[llvm-dev] InstCombine, graph rewriting, Equality saturation

[Daniel Berlin via llvm-dev]

On Mon, Sep 11, 2017 at 9:48 AM, Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> On 09/11/2017 10:50 AM, Sean Silva via llvm-dev wrote:
>
>
>
> On Mon, Sep 11, 2017 at 8:14 AM, Daniel Neilson via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>> Just thinking out loud…. I’m really not familiar with the vast majority
>> of what instcombine does, so please excuse me if my naiveté is too obvious.
>> I can’t help but notice all of the uses of ‘and’ in Daniel B’s description
>> of what instcombine is right now:
>>
>> > On Sep 8, 2017, at 11:27 PM, Daniel Berlin via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>> >
>> > FWIW, Before getting to "how to do it", I think InstCombine needs an
>> actual goal.
>> > Right now it's "do a bunch of instruction combination and
>> canonicalization and random kinds of semi-local optimization with some
>> weird analysis and other stuff in the middle.
>> > Iterate this as hard as we can"
>> > Nobody can really draw any real dividing line for what should be there
>> or not except based on how easy or expensive it is to shove it in.
>> > That's a recipe for pass creep.
>>
>> This makes me wonder… is it sensible to be talking about splitting up
>> instcombine into multiple separate passes? Would such a thing even be
>> possible? For example, split by functionality into separate passes that
>> each do one of:
>> * instruction combinations
>> * canonicalization
>> * semi-local optimizations
>> * etc…
>>
>
> The problem is that all of these are really the same thing. Almost all
> canonicalizations are also simplifications, the common underlying factor
> being that they're mostly-local transformations that likely enable other
> optimizations.
>
>
>>  Or something like that.
>>
>>  As separate passes, each would probably have a natural way to be
>> implemented effectively and those implementations might vary.
>>
>
> One obstacle to that is that currently instcombine has an internal
> fixed-point iteration that it does.
>
> So when splitting it into separate passes we would need to either add
> fixed-point iteration to the pass manager running the separate instcombine
> passes (extending the pass management in this way is doable and has been
> explored in the past, e.g. https://www.youtube.com/watch?v=c7iP43an5_Q )
> or demonstrate/measure that we don't regress without the fixed-point
> iteration across separate instcombine passes.
>
>
> I agree, but I'll add that I view the fixed-point iteration here to be an
> asset. It increases the robustness and consistency of the compiler, and
> allows later passes to depend on the canonicalization (*) without worrying
> about phase-ordering effects (**).
>
FWIW: I completely agree.  At least as you  are using it here, which is in
the sense of "system is built to apply a set of transformation. Fixed point
iteration ensures that all such transformations that can occur, do, even if
they are only exposed as a result of earlier transformations".

In that sense, a vast majority of our passes do or should be using fixed
point iteration.
The usual concern is cost, and it requires careful engineering (as you say
below) to ensure you are doing this in an effective manner.  IE ad-hoc
solutions work but rarely scale, it requires a good understanding of theory
and engineering to get something that will scale really well and work well

Integrating multiple transformations/capabilities into a single pass is
much trickier than one would expect.  You can easily screw stuff up by
doing things you think might only improve things.

A simple example from value numbering https://pastebin.com/Tp11bfCa

Note that if we ignore unreachable edges at first and just shove it in the
algorithm, we no longer get optimal answers in certain cases.  Even though
we only changed exactly one initial state, and we're trying to make things
better!
This is fixable, but requires a lot of thought to see that this will happen
in the first case.

(the reason in this case is because the algorithm relies on the TOP state
to resolve cycles. There is no perfect comes-before iteration ordering in
this case.  By changing the initial state that fed the cycle, without
resetting that cycle to TOP, it can no longer resolve the cycle to the
optimal answer. )



> I'd like to see us do more fixed-point iteration in our optimization
> pipeline, and our past work has shown this would be practical (i.e., only a
> few iterations will be needed in practice), but even that won't remove the
> advantages of using a fixed-point iteration within InstCombine.
>

Completely agree on this

>
> Regardless, I think that if we had a model for what InstCombine did (i.e.,
> as a graph-rewriting system), then it would be clear what could be part of
> that system and what couldn't. Otherwise, I think that the real challenge
> here is figuring out the underlying structural foundations to make the
> process efficient and sound.
>

Also completely agree.


>
>  -Hal
>
>
> -- Sean Silva
>
>
>>
>> -Daniel
>>
>> ---
>> Daniel Neilson, Ph.D.
>> Azul Systems
>>
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>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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