inference: remove CachedMethodTable (JuliaLang#44240)

Since we couldn't confirm any performance benefit from `CachedMethodTable`
in the current infrastructure (see the benchmark results in JuliaLang#44240), now
I'd like to propose to eliminate that entirely and save a bit of space.

base/compiler/inferencestate.jl

@@ -65,7 +65,7 @@ mutable struct InferenceState
     # The place to look up methods while working on this function.
     # In particular, we cache method lookup results for the same function to
     # fast path repeated queries.
-    method_table::CachedMethodTable{InternalMethodTable}
+    method_table::InternalMethodTable
 
     # The interpreter that created this inference state. Not looked at by
     # NativeInterpreter. But other interpreters may use this to detect cycles
@@ -141,7 +141,7 @@ mutable struct InferenceState
             cache === :global, false, false,
             Effects(consistent, ALWAYS_TRUE, ALWAYS_TRUE, ALWAYS_TRUE,
                    inbounds_taints_consistency),
-            CachedMethodTable(method_table(interp)),
+            method_table(interp),
             interp)
         result.result = frame
         cache !== :no && push!(get_inference_cache(interp), result)

base/compiler/methodtable.jl

@@ -2,22 +2,6 @@
 
 abstract type MethodTableView; end
 
-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
-
 """
     struct InternalMethodTable <: MethodTableView
 
@@ -39,19 +23,21 @@ struct OverlayMethodTable <: MethodTableView
     mt::Core.MethodTable
 end
 
-"""
-    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
+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
-CachedMethodTable(table::T) where T =
-    CachedMethodTable{T}(IdDict{Any, Union{Missing, MethodLookupResult}}(),
-        table)
+getindex(result::MethodLookupResult, idx::Int) = getindex(result.matches, idx)::MethodMatch
 
 """
     findall(sig::Type, view::MethodTableView; limit=typemax(Int))
@@ -91,13 +77,6 @@ function findall(@nospecialize(sig::Type), table::OverlayMethodTable; limit::Int
     return MethodLookupResult(ms::Vector{Any}, WorldRange(_min_val[], _max_val[]), _ambig[] != 0)
 end
 
-function findall(@nospecialize(sig::Type), 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, view::MethodTableView)::Union{Tuple{MethodMatch, WorldRange}, Nothing}
 
@@ -121,10 +100,6 @@ function findsup(@nospecialize(sig::Type), table::InternalMethodTable)
     (result.method, WorldRange(min_valid[], max_valid[]))
 end
 
-# This query is not cached
-findsup(@nospecialize(sig::Type), table::CachedMethodTable) = findsup(sig, table.table)
-
 isoverlayed(::MethodTableView)     = error("unsatisfied MethodTableView interface")
 isoverlayed(::InternalMethodTable) = false
 isoverlayed(::OverlayMethodTable)  = true
-isoverlayed(mt::CachedMethodTable) = isoverlayed(mt.table)

base/compiler/types.jl

@@ -263,7 +263,6 @@ struct NativeInterpreter <: AbstractInterpreter
         # incorrect, fail out loudly.
         @assert world <= get_world_counter()
 
-
         return new(
             # Initially empty cache
             Vector{InferenceResult}(),