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Refactor compiler method lookup interface (JuliaLang#36743)
The primary motivation here is to clean up the notion of "looking up a method in the method table" into a single object that can be passed around. At the moment, it's all a bit murky, with fairly large state objects being passed around everywhere and implicit accesses to the global environment. In my AD use case, I need to be a bit careful to make sure the various inference and optimization steps are looking things up in the correct tables/caches, so being very explicit about where things need to be looked up is quite helpful. In particular, I would like to clean up the optimizer, to not require the big `OptimizationState` which is currently a bit of a mix of things that go into IRCode and information needed for method lookup/edge tracking. That isn't part of this PR, but will build on top of it. More generally, with a bunch of the recent compiler work, I've been trying to define more crisp boundaries between the various components of the system, giving them clearer interfaces, and at least a little bit of documentation. The compiler is a very powerful bit of technology, but I think people having been avoiding it, because the code looks a bit scary. I'm hoping some of these cleanups will make it easier for people to understand what's going on. Here in particular, I'm using `findall(sig, table)` as the predicate for method lookup. The idea being that people familiar with the `findall(predicate, collection)` idiom from regular julia will have a good intuitive understanding of what's happening (a collection is searched for a predicate), an array of matches is returned, etc. Of course, it's not a perfect fit, but I think these kinds of mental aids can be helpful in making it easier for people to read compiler code (similar to how JuliaLang#35831 used `getindex` as the verb for cache lookup). While I was at it, I also cleaned up the use of out-parameters which leaked through too much of the underlying C API and replaced them by a proper struct of results.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,93 @@ | ||
| abstract type MethodTableView; end | ||
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| struct MethodLookupResult | ||
| # Really Vector{Core.MethodMatch}, but it's easier to represent this as | ||
| # and work with Vector{Any} on the C side. | ||
| matches::Vector{Any} | ||
| valid_worlds::WorldRange | ||
| ambig::Bool | ||
| end | ||
| length(result::MethodLookupResult) = length(result.matches) | ||
| function iterate(result::MethodLookupResult, args...) | ||
| r = iterate(result.matches, args...) | ||
| r === nothing && return nothing | ||
| match, state = r | ||
| return (match::MethodMatch, state) | ||
| end | ||
| getindex(result::MethodLookupResult, idx::Int) = getindex(result.matches, idx)::MethodMatch | ||
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||
| """ | ||
| struct InternalMethodTable <: MethodTableView | ||
| A struct representing the state of the internal method table at a | ||
| particular world age. | ||
| """ | ||
| struct InternalMethodTable <: MethodTableView | ||
| world::UInt | ||
| end | ||
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||
| """ | ||
| struct CachedMethodTable <: MethodTableView | ||
| Overlays another method table view with an additional local fast path cache that | ||
| can respond to repeated, identical queries faster than the original method table. | ||
| """ | ||
| struct CachedMethodTable{T} <: MethodTableView | ||
| cache::IdDict{Any, Union{Missing, MethodLookupResult}} | ||
| table::T | ||
| end | ||
| CachedMethodTable(table::T) where T = | ||
| CachedMethodTable{T}(IdDict{Any, Union{Missing, MethodLookupResult}}(), | ||
| table) | ||
|
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||
| """ | ||
| findall(sig::Type{<:Tuple}, view::MethodTableView; limit=typemax(Int)) | ||
| Find all methods in the given method table `view` that are applicable to the | ||
| given signature `sig`. If no applicable methods are found, an empty result is | ||
| returned. If the number of applicable methods exeeded the specified limit, | ||
| `missing` is returned. | ||
| """ | ||
| function findall(@nospecialize(sig::Type{<:Tuple}), table::InternalMethodTable; limit::Int=typemax(Int)) | ||
| _min_val = RefValue{UInt}(typemin(UInt)) | ||
| _max_val = RefValue{UInt}(typemax(UInt)) | ||
| _ambig = RefValue{Int32}(0) | ||
| ms = _methods_by_ftype(sig, limit, table.world, false, _min_val, _max_val, _ambig) | ||
| if ms === false | ||
| return missing | ||
| end | ||
| return MethodLookupResult(ms::Vector{Any}, WorldRange(_min_val[], _max_val[]), _ambig[] != 0) | ||
| end | ||
|
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| function findall(@nospecialize(sig::Type{<:Tuple}), table::CachedMethodTable; limit::Int=typemax(Int)) | ||
| box = Core.Box(sig) | ||
| return get!(table.cache, sig) do | ||
| findall(box.contents, table.table; limit=limit) | ||
| end | ||
| end | ||
|
|
||
| """ | ||
| findsup(sig::Type{<:Tuple}, view::MethodTableView)::Union{Tuple{MethodMatch, WorldRange}, Nothing} | ||
| Find the (unique) method `m` such that `sig <: m.sig`, while being more | ||
| specific than any other method with the same property. In other words, find | ||
| the method which is the least upper bound (supremum) under the specificity/subtype | ||
| relation of the queried `signature`. If `sig` is concrete, this is equivalent to | ||
| asking for the method that will be called given arguments whose types match the | ||
| given signature. This query is also used to implement `invoke`. | ||
| Such a method `m` need not exist. It is possible that no method is an | ||
| upper bound of `sig`, or it is possible that among the upper bounds, there | ||
| is no least element. In both cases `nothing` is returned. | ||
| """ | ||
| function findsup(@nospecialize(sig::Type{<:Tuple}), table::InternalMethodTable) | ||
| min_valid = RefValue{UInt}(typemin(UInt)) | ||
| max_valid = RefValue{UInt}(typemax(UInt)) | ||
| result = ccall(:jl_gf_invoke_lookup_worlds, Any, (Any, UInt, Ptr{Csize_t}, Ptr{Csize_t}), | ||
| sig, table.world, min_valid, max_valid)::Union{Method, Nothing} | ||
| result === nothing && return nothing | ||
| (result, WorldRange(min_valid[], max_valid[])) | ||
| end | ||
|
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||
| # This query is not cached | ||
| findsup(sig::Type{<:Tuple}, table::CachedMethodTable) = findsup(sig, table.table) |
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