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DynamoRIO
i#5121: Add test for getting app value in multi-phase drreg
Adds a test that demonstrates how to get the app value for a reg that was reserved in multiple phases using drreg. This is complex as in the current design a phase doesn't know about spills done in earlier phases. So, getting the app value in insertion phase for a reg that was reserved in app2app and insertion phases both requires both the phases to work together.
Adds to the documentation of drreg_get_app_value and drreg_statelessly_restore_app_value that they allow obtaining the reg's meta value from the last phase where it was reserved, OR the app value if there's no such previous phase.
Issue: #5121
I think it should be possible to avoid this multi-step complexity at the client side. In the insertion phase, we can find the spill slot used in the app2app phase for the actual app value. It should be the first spill after the last app write to the reg (or start of bb). If this spill was added by insertion phase, then there's no overlapping spill regions, otherwise we restore from that earlier spill slot.
The implementation might be tricky though, as we're restoring from a slot that the current phase doesn't know contains anything. What would stateful restoration mean in this case? Because the insertion phase needs to preserve the app2app value of the reg too.
Actually, using the first spill slot after the last app write won't work if that spill slot was later recycled. I remember this being a corner case also in the drreg state restoration code. I guess we can invoke the state restoration code itself in some form to figure out the correct spill slot for the to-be-restored reg; only when the BB has the multi phase use flag set.
@derekbruening I thought it'd be useful to add an example for how the app2app phase should restore a spilled reg value so that the insertion phase can see it.
This is still marked as a draft.
@derekbruening I thought it'd be useful to add an example for how the app2app phase should restore a spilled reg value so that the insertion phase can see it.
If the idea is for users to see how to write proper app2app changes, should it be in api/samples/ then instead of subtest number 39 in a large drreg test?
@abhinav92003 -- trying to clean up stil-open PR's: is still still valid?