Sharing on patricia trees #4183
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This patch introduces a new `union`-like function to the patricia tree (and containers) interface, `diff`. It also provides `_shared` and `_sharing` variants of both `diff` and `union` to exploit internal sharing, and a typed interface to help ensure sharing is not accidentally lost. The `diff` function is an `union`-like generalization of the existing `diff_domains`. `diff f m1 m2` is similar to `merge f m1 m2` with `f None (Some _) = None` and `f (Some v) None = Some v`. The `_shared` variants (`diff_shared`, really) are the main motivation for this patch, and have a transformative impact on performance: `diff_shared` and `union_shared` exploit physical equality to simplify `diff_shared f m m = empty` and `union_shared f m m = m` in constant time. They also ensure maximal sharing of the result with their first argument. The `_sharing` variants do not perform this optimisation but still ensure maximal sharing of the result with their first argument. They are useful to add or remove entries into a map while preserving sharing. The `Shared_set` and `Shared_map` are thin wrapper modules that only expose sharing-preserving functions to avoid accidental loss of sharing.
| lines of Haskell's implementation; this would let us preserve sharing (see | ||
| Haskell's [Data.IntMap] for details). *) | ||
| let rec union f t0 t1 = | ||
| let[@inline always] universal_merge ?shortcut ?check0 ?check1 ~only_left |
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We agreed on renaming check0 and check1 to something more sensible — maybe phys_eq_left and phys_eq_right?
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| let update_many f t0 t1 = | ||
| let iv = is_value_of t0 in | ||
| universal_merge |
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Following offline discussion, we should add check0 here for memory optimisation, and either document it as a guarantee or add update_many_sharing with that guarantee.
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Extends the #4073 PR.
This adds some important inlining annotations, and add a new helper function.
This removes Shared_set and map as the type is not obviously the good one.
It also add the update_and_extend function that is useful to optimize a quadratic case in dataflow