[llvm-dev] Writing loop transformations on the right representation is more productive

[Albert Cohen via llvm-dev]

Thanks Chris for CCing us.

I remember Michael's presentation and suggestion to consider Roslyn's
design and experience. I'll be glad to discuss further in April. Michael,
we can also talk later this week if you'd like. I'll send you a separate
email.

Loop transformations in MLIR take many different paths. Polyhedral
affine-to-affine, generative/lowering in linalg, and also exploring lifting
lower level constructs into affine and further into linalg and tensor
compute ops. I'm all for exchanging on the rationale, use cases, and design
of these paths, alongside with LLVM LNO. One practical option being to
compose these lifting, affine transformations and lowering steps to build
an LLVM LNO. Ideally this could be one generic effort that would also
interop with numerical libraries or specialized hardware ops where they
exist, and that can be used to implement domain-specific code generators.

More on Roslyn: I'm not convinced yet about the added value of red-green
trees. I see them as an implementation detail at the moment: much like sea
of nodes is a convenient abstraction for implementing SSA-based global
optimization passes, red-green trees may improve on the practice of
AST/loop-nest transformations, but I don't see much fundamental or solid
engineering benefits... yet.

Albert



On Sat, Feb 8, 2020 at 6:11 PM Chris Lattner <clattner at nondot.org> wrote:

>
>
> On Feb 7, 2020, at 10:16 PM, Michael Kruse <llvmdev at meinersbur.de> wrote:
>
> Am Fr., 7. Feb. 2020 um 17:03 Uhr schrieb Chris Lattner <
> clattner at nondot.org>:
>
>> > The discussion here is valuable for me, helping me to make my
>> > presentation about it at EuroLLVM as relevant as possible. My current
>> > idea is to take a complex loop nest, and compare optimizing it using
>> > red/green DAGs and traditional pass-based optimizers.
>>
>> Cool.  I’d really recommend you connect with some of the loop
>> optimization people working on MLIR to learn more about what they are
>> doing, because it is directly related to this and I’d love for there to be
>> more communication.
>>
>> I’ve cc'd Nicolas Vasilache, Uday Bondhugula, and Albert Cohen as
>> examples that would be great to connect with.
>>
>
> You may have already seen my discussion with Uday on the mailing list. I
> would like to discuss approaches with all 3 of them, at latest at EuroLLVM
> (or contact be before that, e.g. on this mailing-list thread).
>
>
> Great!  I’d be happy to join a discussion about this at EuroLLVM too.  I
> think it is really important to improve the loop optimizer in LLVM, and I’m
> glad you’re pushing on it!
>
> -Chris
>
>
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