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

[Chris Lattner via llvm-dev]

> On Feb 5, 2020, at 8:22 PM, Michael Kruse <llvmdev at meinersbur.de> wrote:
> 
> Am Mi., 5. Feb. 2020 um 18:13 Uhr schrieb Chris Lattner <clattner at nondot.org>:
>> If I understand your claims, you are claiming both that red/green trees are essential for a loop optimizer, and that this essential nature “justifies” the cost of reinventing an entire compiler infrastructure is lower than the benefit of using (e.g.) MLIR to do this.  I haven’t seen evidence of either point: lots of loop optimizers exist that don’t use red/green trees.
> 
> I have spoken with multiple compiler vendors and we all could agree
> that the approach of pass/transformation-local decision making is
> limiting the global optimization goal the more transformations there
> are.
> seriously limiting the quality of global optimizations. Of those who
> gave me some details, have a loop-specific data structure (although
> not "red/green" trees). I don't suggest to replace anything in the
> existing toolchain/pipeline, so there is no risk of regressing
> anything that already works today. Every advancement will need to show
> its advantages at one point, this was already the case with e.g. SSA
> form.

Sure, to reiterate, I think that having an explicit loop nest representation is super important.  My question was about red/green trees specifically.

>> Furthermore, my experience is that specialized IRs never get the investment in (e.g.). testing, location propagation for debugging optimized code, textual round tripping, pass management, and the multitude of other things that are required for a world class compiler implementation.
> 
> With writing the RFC I also try to win contributors about the
> usefulness of the idea and I think it is feasible. For example, Polly
> started as a research project, attracting contributors, but did not
> succeed to make it a main component of LLVM because of other reasons.
> I think if LLVM also want's a world class loop optimizer, some time
> needs to be invested.
> 
> 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.

-Chris