[LLVMdev] IR Liveness Analysis?

[Hal Finkel]

Philip,

To clarify, you want an analysis that answers this question: Are there are any uses of a value, V, that are dominated by an instruction, I. Is that right?

 -Hal

----- Original Message -----
> From: "Philip Reames" <listmail at philipreames.com>
> To: LLVMdev at cs.uiuc.edu
> Sent: Wednesday, July 16, 2014 4:51:42 PM
> Subject: [LLVMdev] IR Liveness Analysis?
> 
> 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
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory