[llvm-dev] RFC: Introducing CfgTraits and type-erased CfgInterface / CfgBlockRef / CfgValueRef

Mon Jul 6 21:24:52 PDT 2020

Hi Nicolai,

Thanks for sharing the proposal and the patches. I looked at most of them
and think this is a good general direction.

There's a lot of heavily templated code in generic DomTee
construction/updater, MemSSA updater, and GraphDiff that has become really
hard to modify. For the context, Alina (cc'd) was recently looking into
making the domtree code work with 'CFG views'; the basic idea is to take a
concrete CFG and a series of updates and create a view over this CFG, as if
the updates were applied, add more updates and repeat [1]. The initial
attempt was based on GraphDiff and GraphTraits over GraphDiff, but this
gets prohibitively verbose very quickly and caused difficult to fix
compilation time regressions related to nested fancy iterator types
(something like concat<filter<pair<vector<>::iterator, T*>>, ...>) [2]. We
got to the idea of trying  polymorphic iterators, which I think is kind of
similar to this RFC, except that your proposal does type erasure at the
bottom, while polymorphic iterators from the top. Alina should know this
problem space best.

I think that generic DomTree construction/updater could be rewritten to
work on CfgTraits directly, while GraphDiff would provide CfgTraits
directly usable by the updaters. Same with the generic IDF calculator. The
biggest question for me here is how all of that would affect compilation
times. What are your thoughts on more refactoring like this? How do you
plan to evaluate compilation times impact of your changes?

Thanks,
Jakub

[1]
https://github.com/llvm/llvm-project/commit/a90374988e4eb8c50d91e11f4e61cdbd5debb235
[2]
http://llvm-compile-time-tracker.com/compare.php?from=37bcf2df01cfa47e4509a5d225a23e2ca95005e6&to=a90374988e4eb8c50d91e11f4e61cdbd5debb235&stat=instructions

On Thu, Jul 2, 2020 at 6:46 PM Nicolai Hähnle via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
>
> This is a request for comment on a series of patches which introduce a
> new way of writing algorithms that are generic over different types of CFG.
>
>
> What is this?
> =============
> This series of patches introduces a set of classes and templates for:
>
> 1. Working on basic blocks and values generically, in particular with
> the same algorithm implementation on both LLVM IR and MachineIR (in SSA
> form), and
>
> 2. Doing so using type erasure, i.e. the bulk of the algorithm you're
> writing doesn't have to be a template (at the cost of using some virtual
> methods).
>
> The second point is achieved by
>
> a) having algorithms refer to blocks and values using CfgBlockRef and
> CfgValueRef types which wrap a `void *` whose interpretation depends on
> the concrete CFG (for example, CfgValueRef wraps a `Value *` for LLVM IR
> but a `Register` for MachineIR), and
>
> b) querying the CFG via virtual methods in a CfgInterface interface class.
>
> Please take a look at the following Phabricator reviews and related
> changes you can navigate to via the "Stack" tab:
>
> 1. https://reviews.llvm.org/D83088: Introduces the classes and templates
> that enable this new way of writing analyses (CfgTraits, CfgInterface,
> CfgInterfaceImpl<CfgTraitsT>, ...).
>
> 2. https://reviews.llvm.org/D83089: Refactors the dominator tree
> implementation so that algorithms that operate on CfgBlockRef will be
> able to use a standard dominator tree as input. The calculation of
> dominator trees remains unchanged as a template.
>
> 3. https://reviews.llvm.org/D83094: Adds a GenericCycleInfo analysis as
> a first "full" user of the CfgInterface machinery. Unlike LoopInfo, this
> analysis computes a nesting of both reducible and irreducible loops
> (based on Havlak (1997)).
>
>
> Why now?
> ========
> The concrete need that prompted this development is a reworking of the
> control flow lowering in the AMDGPU backend. Part of this rework is a
> new divergence/uniformity analysis that can be used on both LLVM IR and
> MachineIR. This analysis has to be able to operate generically on both
> basic blocks and values.
>
> If you're interested in this larger context, feel free to take a look
> here: https://github.com/nhaehnle/llvm-project/tree/controlflow-wip-v4.
> There are still some missing bits and pieces and more cleanup to be done
> for the parts which are not yet on Phabricator. I am going to tackle
> those over the next weeks.
>
>
> Templates vs. interfaces
> ========================
> At a high level, there are two parts to the proposed framework:
>
> - CfgTraits, which provide generic operations on concrete CFG types. For
> example, CfgTraits allow iterating over all values defined in a block.
> The implementations of CfgTraits are typically used as template parameters.
>
> - CfgInterface, which provides generic operations on opaque, type-erased
> objects of type CfgBlockRef and CfgValueRef.
>
> It is this second part which enables generic algorithms to be written
> using virtual method-based interfaces instead of as templates.
>
> Both approaches for writing generic algorithms have their advantages and
> disadvantages. Templates don't have the (direct and indirect) runtime
> overhead of virtual methods. Code that uses abstract interfaces is
> easier to maintain and causes less binary bloat. It is not always clear
> what the right trade-off is.
>
> Existing generic analyses are all written as templates. It's not clear
> to me how conscious that choice was, or whether the alternatives were
> ever fully explored, but in any case I am not proposing to change the
> already chosen trade-off there. Instead, I am proposing that we make the
> alternative _possible_, and I have a whole set of analyses that are
> implemented using this alternative waiting to be upstreamed.
>
>
> What about MLIR / other CFGs?
> =============================
> While I personally only work in LLVM IR and MachineIR and my changes
> only provide the minimum required to ensure that dominator trees
> continue to work for everything else, the framework is designed with
> other CFGs in mind. In particular, since `mlir::Value` is pointer-sized,
> it shouldn't be too much effort to allow it to be wrapped in a
> CfgValueRef as well. This would enable analyses written on CfgTraits /
> CfgInterface (such as my upcoming divergence/uniformity analysis) to be
> used from MLIR as well.
>
> Cheers,
> Nicolai
>
> --
> Lerne, wie die Welt wirklich ist,
> Aber vergiss niemals, wie sie sein sollte.
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>

-- 
Jakub Kuderski
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