[PATCH] D36352: [LCG] Switch one of the update methods for the LazyCallGraph to support limited batch updates.

Sean Silva via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Mon Aug 7 14:04:51 PDT 2017

silvas added a comment.

One way to break up this large RefSCC is to notice that the fanout from the vtable ref edges in the ctors/dtors is contributing a lot to it being so large. If those edges are made more precise, then it can reduce the size. For example, by breaking the vtable ref edges we prevent the ref edges to virtual GetMetadata methods and so the huge RefSCC breaks up as I mentioned earlier.

This can basically be seen as increasing the precision of the reference visitation; right now, it just assumes that all transitively reachable function pointers are ref edges. This leads to a lot of false positives, as ref edges are just meant to be a conservative superset of all edges that we might eventually discover through static optimization as call edges. In this case, the C2 constructors for Foo, Baz, and Metadata3 can be seen to contain no indirect calls and all their callees are either external or contain no indirect calls. So we can avoid adding any ref edges at all. A simple context-insensitive bottom-up RPO tracking whether an indirect call instruction is present be enough for this case (it would be another cache-busting walk over all instructions in the module though, but that may be worth it to decrease the number of ref edges / ref edge operations).

I'm sure the tradeoffs and stuff for doing this kind of "may call through this function pointer" analysis are well studied in the Java literature, but I'm not really very familiar with it. The main difference for our use case here is that external functions count as "contributes no ref edges" instead of "could dynamically call potentially everything" (since we aren't concerned with runtime calls, but rather a conservative superset of statically discoverable direct calls).
But of course the point of this is to be fast and get rid of the most egregious false positive ref edges so something simple is probably enough.


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