Zero alloc: don't loose witnesses #2353
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This allows us to get rid of a lot of machinery for tracking and resolving unresolved depedencies. Conservative on functions that have mutually recursive calls or forward calls.
1) Keep unresolved functions around until the end of the compilation unit 2) Naively compute fixed point at the end of the unit
t.approx should only be used for self rec that has no other unresolved dependencies.
Pass [keep_witnesses ] as an argument to [Analysis.create] instead of using default initial value and mutating it later.
otherwise we loose witnesses sometimes
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xclerc
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Mar 13, 2024
I am collecting some statistics on this RP and it's baseline #2290. |
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I messed up the repo/branch in this PR, sorry. I am closing this in favor of #2354 to reduce the confusion. We can continue the discussion there. |
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There is a mismatch in handling witnesses:
V.lessequalignores them andV.joindoes not. As a result, witnesses discovered in the last iteration of a fixed point computation might be lost, if there are already other witnesses for the same component.The analysis uses
V.lessequal new_value valueto detect fixed points and will not join thenew_valueto the preiously computed value if the only difference is in witnesses.This is not a soundness problem in the sense that the check will not miss allocations, but it might not return all allocations as witnesses. We do not guarantee that all witnesses will be reported, but it is a nice property to have. We rely on the witnesses produced by the analysis to visualize allocations and not only to check annotations. Having consistent output also makes testing easier.
This PR fixes it "properly" by taking witnesses into account in
V.lessequaland refactoring some other witness related checks to make it clear that we are allowed to ignore witnesses of "expected" values that come from annotations, and not the computed values. The downside of this approach is that the fixpoint can take longer: the height of the abstract domain now depends on the size of the input file in the worst case, instead of being constant height of3. I think that the bad case cannot happen in practice because of the order or propagation of witnesses along all the control flow paths within a loop before checking for a fixed point.An alternative is to call
joineven whenlessequalis true. It works too but feels like a hack.For example, the output of existing test
fail19.mlchanges depending on the order in which functions are emitted in the file:-function-layout topologicalincludesprobewitness and with thesourcelayout it doesn't. This happens because the fixpoint for "forward defined" functions discovered a new witness but the value was alreadyTop, so the new witness is not recorded. I haven't found a way to reproduce this on main yet.On top of #2290.