allow external lattice to provide its own field-merge implementation #47910
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base/compiler/abstractlattice.jl
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| This is an opt-in interface to allow external lattice implementation to provide its own | ||
| field-merge strategy. If it returns `nothing`, `tmerge(::PartialsLattice, ...)` | ||
| will use the default aggressive type merge implementation that does not use `tmerge` | ||
| recursively to accelerate convergence. |
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accelerate is probably the wrong term here, since the later computations this skips were demonstrated to be necessary to reach convergence at all
also, confusingly the tmerge function does not directly operate on the AbstractLattice subtypes, but rather the complexity lattice, and merely widens using the AbstractLattice
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also, confusingly the tmerge function does not directly operate on the AbstractLattice subtypes, but rather the complexity lattice, and merely widens using the AbstractLattice
Could you elaborate this? Maybe you mean we want to set up another lattice for representing complexity and tmerge should work on it rather than the AbstractLattice?
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#44404 was the change that made this more aggressive for future reference. |
When merging `PartialStruct`, we are currently merging their fields a bit aggressively (#47307) in order to accelerate convergence. However, when `PartialStruct` wraps external lattice elements, this can be too aggressive since it does not use `tmerge(𝕃, fields...)` recursively and thus the external lattice elements are not merged as expected. This commit adds an additional lattice hook, `tmerge_field`, inside `tmerge(::PartialsLattice)` so that external lattice implementation can provide its own field-merge strategies.
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| @@ -515,8 +515,12 @@ function tmerge(lattice::PartialsLattice, @nospecialize(typea), @nospecialize(ty | |||
| tyi = ai | |||
| elseif is_lattice_equal(lattice, bi, ft) | |||
| tyi = bi | |||
| elseif (tyi′ = tmerge_field(lattice, ai, bi); tyi′ !== nothing) | |||
| # allow external lattice implementation to provide a custom field-merge strategy | |||
| tyi = tyi′ | |||
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| tyi = tyi′ | |
| if issimplertype(tyi′, ai) && issimplertype(tyi′, bi) | |
| tyi = tyi′ | |
| else | |
| tyi = Any # the custom tmerge_field implementation violates the requirements of this lattice: abort | |
| end |
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That doesn't seem right. The requirement is that the total tmerge you can get is bounded, not that any individual tmerge produces something simpler than the things you're merging.
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I agree that an external tmerge_field should apply some complexity-limiting mechanisms, such as limiting internal Union-type to split up to 4, but currently we don't have a good way to detect if such a limitation has been applied to individual operations.
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In practice, we have found that to be a poor heuristic for Unions, since it is too big for practical purposes of similar things and too small for distinct things. It also does not converge in total, unless you had also applied some complexity bounding to each individual part of the Union
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Note that everywhere else, this algorithm is mandating issimplertype of each result (though || instead of && would also suffice here), which is lost here in the external call implementation now
| """ | ||
| tmerge_field(𝕃::AbstractLattice, a, b) -> nothing or lattice element | ||
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| Compute a lattice join of elements `a` and `b` over the lattice `𝕃`, |
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This is still not quite the lattice that tmerge refers to. It is a complexity reduction join, like tmerge. But it deviates from tmerge because it is a join of invariant parameters instead of a covariant join, so the requirements are even a bit stricter
When merging
PartialStruct, we are currently merging their fields a bit aggressively (#44404) in order to accelerate convergence. However, whenPartialStructwraps external lattice elements, this can be too aggressive since it does not usetmerge(𝕃, fields...)recursively and thus the external lattice elements are not merged as expected.This commit adds an additional lattice hook,
tmerge_field, insidetmerge(::PartialsLattice)so that external lattice implementation can provide its own field-merge strategies.