[llvm-dev] RFC: Dynamic dominators
Tobias Grosser via llvm-dev
llvm-dev at lists.llvm.org
Tue Jun 13 00:05:13 PDT 2017
On Tue, Jun 13, 2017, at 08:47 AM, Jakub (Kuba) Kuderski via llvm-dev
wrote:
> Hi Tobias,
>
> 1) Daniel and Chandler have for a long time been talking about computing
> > dominance and post-dominance in one shot to reduce the cost of
> > post-dominance and make it (freely) available everywhere. Is this
> > covered by your current (or planned) work?
>
> I'm not sure what you exactly mean by one shot; I'll ask around tomorrow.
Not sure what algorithm Daniel had in mind, but he was talking about
some approach that basically gives post-dominance "for free" when
computing dominance. Not sure how exactly this would work.
> I wanted to play a little bit with your work, but run in some issues:
> > > bin/llvm-as ../test/Analysis/Dominators/2007-07-11-SplitBlock.ll
> > > bin/dominators -to-graph ../test/Analysis/Dominators/2007-07-11-SplitBlock.bc
> > | tee graph
> > p 5 5 1 1
> > a 1 3
> > a 1 5
> > a 2 3
> > a 3 2
> > a 3 4
> > e
> > > bin/dominators graph
> > Unknown file format for graph
> > Invalid input graph
> > I must do something obvious wrong. Any idea what?
>
>
> You almost got it right -- 'graph' files must end with ".txt"... :P
I got it. Thank you! I also realized I can directly read in .bc files.
I have a couple of immediate (but rather minor) comments:
- It would be convenient to allow .ll files to be read.
- You do not use the BB names in the output, right? This would certainly
improve readability!
- My output prints "New Dominator Tree" to times in a row. What is the
difference? What is the difference to Inorder Dominator Tree?
- How do I get the post-dominator tree with your tool? Do you already
have test cases? In fact, normal IR test cases would be really cool. We
do not have a lot of test coverage for all the dominance stuff.
Best,
Tobias
>
> Best,
> Kuba
>
> On Mon, Jun 12, 2017 at 11:24 PM, Tobias Grosser
> <tobias.grosser at inf.ethz.ch
> > wrote:
>
> > Hi Jakub,
> >
> > thanks for the update. I was already eagerly waiting to hear what your
> > internship on dominance brings. I think the numbers are already very
> > encouraging and I believe moving incrementally over to the new algorithm
> > is exactly the right approach.
> >
> > I have a couple of questions:
> >
> > 1) Daniel and Chandler have for a long time been talking about computing
> > dominance and post-dominance in one shot to reduce the cost of
> > post-dominance and make it (freely) available everywhere. Is this
> > covered by your current (or planned) work?
> >
> > 2) I tried to use tools/dominators
> >
> > I wanted to play a little bit with your work, but run in some issues:
> >
> > > bin/llvm-as ../test/Analysis/Dominators/2007-07-11-SplitBlock.ll
> > > bin/dominators -to-graph ../test/Analysis/Dominators/2007-07-11-SplitBlock.bc
> > | tee graph
> > p 5 5 1 1
> > a 1 3
> > a 1 5
> > a 2 3
> > a 3 2
> > a 3 4
> > e
> > > bin/dominators graph
> > Unknown file format for graph
> > Invalid input graph
> >
> > I must do something obvious wrong. Any idea what?
> >
> > Best,
> > Tobias
> >
> > On Tue, Jun 13, 2017, at 02:12 AM, Jakub (Kuba) Kuderski via llvm-dev
> > 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/1wPYeWykeO51YDPLYQEg4KNTlDIGId
> > yF65OTfhSMaNHQ/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.
> > >
> > > 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. 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/1wPYeWykeO51YDPLYQEg4KNTlDIGId
> > yF65OTfhSMaNHQ/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).
> > >
> > >
> > >
> > > [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/1wPYeWykeO51YDPLYQEg4KNTlDIGId
> > yF65OTfhSMaNHQ/edit?usp=sharing
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > llvm-dev at lists.llvm.org
> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
>
>
> --
> Jakub Kuderski
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