[llvm-dev] more reassociation in IR

[Daniel Berlin via llvm-dev]

On Tue, May 8, 2018 at 10:38 AM, Hiroshi Yamauchi via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> (
> ​I came across this issue in the context of
>  D46336 <https://reviews.llvm.org/D46336>.
> ​ ​
> Thanks, Sanjay, for starting this discussion.)
>
> If
> ​we will
>  move
> ​reassociation,
> or keep additional ones
> ​,​
> out of instcombine,
> ​open questions for me would be
> ​​:
>
>
> 1. Since -reassociate isn't a fixed point pass,
>

This is fixable, fwiw, without fixpointing it.


> we might need to repeat "-instcombine -reassociate" multiple times to
> ​fold
>  down to what we want (relating to my comment here
> <https://reviews.llvm.org/D46336#1087082>). I assumed this isn't not what
> we want to do
> ​? My impression is we don't do a fixed-point with passes?
>

Well, i mean there is no practical difference between passes that we
fixpoint externally and fixpoint internally.


>
>
2.
> ​Since -reassociate needs to come up with one operand order (at least
> currently as the only reassociate pass), would there exist a single,
> unique operand order that would enable all reassociative/commutative
> foldings that we want?
>

In what way?
Are you asking whether there is a single reassociation order that makes all
foldings occur in the same operation or something?
I don't feel like i understand what you are asking.


> If not, could there be conflicts among different reassociation orders
> depending on what folding we want?
>

We actually do try to have a single canonical form at various points in the
compiler.

We may want to canonicalize differently in different parts of the compiler,
but it would be nice to at least be self consistent.

>
>
> On Tue, May 8, 2018 at 9:19 AM Sanjay Patel <spatel at rotateright.com>
> wrote:
>
>> There are at least 3 active proposals to add reassociative optimizations
>> in IR:
>> [1] D41574 <https://reviews.llvm.org/D41574>- a new pass for
>> reassociation/factoring
>> [2] D46336 <https://reviews.llvm.org/D46336> - enhance -instcombine to
>> do more reassociation/factoring
>> [3] D45842 <https://reviews.llvm.org/D45842> - add to the existing
>> -reassociate pass to enable factoring
>>
>> Here's a basic motivating example that we fail to optimize:
>> https://godbolt.org/g/64NmJM
>> https://rise4fun.com/Alive/wec
>>
>> Currently, responsibility for reassociative transforms is split between
>> -reassociate and -instcombine. I don't know if that split is intentional or
>> just how the code evolved. Debug stats for test-suite compiled with -O3
>> show:
>> 78K "reassociations" by -instcombine.
>> 58K "reassociated" by -reassociate
>>
>> A debug stat with D45842 showed that that transform fired 1.3K times.
>>
>> Keep in mind that instcombine runs 1st, runs to fixed-point, and runs 8
>> times in the -O2 pipeline. Reassociate runs 1 time and doesn't run to
>> fixed-point. Some transforms are unique to 1 pass or the other, but there
>> is some overlapping functionality already there.
>>
>> So the questions are:
>> 1. How do we solve these remaining reassociation optimizations?
>> 2. Do we want to consolidate existing reassociation transforms in 1 pass
>> or is there value in splitting the folds across multiple passes?
>>
>>
>>
>>
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