[llvm-dev] RFC: Dynamic dominators

[Tobias Grosser via llvm-dev]

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