[llvm-dev] Using [GlobalISel] to provide peephole optimizations

Amara Emerson via llvm-dev llvm-dev at lists.llvm.org
Mon Aug 26 13:23:06 PDT 2019

+Daniel who’s prototyping the new combiner framework.

> On Aug 23, 2019, at 4:51 AM, via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hi,
> GlobalISel is fantastic, but obviously lacks a lot of the transforms that
> makes SelectionDAG so good. Whilst it's plenty usable, you'll find yourself
> wanting/needing to add a lot of manual little transforms to clean things up.
> I know of the RFC for a new Combiner with its own syntax
> (https://reviews.llvm.org/D54286 is the latest I can find of it), but after
> manually adding my Nth manually coded pass for a niggling important
> transform, and then needing to add more cases to
> FoldImmediate/MemoryOperand/OptimizeLoad after it. I wondered how hard it
> would be to allow GlobalISel to reselect machine patterns, eg after they've
> been made available by other passes.
> What I was thinking is in addition to anything else that's coming, allowing
> Instructions to exist on the input side of Pat<>, and using the same
> InstructionSelector we already have to reselect.
> To my surprise... not many changes are required to seemingly make this work:
>    // fold loads in to compare instructions
>    def	: Pat<(CPw_sr i32:$k, (MOVw_wf iPTR:$s)), (CPw_sf i32:$k, iPTR:$s)>;
> And it looks like SDNodeXForms will work off the bat, along with complex
> renderers. The main catches being with constants that require checking (due
> G_CONSTANT being handled differently to immediates), along with needing to
> add checks that the instruction has the same implicits, and that they're
> dead where appropriate. But it seems viable and is definitely easy to use so
> I'm just wondering... is this something that's being considered/is appealing
> to people? And/or is the restriction of not allowing Instructions on the LHS
> quite an intentional design decision?
> Because it seems that this would provide some value even for those not using
> GlobalISel as their primary selector, just as a way of quickly describing
> peephole optimizations and leveraging the very nifty little VM there to
> implement them. In theory, a lot of pattern fragments could even be added
> automatically, by comparing pattern fragments and the machine opcodes they
> represent - giving a free automatically generated "foldImmediate", among
> other things.
> A diff of the proof-of-concept can be found here:
> (https://paste.ee/p/aDHIg), note though it's really just a curiosity to get
> some conversation going, that other Selectors will reject these patterns (I
> haven't added their escape routes), and that you're likely to break it in
> many different ways. And of course that they should exist as their own
> table, and that Reselect ought return the newly selected instruction.
> But it's neat enough for me as is to get some useful patterns added already,
> with these caveats in mind.
> Any thoughts? 
> Regards,
> Alex Davies
> PS Apologies if the diff doesn't work first go - had to wrestle with quite a
> few out-of-tree things back to get things in order. Note, includes fix for
> https://bugs.llvm.org/show_bug.cgi?id=42032 also. Oh, and if there's already
> a peephole pattern emitter that I've missed, please do let me know, I'll
> easily survive the shame. :)
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