[PBQP] Use a local bit-matrix to speedup searching an edge in the graph

Lang Hames lhames at gmail.com
Wed Mar 4 20:31:01 PST 2015

Hi Arnaud,

This looks great. Please go ahead and commit. :)

I'm very surprised to hear that the llvm_unreachable never triggered. I
will have to have a closer look at that. The Interference::apply algorithm
is derived from Linear Scan. It walks the segments of the live-intervals of
the function according to their starting points and maintains a set (the
active set) of the live segments at the current point. The list of segments
whose start point is further down in the function is called the inactive
set. A duplicate interference detection should occur any time two intervals
have separate overlapping segments. E.g.:

|   <- A (segment 1) joins active set.
| | <- B (segment 1) joins active set, interference between A and B
    <- A and B both leave the active set.
  | <- B (segment 2) joins the active set.
| | <- A (segment 2) joins the active set, interference between A and B

It is the second detection that the findEdge call is guarding against.

I'll see if I can replicate your results in the next few days. I want to
revisit this code anyway: I have a suspicion that there are further
performance improvements to be had, though I don't know if they'll be

- Lang.

On Wed, Mar 4, 2015 at 9:05 AM, Arnaud A. de Grandmaison <
arnaud.degrandmaison at arm.com> wrote:

> Hi Lang,
> The attached patch addresses a FIXME in RegAllocPBQP.cpp, in the
> Interference class when building the graph.
> This indeed improves compilation time when building llvm+clang+lldb.
> However, I wonder if this check+continue for an already seen edge should
> exist at all.
> I may have missed something in my reading / understanding of
> Interference::apply(), but I do not think the algorithm can exhibit such a
> case --- or it would be an error, which we should trap with an assert,
> either in Interference::apply, or even better in Graph::addEdge. I replaced
> the continue statement with an llvm_unreachable, and it never triggered
> while building llvm+clang+lldb (granted, this is not a proof, just an
> hint). Removing this check+continue brings yet another nice speedup, making
> pbqp build llvm+clang+lldb slightly faster than the default allocator in
> release mode.
> Cheers,
> --
> Arnaud A. de Grandmaison
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