[llvm-dev] RFC: Dynamic dominators

Jakub (Kuba) Kuderski via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 13 11:14:55 PDT 2017


>
> I tried to use a bitcode file directly:
>
> ...
>
> It seems the names "inloop" and "next" are not used.


My bad, I almost always use the tool with -view-cfg and didn't notice that
BB names were different. I fixed that and now when you just run the
dominators tool on a module it should work fine. It now also supports
reading .ll files.

Interesting. Probably it is just a testing/debugging tool, but maybe
> clarifying this in the output may avoid confusion for others who try
> your changes.

Yep, I briefly mentioned that in the first line of the tool's source file,
but I should probably add a note in some more visible place. Sorry for
making it confusing.

We could even have a tool that records the dom-tree modification
> requests, and dumps a corresponding IR-file.
>
> Obviously, this is just an idea.

I'll think about it.

Btw, here is another interesting paper about post-dominators and control
> dependence:
>
 https://pdfs.semanticscholar.org/cbb2/9a0e4895025bd9df24f9263217df12
> f1ed1e.pdf

Interesting, thanks for the link, I'll try to take a look at it and
probably report back with some ideas and more questions.

 Best,
Kuba

On Tue, Jun 13, 2017 at 1:27 AM, Tobias Grosser <tobias.grosser at inf.ethz.ch>
wrote:

> Btw, here is another interesting paper about post-dominators and control
> dependence:
>
> https://pdfs.semanticscholar.org/cbb2/9a0e4895025bd9df24f9263217df12
> f1ed1e.pdf
>
> I think a great outcome of your internship would be some precise
> documentation regarding the guarantees the LLVM dominators give --
> possibly also considering classic and weak control dependence and the
> difference between loop-dominance and dominance.
>
> Please keep me up-to-date with your work. This is really work that has
> long been overdue!
>
> Best,
> Tobias
>
> On Tue, Jun 13, 2017, at 10:23 AM, Tobias Grosser via llvm-dev wrote:
> > On Tue, Jun 13, 2017, at 09:22 AM, Jakub (Kuba) Kuderski via llvm-dev
> > wrote:
> > > 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.
> >
> > Thank you!
> >
> > > - 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.
> >
> > I tried to use a bitcode file directly:
> >
> > [grosser at greina0 build]$ cat ../test/Analysis/RegionInfo/test.ll
> > define void @foo() {
> > entry:
> >   br i1 1, label %infloop, label %next
> >
> > infloop:
> >   br label %infloop
> >
> > next:
> >   ret void
> >
> > [grosser at greina0 build]$ bin/dominators
> > ../test/Analysis/RegionInfo/test.bc
> >
> > New Dominator Tree:
> >   [0] %virtual_entry IO{0, 7}, R{nullptr}, P{nullptr}
> >     [1] %entry_n_1 IO{1, 6}, R{virtual_entry}, P{virtual_entry}
> >       [2] %n_2 IO{2, 3}, R{entry_n_1}, P{entry_n_1}
> >       [2] %n_3 IO{4, 5}, R{entry_n_1}, P{entry_n_1}
> >
> > New Dominator Tree:
> >   [0] %virtual_entry IO{0, 7}, R{nullptr}, P{nullptr}
> >     [1] %entry_n_1 IO{1, 6}, R{virtual_entry}, P{virtual_entry}
> >       [2] %n_2 IO{2, 3}, R{entry_n_1}, P{entry_n_1}
> >       [2] %n_3 IO{4, 5}, R{entry_n_1}, P{entry_n_1}
> > =============================--------------------------------
> > Inorder Dominator Tree:
> >   [1] %entry_n_1 {0,5}
> >     [2] %n_2 {1,2}
> >     [2] %n_3 {3,4}
> >
> > It seems the names "inloop" and "next" are not used.
> >
> > > 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.
> >
> > For from-scratch dominance computation LLVM-IR seems fine, but for all
> > the incremental stuff you are doing, such input indeed might make sense.
> > Especially if you are planning to generate many of these test cases
> > automatically.
> >
> > > - 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
> >
> > Interesting. Probably it is just a testing/debugging tool, but maybe
> > clarifying this in the output may avoid confusion for others who try
> > your changes.
> >
> > > --
> > > I used to use it just for visual comparison.
> >
> > That's what I did.
> >
> > > - 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.
> >
> > I think we certainly should have standard LLVM-IR style test cases,
> > where we use -analyze to dump the domtree for a given piece of IR that
> > clarfies how/what dominator tree is expected for a certain piece of
> > input and where it is easy for others to add test cases.
> >
> > Now, most of the bugs you might expect are likely in the update /
> > invalidation cycle. What would look very nice and readable to me, is to
> > write test cases as follows:
> >
> > ; RUN: opt -domtree -domtree-modifier -dom-tree-modifier-input=%s \
> > ; RUN:   < %s | FileCheck %s
> > ;
> > ; CHECK-LABEL; delete (bb3 -> bb4)
> > ; CHECK: <new-dom-tree>
> >
> > ; CHECK-LABEL; delete (bb4 -> bb5)
> > ; CHECK: <new-dom-tree>
> >
> > ....
> >
> > We could even have a tool that records the dom-tree modification
> > requests, and dumps a corresponding IR-file.
> >
> > Obviously, this is just an idea.
> >
> > Best,
> > Tobias
> > _______________________________________________
> > 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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