[llvm-dev] RFC: Dynamic dominators

[Tobias Grosser via llvm-dev]

Btw, here is another interesting paper about post-dominators and control
dependence:

https://pdfs.semanticscholar.org/cbb2/9a0e4895025bd9df24f9263217df12f1ed1e.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
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