llvm dev - Jun 2017 - RFC: Dynamic dominators

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.

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/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).



[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
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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
>
>
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On Mon, Jun 12, 2017 at 5:58 PM, Sean Silva <chisophugis at gmail.com>
wrote:
>
>
> 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.
>
s/pretty/pretty small overall/

>
>> 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/kuh
>> ar/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/1wPYeWykeO51YDPLYQEg4KNTl
>> DIGIdyF65OTfhSMaNHQ/edit?usp=sharing
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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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/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.
> 
> 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/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).
> 
> 
> 
> [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

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
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>


-- 
Jakub Kuderski
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On Mon, Jun 12, 2017 at 11:24 PM, Tobias Grosser via llvm-dev <
llvm-dev at lists.llvm.org> 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?
>
I think chandler more than me. If me, assume I was wrong :P

I believe you can prove it is not possible to  do this without doing all
the work of the separate passes. The main cost is the DFS walk, and you
can't do an undirected DFS that will work here.
I believe related things have even been proven to show the brokenness of
various algorithms.

See https://hal.inria.fr/hal-00761505 section 4.1

Given the difficulties and subtleties here, I would consider any such
approach that tried to share work between the passes as "fraught with
peril"
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On Mon, Jun 12, 2017 at 5:12 PM, Jakub (Kuba) Kuderski via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> 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]. 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.
Do you mean the algorithm is slower in debug builds of LLVM, or when
LLVM is used to do debug builds of things?

It sounds like you say the algorithm can be 8% slower for debug
builds, but also likely to speed up debug builds? I'm confused :-)

>
> Do you mean the algorithm is slower in debug builds of LLVM, or when
> LLVM is used to do debug builds of things?
>
> It sounds like you say the algorithm can be 8% slower for debug
> builds, but also likely to speed up debug builds? I'm confused :-)
The algorithm can be slower when LLVM is built in Debug mode instead of
Release.
DomTree is only an analysis, so as long as it is correct it doesn't affect
performance of compiled programs.

On Tue, Jun 13, 2017 at 11:44 AM, Hans Wennborg <hans at chromium.org>
wrote:
> On Mon, Jun 12, 2017 at 5:12 PM, Jakub (Kuba) Kuderski via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > 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]. 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.
>
> Do you mean the algorithm is slower in debug builds of LLVM, or when
> LLVM is used to do debug builds of things?
>
> It sounds like you say the algorithm can be 8% slower for debug
> builds, but also likely to speed up debug builds? I'm confused :-)
>


-- 
Jakub Kuderski
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Hi Kuba,

On Mon, Jun 12, 2017 at 7:12 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 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.
>
Another motivating point is that with dominators automatically updated,
the JumpThreading pass can be extended to reason about loops: i.e.,
jump-threading across back-edges of a loop is an important extension
to increase the performance of finite state automata usually implemented
with a loop and a switch stmt within.  Brian Rzycki and I have been toying
with keeping the dominators updated across the JumpThreading pass and
that seemed quite involved with the current implementation of domtree.
I'm glad to see work in this area, and we will help testing your prototype
and patches.

Thanks,
Sebastian

On 06/12/2017 07:58 PM, Sean Silva via llvm-dev wrote:
>
>
> On Mon, Jun 12, 2017 at 5:12 PM, Jakub (Kuba) Kuderski via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>
>     Hi folks,
>     ...
>     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.
+1

  -Hal
>
> -- Sean Silva
>
>
>
>
>     [1] Georgiadis et al., An Experimental Study of Dynamic
>     Dominators, https://arxiv.org/pdf/1604.02711.pdf
>     <https://arxiv.org/pdf/1604.02711.pdf>
>     [2] llvm-dominators LLVM fork on Github,
>     https://github.com/kuhar/llvm-dominators
>     <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
>     <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
>    
<https://docs.google.com/document/d/1wPYeWykeO51YDPLYQEg4KNTlDIGIdyF65OTfhSMaNHQ/edit?usp=sharing>
>
>     _______________________________________________
>     LLVM Developers mailing list
>     llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>     http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>     <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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