[llvm-dev] RFC: Dynamic dominators

[Sean Silva via llvm-dev]

On Mon, Jun 12, 2017 at 5:12 PM, Jakub (Kuba) Kuderski via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi folks,
>
> This summer I'm working on improving dominators during my internship at
> Google. Below is an RFC on switching to dynamic dominators, which you can
> also read as a Google Doc
> <https://docs.google.com/document/d/1wPYeWykeO51YDPLYQEg4KNTlDIGIdyF65OTfhSMaNHQ/edit?usp=sharing>
> if you prefer so. Please let us know what you think.
>
> ~Kuba
>
> =======================================================================
>
> *1. Context*
>
> Currently LLVM uses the Simple Lengauer-Tarjan algorithm to build the
> (post)dominator tree. The only way to update it is by manually setting
> IDoms which is not obvious in many cases and can be extremely error-prone.
> And because updating it manually is so difficult, programmers tend to just
> recompute it after changing the CFG (by not AU.addPreserved()'ing the
> DomTree). This causes DomTree calculation to fire very often even if only a
> very small portion of it gets really affected by the changes in CFG. As an
> example, when optimizing a Full LTO clang bitcode, DominatorTreeWrapperPass
> alone calls DT.recalculate over 6.5 million times, which takes 20s on my
> machine.
>

Assuming an optimistic 10min clang FullLTO build time, this is about 3%, so
overall this is actually a pretty perf improvement I think.

>
> Using an incremental algorithm it would be much easier to keep an
> up-to-date DomTree without custom update logic, which will save us the time
> currently spent during DomTree recalculations and reduce the number of bugs
> caused by manual updates.
>

Avoiding bugs seems really compelling. (IIRC talking with you, Davide, and
DannyB at the social, there are tons of such bugs, so eliminating them by
mechanically just using a new API seems awesome)


> It would also make it feasible to maintain postdominators and use them
> more broadly, which currently can be too complicated and expensive.
>
> *2. Proposal*
>
> The proposal is to make dominators use the incremental algorithm that
> allows to keep (post)dominator tree up to date as CFG changes. To achieve
> that, we would implement the dynamic depth based search algorithm (DBS)
> described in this paper [1] <https://arxiv.org/pdf/1604.02711.pdf> and
> expose 2 main new functions: insertArc(From, To) and deleteArc(From, To).
> The algorithm uses SemiNCA under the hood which would replace
> Lengauer-Tarjan implementation currently used.
>
> The second part of the proposal is to gradually deprecate and remove the
> existing API for manually manipulating dominator tree
> (changeImmediateDominator, addNewBlock) and replace its use within LLVM
> with the new incremental API.
>

> *3. Proof of concept*
>
> The prototype implementation can be found in my LLVM fork [2]
> <https://github.com/kuhar/llvm-dominators>. It comes with several layers
> of verification and was tested on clang, llvm test suite and a few open
> source libraries.
> The code provides the basic functionality and tries be ‘API-equivalent’
> with the current DomTree implementation. The NewDomTree is also able to
> convert itself to the current one for testing and verification purposes.
> Dynamic dominators are hacked into 3 existing passes (DomTreeWrapperPass,
> LoopDeletion, LoopSimplifyCFG) to test correctness and experiment with the
> use of the new API.
>
> The prototype also comes with a bunch of testing utilities and a simple
> benchmarking tool.
>
> *4. Performance*
>
> The real life performance of full dynamic DBS-based DomTree recalculation
> is between 20% and 2% better on a machine with two Xeon E5-2680v2 CPUs than
> the existing SLT algorithm, which is consistent with the findings from this
> thesis [3] <ftp://ftp.cs.princeton.edu/reports/2005/737.pdf>. The
> advantage is not that visible on debug builds, where the DBS-algorithm can
> be up to 8% slower. It is most like possible to speed up debug builds, but
> I haven’t looked into that yet.
> The benchmark performed [4]
> <https://docs.google.com/document/d/1wPYeWykeO51YDPLYQEg4KNTlDIGIdyF65OTfhSMaNHQ/edit?usp=sharing>
> loads of a (full LTO) bitcode file and builds DomTress of all functions 15
> times.
>
> The current DomTree updates DFS in-out numbers eagerly upon construction,
> while the new one postpones is until they are actually needed. To make the
> benchmark fair, numbers were collected for both eager and lazy strategy for
> the new DomTree.
>
> The main advantage of the incremental algorithm comes from the fact that
> it allows incremental updates without rebuilding the whole tree, not from
> the slightly faster construction.
> What’s more, the insertArc / deleteArc API doesn’t force updates to happen
> immediately -- they can be queued behind the scenes and happen in batches
> if we decide to pursue that design later.
>
> *5. Transition plan*
>
> There are several possibilities when it comes to transition. The biggest
> problem is that the current DomTree exposes an interface for manual updates
> (setIDom, changeImmediateDominator), and for manual construction
> (addNewBlock). Because of the additional data stored in the incremental
> algorithm (relative dominators, preorder parents, levels) it is not really
> possible to use the old API while keeping this data up-to-date. The
> incremental algorithm relies on this information when performing fast arc
> deletions; It is still able to perform them without it -- deletions are
> then just slower in some cases.
> The most straightforward solutions are:
>
> a) Keep the existing DomTree and gradually replace its uses with the new
> one. It is possible to convert the DBS-based dominators to the old ones.
> b) Replace the existing DomTree with the new, dynamic dominators. Nuke all
> of the old update functions and replace their uses with the new incremental
> API in one commit.
> c) Replace the existing DomTree with the new one, but keep the old API
> around and mark it as deprecated. If someone invokes one of the old update
> functions, mark the the additional information as invalid for dynamic
> deletions. Follow the pessimistic but correct dynamic deletion path if the
> additional information is marked as invalidated. Gradually modernize the
> projects to use the new API instead and then remove the old update
> functions.
>
> Maintaining the old DT and the new one simultaneously seems like a waste
> of time as the DBS offers better or similar performance to the existing
> SLT-based implementation.
> The problem with replacing the old API with the new one is that it’s used
> in many places (~100) across the project and I believe that doing that all
> at once would be tricky to get correct. Gradual update seems much easier to
> ensure correctness and the transitional API (invalid additional data check)
> is trivial to implement. Because of that, I propose to follow the option
> (c).
>

(c) sounds like the best choice to me too. That would also allow fixing
dominator verification failures first, giving a nice immediate benefit to
motivate and kickstart the transition.

-- Sean Silva


>
>
>
> [1] Georgiadis et al., An Experimental Study of Dynamic Dominators,
> https://arxiv.org/pdf/1604.02711.pdf
> [2] llvm-dominators LLVM fork on Github, https://github.com/
> kuhar/llvm-dominators
> [3] L. Georgiadis, Linear-Time Algorithms for Dominators and Related
> Problems, ftp://ftp.cs.princeton.edu/reports/2005/737.pdf p. 38
> [4] Google Doc with the performance numbers, https://docs.google.com/
> document/d/1wPYeWykeO51YDPLYQEg4KNTlDIGIdyF65OTfhSMaNHQ/edit?usp=sharing
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170612/98b82344/attachment.html>