Ghost functions have a model supporting their erasure #10
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…stconditions on freshness
nikswamy
commented
Feb 29, 2024
| @@ -187,7 +187,7 @@ let lift_ghost_neutral | |||
| (e:stt_ghost a pre post) | |||
| (reveal_a:non_informative_witness a) | |||
| : stt_atomic a #Neutral emp_inames pre post | |||
| = admit() //This is the main axiom about ghost computations; in Steel, this axiom is implemented within the effect system | |||
| = Action.lift_erased reveal_a e | |||
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This is the main payoff
nikswamy
commented
Feb 29, 2024
| #Unobservable emp_inames | ||
| (ghost_pts_to r v) | ||
| (fun v1 -> ghost_pts_to r v ** pure (fact v1)) | ||
| // Unused? |
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These are unused, but @mtzguido tells me that they are expected to be required soon. I will restore them in a follow-up PR.
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In PulseCore, Ghost functions were previously not semantically distinguished from other atomic .actions, although they were marked as erased. This meant that we needed an axiom to lift ghost functions (with non-informative results) to regular actions. With this PR, we no longer need that axiom.
The technical mechanism enabling this is to move to a bipartite heap PulseCore.Heap2, consisting of a concrete heap and a ghost erased heap. Ghost functions are proven to only mutate the erased heap, and ghost functions now have a model starting from Heap2 and going up through Memory, Action, and Atomic layers.
Note, although this led to removing the axiom, the proofs require quite a lot of boilerplate and duplication between the concrete and ghost heaps. Expect a follow-up PR trying to add some abstraction there to reduce the boilerplate.