[LLVMdev] IR Liveness Analysis?

[Philip Reames]

Is anyone aware of an existing framework for asking liveness questions 
about SSA values in the IR?  I'm looking for something more precise than 
the trivial definition provided by SSA itself.  I can write something 
myself (and will if need be), but it seemed like a generic enough 
problem that I was surprised I couldn't find something already in tree.  
Anyone know of something I've missed?

The problem I'm trying to solve is identifying pointers which might be 
live at a garbage collection safepoint.  This in the context of 
transforming the IR to insert a statepoint intrinsic to explicitly 
represent a possible object relocation.  We've been using a very straigh 
forward and, due to implementation simplicity, expensive estimation 
based on simple reachability*.  This suffices for correctness, but is 
not ideal from a performance perspective.  (To put it mildly.)

* For those curious, you can find the current implement by searching for 
"isLiveAtSafepoint" in 
https://github.com/AzulSystems/llvm-late-safepoint-placement/blob/master/lib/Transforms/Scalar/SafepointPlacementPass.cpp. 
It's a trivial reachability algorithm with special cases for inbound 
values into a phi and uses that are encountered along paths which 
contain the original definition.  We rerun the search once per 
potentially live value.  As you might imagine, that's a bit expensive.  :)

Philip