[PATCH] D20584: [RewriteStatepointsForGC] All constant should have null base pointer

Igor Laevsky via llvm-commits llvm-commits at lists.llvm.org
Tue May 24 11:38:58 PDT 2016

igor-laevsky created this revision.
igor-laevsky added reviewers: mjacob, sanjoy, reames.
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Herald added a subscriber: sanjoy.

Currently we consider that each constant has itself as a base value. I.e "base(const) = const". This introduces couple of problems when we are trying to avoid reporting constants in statepoint live sets:

1. When querying "base( phi(const1, const2) )" we will get "phi(const1, const2)" as a base pointer. Since it's not a constant we will record it in a stack map. However on practice we don't want this to happen (constant are never relocated).
2. base( phi(const, gc ptr) ) = phi( const, base(gc ptr) ). This particular case imposes challenge on our runtime - we don't expect to see constant base pointers other than null.

This problems can be avoided by treating all constant as if they were derived from null pointer base. I.e in a first case we will not include constant pointer in a stack map at all. In a second case we will get "phi(null, base(gc ptr))" as a base pointer which is a lot more convenient.

@Manuel, I understand that your use case of the RS4GC is sensitive to how we handle global variables. Could you please take a look so that this change doesn't bring you any problems?



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