Implement a variation of scalar replacement for non-escaping allocations.
AllocationSinking pass discovers non-escaping allocations that have no input uses other than uses in the stores into its own fields.
Every environment use of such allocation is replaced by a state snapshot (MaterializeObject instruction) that describes the state of each initialized field in the object. State snapshots are computed through an additional round of load-forwarding.
Once snapshots are computed allocations are removed from the graph.
MaterializeObject instructions are not compiled into native code but produce deoptimization instructions instead that describe how object should be materialized at deoptimization.
Deoptimization instructions now follow the following format:
[mat obj #1]...[mat obj #N][ret addr][... mat arguments ...][... real frames ...]
- the prefix describes each object to materialize on deopt via kMaterializeObject instruction;
- actual values that are needed for materialization are emited as a part of bottom-most stack frame. This is done to simplify implementation: they need to be discoverable by a GC during materialization phase. At the end of deoptimization they will be removed from the stack;
- normal stack slots can refer to materialized objects via kMaterializedObjectRef instruction.
Additionally this change contains fixes in load-forwarding that are needed to guarantee that all artificial LoadField instructions inserted during AllocationSinking are correctly replaced with actual values.
Limitations of the current implementation:
- can't eliminate allocations that flow into phis but otherwise don't actually escape;
- can't sink allocations out of loops;
- allocation with type arguments are not handled.
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