[llvm-dev] Dominator tree side effect or intentional

[Jakub (Kuba) Kuderski via llvm-dev]

Hi Guus,

In (Post)DominatorTree children (of the same parent) are unordered. The
thing you observed is the result of the initial DFS walk on the CFG that
uses the `successors` and `predecessors` functions.

There is no strong guarantee of the order of children in a freshly
constructed (post)dom tree, and it was in fact changed for postdominators a
few months ago without any breakage that I know of. What's more, the order
can be changed during a domtree update.

On top of that, you have to keep in mind that the LLVM's implementation of
the DominatorTree doesn't store nodes (forward) unreachable basic blocks,
and it claims that every node dominates a (forward) unreachable node.

Wouldn't a BFS RPO perhaps work in your case?

Hope that this makes things a bit more clear,
Kuba


On Fri, Oct 27, 2017 at 8:03 AM, Leijsten, G.H.P. via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
> I was wondering whether or not some behaviour that I am seeing is expected
> behaviour and that it has been designed like this, or not.
>
> A dominator relation is given by "if A dominates B", then all paths to B
> go through A.
>
> For example, take the CFG below (which is a directed graph (couldn’t make
> the arrow heads but ok.):
>    A
>  /   \
> B     C
>  \   /
>    D
>    |
>    E
>
> We can construct the following dominator tree for the CFG above, DomTree:
>     A
>  /  |  \
> B   D   C
>     |
>     E
>
> I want my pass to do a top-down traversal on the basic blocks in a
> function. I now have an approach where you start at the root of the
> dominator tree and build a work list by visiting all children, similar or
> the same as in MachineCSE. Now I see the following behaviour: when I visit
> the nodes of the root (A in this case), its children have the nice property
> that B and C come before D. This is actually what I want, but is this on
> purpose, or not? It is kind of hard to proof that B and C come before D
> when iterating through the children of the root in this dominator tree.
>
> Is it safe to assume that this always happens? If you look at dominator
> theory only, then D can just as well come before C, since they are on the
> same level, namely, as children of A.
> Has the topological order been taken into account such that iterating
> through children of a dominator tree node, visits children first that would
> be strictly before other children in topological order?
>
> Hope my question is clear enough.
>
> Kind regards,
> Guus Leijsten
>
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>


-- 
Jakub Kuderski
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