This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

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 <> wrote:
> Richard Biener wrote:
>> On Tue, Aug 15, 2017 at 4:11 PM, Wilco Dijkstra <> 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
>>> 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.


> Wilco

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]