[PATCH] Add a pass for constructing gc.statepoint sequences w/explicit relocations

Philip Reames listmail at philipreames.com
Wed Jan 14 12:45:19 PST 2015

Hi whitequark, sanjoy, hfinkel, atrick, ributzka,

This patch consists of a single pass whose only purpose is to visit previous inserted gc.statepoints which do not have gc.relocates inserted yet, and insert them.  This is the meat of what I've been terming 'late safepoint placement'.  The last remaining major piece is identifying where the safepoints (in the form of gc.statepoints) should be placed.  That code will follow in a separate patch in the next week or so.  

NOTE ON REVIEW: I would *strongly* prefer to work on this incrementally in tree.  In review comments, please try to distinguish between 'must fix before submission' and 'please address within near term'.  In particular, there are a number of style naming violations I'm aware of and will fix, but would prefer to do so after submission.

The pass has several main subproblems it needs to address:
- First, it has identify any live pointers.  In the current code, the use of address spaces to distinguish pointers to GC managed objects is hard coded, but this will become parametrizable in the near future.  The actual liveness analysis is trivial (simply dominance).  A follow on change will implement a more precise analysis.
- Second, it has to identify base pointers for each live pointer.  This is a fairly straight forward data flow algorithm.  The current implementation of said algorithm is bit messy, but unless someone *strongly* objects, I'd prefer to leave that is for the moment.  This code has been fairly well exercised out of tree and I'd really like to not make any changes to it until a) we have better in tree tests covering this area and b) I've had a chance to back merge this into our private tree.  
- Third, the information in the previous steps is used to actually introduce rewrites.  Rather than trying to do this by hand, we simply re-purpose the algorithm behind Mem2Reg to do this for us.  

Other planned follow up changes includes:
- The gc.statepoint insertion ('safepoint placement') pass mentioned previously.
- The liveness algorithm improvements mentioned previously.
- A per statepoint flag to indicate whether a particular statepoint has been rewritten or not.
- A GCStrategy hook to early exit the pass for non-gc code.  This is required before adding the pass to the actual pass order.



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