[llvm-dev] RFC: Dynamic dominators

Tue Jun 13 00:22:57 PDT 2017

Tobias,

Thanks for the comments and taking a look at my fork.

>  -  It would be convenient to allow .ll files to be read.

Sure, I can add it tomorrow.

- You do not use the BB names in the output, right? This would certainly
>    improve readability!

You actually don't have to convert you bitcode files to 'graph' files (as
long as they contain a single function) -- then names should be preserved,
but you don't get to play with updates. The format I'm using isn't very
good, but it's compatible with some other implementation by Loucas -- the
author of the algorithm. Do you think that having a format like that in
LLVM would make sense? Danny and I though about in the context of quickly
writing and modifying tests for dominators and things like the NewGVN.

- My output prints "New Dominator Tree" to times in a row. What is the
> difference? What is the difference to Inorder Dominator Tree?
>
 When you graph files have updates (i numFrom numTo and d numFrom numTo)
then the first one is the original one and the second one is the one after
applying all the updates. Inorder Dominator Tree is the existing DomTree --
I used to use it just for visual comparison.

- 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.

 I haven't really played with postdominators yet. Do you have any specific
ideas on how to test it in mind? And I definitely agree, dominators need to
be tested more thoroughly. I think that because of the manual updates (of
questionable correctness) and frequent recalculations there must be many
undiscovered bugs that we just haven't had a chance to observe yet.

Best,
Kuba

On Tue, Jun 13, 2017 at 12:05 AM, Tobias Grosser <tobias.grosser at inf.ethz.ch
> wrote:

>
>
> 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
> > _______________________________________________
> > 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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