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17.3 Non-local exit

If the starting and ending continuations are not in the same environment, then the control transfer is a non-local exit. In this case just call Unwind with the appropriate stack pointer, and let the code at the re-entry point worry about fixing things up.

It seems like maybe a good way to organize VMR conversion of NLX would be to have environment analysis insert funny functions in new interposed cleanup blocks. The thing is that we need some way for VMR conversion to: 1] Get its hands on the returned values. 2] Do weird control shit. 3] Deliver the values to the original continuation destination. I.e. we need some way to interpose arbitrary code in the path of value delivery.

What we do is replace the NLX uses of the continuation with another continuation that is received by a MV-Call to %NLX-VALUES in a cleanup block that is interposed between the NLX uses and the old continuation’s block. The MV-Call uses the original continuation to deliver its values to.

[Actually, it’s not really important that this be an MV-Call, since it has to be special-cased by LTN anyway. Or maybe we would want it to be an MV call. If we did normal LTN analysis of an MV call, it would force the returned values into the unknown values convention, which is probably pretty convenient for use in NLX.

Then the entry code would have to use some special VOPs to receive the unknown values. But we probably need special VOPs for NLX entry anyway, and the code can share with the call VOPs. Also we probably need the technology anyway, since THROW will use truly unknown values.]

On entry to a dynamic extent that has non-local-exists into it (always at an ENTRY node), we take a complete snapshot of the dynamic state:

We insert code at the re-entry point which restores the saved dynamic state. All TNs live at an NLX EP are forced onto the stack, so we don’t have to restore them, and we don’t have to worry about getting them saved.


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