[llvm-dev] RFC: Dynamic dominators

Mon Jun 12 23:47:38 PDT 2017

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.

 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

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
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>

-- 
Jakub Kuderski
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