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

Sat Feb 8 10:22:08 PST 2020

On Mon, 3 Feb 2020 at 12:06, Michael Kruse <llvmdev at meinersbur.de> wrote:

> Am Do., 30. Jan. 2020 um 04:40 Uhr schrieb Uday Kumar Reddy Bondhugula
> <uday at polymagelabs.com>:
> > There are multiple ways regions in MLIR can be viewed, but the more
> relevant point here is you do have a loop tree structure native in the IR
> with MLIR. Regions in MLIR didn't evolve from modeling inlined calls - the
> affine.for/affine.if were originally the only two operations in MLIR that
> could hold blocks (which in turn are a list of operations as you know) and
> there wasn't anything by the name region. Later, "a list of blocks" was
> renamed "region" in order to generalize and unify it with other concepts
> that could be captured with "ops with regions", one of which is isomorphic
> to a "just inlined" call as you view it. But that doesn't mean a loop tree
> doesn't exist as a first class thing in the IR when you have the relevant
> ops around -- there is a hierarchy.
>
> Thanks for the interesting insights into the development history of MLIR.
>
>
> >> Regarding the affine dialect, I see the same problem that Polly has
> when creating a schedule tree representation: A lot of work has to be done
> to make IR originating from Clang compatible. Everything becomes easy if
> the front-end can generate an affine dialect out-of-the box.
> >
> > Right - but for the purposes of your proposal, this isn't really
> relevant - for that matter, one could just use the loop.for, loop.if ops if
> you don't want to leverage affine restrictions. Moreover, with
> affine.graybox ops, you can always use affine.for/if wherever you have
> structured loops (otherwise, you would fall back to a flat list of blocks
> inside the region of the graybox.) While directly generating the affine
> dialect maximally from the frontend / Clang is one option, the other is to
> just generate grayboxes with trivial affine.for/if (or just loop.for/if),
> and then eliminate the grayboxes maximally within MLIR. This way things are
> reusable across different frontends, and it would be similar to Polly's
> approach except that you would be dealing with loops/multi-dimensional
> arrays where possible instead of flat list of CFGs and GEPs.
>
> I think there is a relevant difference of whether we come from a
> high-level code generator and then lift restrictions or from a
> low-level IR which has to be raised. If starting with the high-level,
> we will have to bail out on representing things because we cannot
> ensure the expected semantics of the high-level idioms (while-,
> do-loops, coroutines, possibly infinite loops, non-returning elements
> in the loop body, ...) and have to work towards poking holes into the
> high-level representation that existing passes must me able to handle.
> When starting with a low-level approach, useful guarantees are added
> to the representation, but everything can be represented at the
> beginning.
> I am not saying that the two approaches cannot meet, but I am afraid
> that the high-level approach, like Polly, adds many bail-outs making
> it difficult to use in practice. For instance, we want to apply
> strip-mining to a loop. Since it does not change the execution order
> of any body code, it is always valid, yet we'd have to bail out if we
> cannot guarantee the representation's guarantees. I would like to
> avoid that.
>
> However, I agree that MLIR has the expressiveness requires for
> hierarchical loop structures. We don't think we need to argue about
> that.
>
>
> > That's actually not how I read it. Red/green trees was *one* of the nine
> items you mentioned in your list and this didn't come out as the central
> idea in your opening paras, but let's go with this now that it's clearer to
> me.
>
> Indeed, red/green trees (or DAGs) are one one of the ideas to improve
> loop optimizations, but does justify its use by itself. They happen to
> be effectively necessary for others in the list (e.g. versioning,
> profitiability heuristic by cost function, etc...) and the reason why
> I think the same cannot be done with MLIR. In hindsight, I could have
> pointed this out more in the original RFC. Note that a hierarchical
> representation was not an explicit feature in the list.
>
> To convince me that MLIR is the better IR for loop optimizations,
> might show that each of the features enabled by cheap subtree reuse
> can also be done sufficiently efficient and easily on MLIR (or that a
> feature is not what would actually would want).
>

I suspect that you are looking at this as "MLIR vs your proposal", but the
point I've been trying to make is to compare "your proposal on MLIR" vs
"your proposal on LLVM", i.e., you may want to evaluate the cost/benefit of
hosting your approach on MLIR vis-a-vis LLVM while accounting for the fact
that you'll need the conversion for LLVM to MLIR and back for the relevant
IR units with the former. On this note, in my opinion, MLIR doesn't have a
unified/clear strategy for loop optimization yet (even op representation
wise); there are different ops/representations that are being pedalled by
different folks, and some I believe are redundant or are simply duplicating
infrastructure in less powerful ways. I'm saying this because if you choose
MLIR to host your stuff, you'll have choices to make on which forms to use.

>
> >> (which I firmly disagree with being close to MLIR's in-memory
> representation), not the textual format.
> >
> > "close" isn't the right word here, "closer" is! Would you agree that the
> representation you are proposing is closer to MLIR's representation (both
> its in-memory and its textual representation) than to LLVM's or is this
> proximity not really relevant for the purposes of your proposal? I think
> it's important to know which among the two is the more important question.
>
> I think MLIR is an evolution of LLVM-IR and Swift-IR, built around
> similar principles such as SSA and Control-Flow Graphs (I understand
> that in addition to CFGs, MLIR also enables structured control flow
> idioms). SSA is a distinguishing feature: It allows to quickly
> traversing def/uses (where compilers without it need a data-flow
> analyses), but make subtree reuse hard.
>
> Does this answer your question?
>

Not directly, but I think I have more information now. You are probably
saying it's nearly as easy/difficult to host your approach on LLVM IR as it
is on MLIR.

All the best!

~ Uday

>
>
> > Note that currently there is really very little difference between
> MLIR's textual format for 'for'/if's and the in-memory form its passes use.
> The passes don't create any in-memory higher order representations for
> these IR units; they directly update them. There is nothing like the kind
> of complementary abstractions you are proposing (for cost models,
> copy-wise, etc.).
>
> The point I was making is that the in-memory format has references to
> related items such as parents and use-def chains. These are implicit
> in the textual format, e.g. the parent of an operation is defined by
> its syntactical location. When reading into memory, it is not
> obligatory for the objects to have all the references to related
> objects.
>
>
> Michael
>

-- 
Founder and Director, PolyMage Labs
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