[llvm-dev] more reassociation in IR

[Roman Lebedev via llvm-dev]

My 5 cent, since i've been recently working on instcombine a bit.

First of all, I do acknowledge and recognize the problem at hand -
the IR is not getting fully optimized. I think that should be solved.

As it is evident, LLVM is quite modular, there are separate passes.
This is good. It allows to test things separately. Unfortunately, that
is also bad, because there are always these things that fall through
the cracks between passes...

instcombine is rather huge as is. Almost "all size fits all".
It tries to do everything. Sometimes it does wonders, sometimes not.
It is not modular. The order of folds is completely hardcoded,
not documented, with no obvious logic to it. And only the full and
final IR produced by the whole pass is testable.

Personally, i think this is bad. I'd prefer something else, like having
*separate* folds within instcombine, each tested on it's own,
(much like clang-tidy checks vs monolithic clang diagnostics)
and then somehow combined, not unlike the usual optimization pipelines.

The second problem is the size/complexity of the instcombine pass.
I do acknowledge that D46336 / D46595 (both in instcombine) is an
improvement, which results in more folds that didn't happen before.
But it makes the already-large inscombine even more complex,
and i'm not sure how it affects the performance.

Then there is D45842.
That code is pretty simple, and takes ~one page. It does not do
any change unless that opens the road for further simplification.
(I hope that is what the D46336 / D46595 do, but i don't know.)

As stated in https://reviews.llvm.org/D46336#1090588, the D45842
does allow instcombine to do //most// of the folds that would D46336 do.

But there is an open problem - several consecutive runs of
instcombine and reassociate passes are needed. So basically we'd need
to have a umbrella-pass which would run -instcombine -reassociate until
they stop making changes.

TLDR: monolithic code is bad. instcombine is monolithic, bloated.
The problem at hand can be solved by a separate pass, which would
reduce(*) the size of instcombine, provided there is a way to have
such an umbrella-pass to combine them.
* I do think that we should not only not do that instcombine, but check what
  else is solved by the separate reassociate pass (not talking about
  commutative matchers here), and consider removing that.

Roman.

On Wed, May 9, 2018 at 6:22 AM, Chris Lattner via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
>
> On May 8, 2018, at 9:50 AM, Daniel Berlin via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>
> 1.  The reassociate pass that exists right now was *originally* (AFAIK)
> written to enable CSE/GVN to do better.
>
>
> Agreed.  The original mindset included a (naive) belief that going with a
> canonical form was better than teaching redundancy elimination to handle
> abstractions (as a matter of separation of concerns).  I think that that was
> a failed experiment at this point :-).  We should still canonicalize x*x*x
> into powi, but the more aggressive canonicalization (like distribution)
> shouldn’t be used IMO.
>
> -Chris
>
>
>
> That particular issue is solvable in other ways, because there are good ways
> to integrate reassociation into CSE/GVN (and at this point, it would
> probably be cheaper than -reassociate since it would modify code less, only
> changing it when there are actual redundancies )
>
> I don't know what other things people are trying to target with reassociate
> these days, it would be good to understand what others see as the use cases.
>
> My only other fear with removing it is that we have a tendency to try to
> make every optimization do everything, so i fear removing reassociate might
> just have people try to improve every pass to do reassociation instead of
> declaring what is good enough.
>
> (For example, GVN already does some reassociation, as do some analysis.
> Other analysis relies more on a canonical form existing and don't, for
> example, try to perform commutativity on their own.  IMHO, we should just
> declare one or the other to be the case, and hold that line, rather than
> generate a mishmash)
>
>
> 2. "Keep in mind that instcombine runs 1st, runs to fixed-point, and runs 8
> times in the -O2 pipeline."
>
> Y'all know how i feel about this one, so i'll leave it alone :)
>
>
>
> --Dan
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