[llvm-commits] [PATCH] Added topological graph sort routine to llvm/ADT

[Chandler Carruth]

On Thu, Aug 18, 2011 at 10:03 AM, Delesley Hutchins <delesley at google.com>wrote:

> AFAICT, the RPO iterator does not do what we need.  For example,
> assume we have the following graph (pardon the bad ASCII graphics):
>
>   /---> (2) ----------\
> (1)                     --> (5)
>   \---> (3) ---> (4)--/
>
> I believe the RPO iterator will traverse this graph as 5-2-4-3-1,
> which is a standard depth-first post-order traversal.
>
> What we want is a topological traversal, which is 1-2-3-4-5; every
> predecessor of a node must be traversed before the node itself.
>

What about the reverse of the RPO violates this constraint? 1-3-4-2-5 looks
like it satisfies your constraint. I think Jakub is aware that we would have
to reverse the order, and was just claiming that such a reversed order
satisfied the constraints of a topological ordering.


> Notice that a topological traversal is neither depth-first (that would
> be 1-2-5-3-4), nor breadth-first (1-2-3-5-4); we want to ensure that 2
> and 4 are traversed before 5.
>
> This patch does three things to achieve the topological traversal:
> (1) Depth-first traversal to identify back-edges and assign depths.
> (2) Sort nodes according to depth.
> (3) Traversal of the sorted list of nodes.
>
>  -DeLesley
>
> On Wed, Aug 17, 2011 at 10:00 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk>
> wrote:
> >
> > On Aug 17, 2011, at 9:17 PM, Chandler Carruth wrote:
> >
> > On Wed, Aug 17, 2011 at 8:56 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk>
> > wrote:
> >>
> >> On Aug 17, 2011, at 3:52 PM, Delesley Hutchins wrote:
> >>
> >> > This patch adds a topological sort routine for indexed graphs to
> >> > llvm/ADT.  This sort routine is currently used to traverse CFGs in
> >> > Clang when doing thread safety analysis (patch to be released
> >> > shortly).
> >>
> >> Could you explain why we need this in addition to
> >> include/llvm/ADT/PostOrderIterator.h?
> >
> > FYI, I'm not terribly familiar w/ PostOrderIterator (so maybe it would
> > actually serve here), but JSYK, the motivating use case is in a Clang
> > patch:
> http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20110815/045262.html
> >
> > I see.
> > AFAICT, the existing RPO iterator will give you the exact same ordering
> of
> > blocks, at least it will give you a topo-sort of the graph after removing
> > back-edges.
> > You can detect back-edges by keeping track of visited blocks in a
> BitVector
> > as you iterate through the RPO.
> > /jakob
> >
>
>
>
> --
> DeLesley Hutchins | Software Engineer | delesley at google.com | 505-206-0315
>
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