we can
transform this to (x * C') / y then it's indeed one.
>>>> > x / (- y) -> (-x) / y
>>>>
>>>> Why? (it's only one of the possible canonicalizations)
>>>
>>> Same here, y is now available for reciprocal optimization. The
>>> negate may now be optimized, for example (a * b) / -y -> (-a*b) / y
>>> will use a negated multiple on various targets.
>>
>> Fair enough. Though if it were x / -(a*b) you'd regress that case.
>
> Possibly. You might still be able to merge the negate if the result is used in an
> addition or multiply, which wouldn't be possible if it were hiding in a subexpression.
> Without global analysis it seems best to move constants/negates to the toplevel
> if they can't be trivially removed in a subexpression. Eg. -x / (a * b * -c).
Sure. So both patterns are canonicalization which is fine for match.pd. Those
followup transforms should be done at a place that can look at more complicated
patterns. We have the reassoc pass, then backprop (not exactly matching),
and the recip pattern matching / cse pass.
Richard.
> Wilco