RFC: Compiler support for optimizing PersistentDict

This is part of the work to address #51352 by attempting to allow
the compiler to perform SRAO on persistent data structures like
`PersistentDict` as if they were regular immutable data structures.
These sorts of data structures have very complicated internals
(with lots of mutation, memory sharing, etc.), but a relatively
simple interface. As such, it is unlikely that our compiler will
have sufficient power to optimize this interface by analyzing
the implementation.

We thus need to come up with some other mechanism that gives the
compiler license to perform the requisite optimization. One way
would be to just hardcode `PersistentDict` into the compiler,
optimizing it like any of the other builtin datatypes. However,
this is of course very unsatisfying. At the other end of the
spectrum would be something like a generic rewrite rule system
(e-graphs anyone?) that would let the PersistentDict
implementation declare its interface to the compiler and the
compiler would use this for optimization (in a perfect world,
the actual rewrite would then be checked using some sort of
formal methods). I think that would be interesting, but we're
very far from even being able to design something like that
(at least in Base - experiments with external AbstractInterpreters
in this direction are encouraged).

This PR tries to come up with a reasonable middle ground, where
the compiler gets some knowledge of the protocol hardcoded without
having to know about the implementation details of the data structure.

The basic ideas is that `Core` provides some magic generic functions
that implementations can extend. Semantically, they are not special.
They dispatch as usual, and implementations are expected to work
properly even in the absence of any compiler optimizations.

However, the compiler is semantically permitted to perform structural
optimization using these magic generic functions. In the concrete
case, this PR introduces the `KeyValue` interface which consists
of two generic functions, `get` and `set`. The core optimization
is that the compiler is allowed to rewrite any occurrence of
`get(set(x, k, v), k)` into `v` without additional legality checks.
In particular, the compiler performs no type checks, conversions, etc.
The higher level implementation code is expected to do all that.

This approach closely matches the general direction we've been taking
in external AbstractInterpreters for embedding additional semantics
and optimization opportunities into Julia code (although we generally
use methods there, rather than full generic functions), so I think
we have some evidence that this sort of approach works reasonably well.

Nevertheless, this is certainly an experiment and the interface is
explicitly declared unstable.

## Current Status

This is fully working and implemented, but the optimization currently
bails on anything but the simplest cases. Filling all those cases in
is not particularly hard, but should be done along with a more invasive
refactoring of SROA, so we should figure out the general direction
here first and then we can finish all that up in a follow-up cleanup.

## Obligatory benchmark
Before:
```
julia> using BenchmarkTools

julia> function foo()
           a = Base.PersistentDict(:a => 1)
           return a[:a]
       end
foo (generic function with 1 method)

julia> @benchmark foo()
BenchmarkTools.Trial: 10000 samples with 993 evaluations.
 Range (min … max):  32.940 ns …  28.754 μs  ┊ GC (min … max):  0.00% … 99.76%
 Time  (median):     49.647 ns               ┊ GC (median):     0.00%
 Time  (mean ± σ):   57.519 ns ± 333.275 ns  ┊ GC (mean ± σ):  10.81% ±  2.22%

        ▃█▅               ▁▃▅▅▃▁                ▁▃▂   ▂
  ▁▂▄▃▅▇███▇▃▁▂▁▁▁▁▁▁▁▁▂▂▅██████▅▂▁▁▁▁▁▁▁▁▁▁▂▃▃▇███▇▆███▆▄▃▃▂▂ ▃
  32.9 ns         Histogram: frequency by time         68.6 ns <

 Memory estimate: 128 bytes, allocs estimate: 4.

julia> @code_typed foo()
CodeInfo(
1 ─ %1  = invoke Vector{Union{Base.HashArrayMappedTries.HAMT{Symbol, Int64}, Base.HashArrayMappedTries.Leaf{Symbol, Int64}}}(Base.HashArrayMappedTries.undef::UndefInitializer, 1::Int64)::Vector{Union{Base.HashArrayMappedTries.HAMT{Symbol, Int64}, Base.HashArrayMappedTries.Leaf{Symbol, Int64}}}
│   %2  = %new(Base.HashArrayMappedTries.HAMT{Symbol, Int64}, %1, 0x00000000)::Base.HashArrayMappedTries.HAMT{Symbol, Int64}
│   %3  = %new(Base.HashArrayMappedTries.Leaf{Symbol, Int64}, :a, 1)::Base.HashArrayMappedTries.Leaf{Symbol, Int64}
│   %4  = Base.getfield(%2, :data)::Vector{Union{Base.HashArrayMappedTries.HAMT{Symbol, Int64}, Base.HashArrayMappedTries.Leaf{Symbol, Int64}}}
│   %5  = $(Expr(:boundscheck, true))::Bool
└──       goto #5 if not %5
2 ─ %7  = Base.sub_int(1, 1)::Int64
│   %8  = Base.bitcast(UInt64, %7)::UInt64
│   %9  = Base.getfield(%4, :size)::Tuple{Int64}
│   %10 = $(Expr(:boundscheck, true))::Bool
│   %11 = Base.getfield(%9, 1, %10)::Int64
│   %12 = Base.bitcast(UInt64, %11)::UInt64
│   %13 = Base.ult_int(%8, %12)::Bool
└──       goto #4 if not %13
3 ─       goto #5
4 ─ %16 = Core.tuple(1)::Tuple{Int64}
│         invoke Base.throw_boundserror(%4::Vector{Union{Base.HashArrayMappedTries.HAMT{Symbol, Int64}, Base.HashArrayMappedTries.Leaf{Symbol, Int64}}}, %16::Tuple{Int64})::Union{}
└──       unreachable
5 ┄ %19 = Base.getfield(%4, :ref)::MemoryRef{Union{Base.HashArrayMappedTries.HAMT{Symbol, Int64}, Base.HashArrayMappedTries.Leaf{Symbol, Int64}}}
│   %20 = Base.memoryref(%19, 1, false)::MemoryRef{Union{Base.HashArrayMappedTries.HAMT{Symbol, Int64}, Base.HashArrayMappedTries.Leaf{Symbol, Int64}}}
│         Base.memoryrefset!(%20, %3, :not_atomic, false)::MemoryRef{Union{Base.HashArrayMappedTries.HAMT{Symbol, Int64}, Base.HashArrayMappedTries.Leaf{Symbol, Int64}}}
└──       goto #6
6 ─ %23 = Base.getfield(%2, :bitmap)::UInt32
│   %24 = Base.or_int(%23, 0x00010000)::UInt32
│         Base.setfield!(%2, :bitmap, %24)::UInt32
└──       goto #7
7 ─ %27 = %new(Base.PersistentDict{Symbol, Int64}, %2)::Base.PersistentDict{Symbol, Int64}
└──       goto #8
8 ─ %29 = invoke Base.getindex(%27::Base.PersistentDict{Symbol, Int64}, 🅰️:Symbol)::Int64
└──       return %29
```

After:
```
julia> using BenchmarkTools

julia> function foo()
           a = Base.PersistentDict(:a => 1)
           return a[:a]
       end
foo (generic function with 1 method)

julia> @benchmark foo()
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
 Range (min … max):  2.459 ns … 11.320 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     2.460 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   2.469 ns ±  0.183 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

  ▂    █                                              ▁    █ ▂
  █▁▁▁▁█▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁█▁▁▁▁█ █
  2.46 ns      Histogram: log(frequency) by time     2.47 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

julia> @code_typed foo()
CodeInfo(
1 ─     return 1
```

base/boot.jl

@@ -959,4 +959,6 @@ arraysize(a::Array) = a.size
 arraysize(a::Array, i::Int) = sle_int(i, nfields(a.size)) ? getfield(a.size, i) : 1
 export arrayref, arrayset, arraysize, const_arrayref
 
+include(Core, "optimized_generics.jl")
+
 ccall(:jl_set_istopmod, Cvoid, (Any, Bool), Core, true)

base/compiler/ssair/passes.jl

@@ -6,6 +6,13 @@ function is_known_call(@nospecialize(x), @nospecialize(func), ir::Union{IRCode,I
     return singleton_type(ft) === func
 end
 
+function is_known_invoke_or_call(@nospecialize(x), @nospecialize(func), ir::Union{IRCode,IncrementalCompact})
+    isinvoke = isexpr(x, :invoke)
+    (isinvoke || isexpr(x, :call)) || return false
+    ft = argextype(x.args[isinvoke ? 2 : 1], ir)
+    return singleton_type(ft) === func
+end
+
 struct SSAUse
     kind::Symbol
     idx::Int
@@ -817,6 +824,81 @@ function lift_svec_ref!(compact::IncrementalCompact, idx::Int, stmt::Expr)
     return
 end
 
+function lift_leaves_keyvalue(compact::IncrementalCompact, @nospecialize(key),
+                             leaves::Vector{Any}, 𝕃ₒ::AbstractLattice)
+    # For every leaf, the lifted value
+    lifted_leaves = LiftedLeaves()
+    maybe_undef = false
+    for i = 1:length(leaves)
+        leaf = leaves[i]
+        cache_key = leaf
+        if isa(leaf, AnySSAValue)
+            (def, leaf) = walk_to_def(compact, leaf)
+            if is_known_invoke_or_call(def, Core.OptimizedGenerics.KeyValue.set, compact)
+                @assert isexpr(def, :invoke)
+                if length(def.args) in (5, 6)
+                    collection = def.args[end-2]
+                    set_key = def.args[end-1]
+                    set_val_idx = length(def.args)
+                elseif length(def.args) == 4
+                    collection = def.args[end-1]
+                    # Key is deleted
+                    # TODO: Model this
+                    return nothing
+                elseif length(def.args) == 3
+                    collection = def.args[end]
+                    # The whole collection is deleted
+                    # TODO: Model this
+                    return nothing
+                else
+                    return nothing
+                end
+                if set_key === key || (egal_tfunc(𝕃ₒ, argextype(key, compact), argextype(set_key, compact)) == Const(true))
+                    lift_arg!(compact, leaf, cache_key, def, set_val_idx, lifted_leaves)
+                    continue
+                end
+                # TODO: Continue walking the chain
+                return nothing
+            end
+        end
+        return nothing
+    end
+    return lifted_leaves, maybe_undef
+end
+
+function lift_keyvalue_get!(compact::IncrementalCompact, idx::Int, stmt::Expr, 𝕃ₒ::AbstractLattice)
+    # TODO: Support variants with callbacks
+    #length(stmt.args) == 4 || return
+
+    collection = stmt.args[end-1]
+    key = stmt.args[end]
+
+    leaves, visited_philikes = collect_leaves(compact, collection, Any, 𝕃ₒ, phi_or_ifelse_predecessors)
+    isempty(leaves) && return
+
+    lifted_result = lift_leaves_keyvalue(compact, key, leaves, 𝕃ₒ)
+    lifted_result === nothing && return
+    lifted_leaves, any_undef = lifted_result
+
+    result_t = Union{}
+    for v in values(lifted_leaves)
+        v === nothing && return
+        result_t = tmerge(𝕃ₒ, result_t, argextype(v.val, compact))
+    end
+
+    lifted_val = perform_lifting!(compact,
+        visited_philikes, key, result_t, lifted_leaves, collection, nothing)
+
+    compact[idx] = lifted_val === nothing ? nothing : lifted_val.val
+    if lifted_val !== nothing
+        if !⊑(𝕃ₒ, compact[SSAValue(idx)][:type], result_t)
+            compact[SSAValue(idx)][:flag] |= IR_FLAG_REFINED
+        end
+    end
+
+    return
+end
+
 # TODO: We could do the whole lifing machinery here, but really all
 # we want to do is clean this up when it got inserted by inlining,
 # which always targets simple `svec` call or `_compute_sparams`,
@@ -1002,7 +1084,7 @@ function sroa_pass!(ir::IRCode, inlining::Union{Nothing,InliningState}=nothing)
     for ((_, idx), stmt) in compact
         # check whether this statement is `getfield` / `setfield!` (or other "interesting" statement)
         isa(stmt, Expr) || continue
-        is_setfield = is_isdefined = is_finalizer = false
+        is_setfield = is_isdefined = is_finalizer = is_keyvalue_get = false
         field_ordering = :unspecified
         if is_known_call(stmt, setfield!, compact)
             4 <= length(stmt.args) <= 5 || continue
@@ -1092,6 +1174,9 @@ function sroa_pass!(ir::IRCode, inlining::Union{Nothing,InliningState}=nothing)
                 lift_comparison!(isa, compact, idx, stmt, 𝕃ₒ)
             elseif is_known_call(stmt, Core.ifelse, compact)
                 fold_ifelse!(compact, idx, stmt)
+            elseif is_known_invoke_or_call(stmt, Core.OptimizedGenerics.KeyValue.get, compact)
+                4 <= length(stmt.args) <= 6 || continue
+                lift_keyvalue_get!(compact, idx, stmt, 𝕃ₒ)
             elseif isexpr(stmt, :new)
                 refine_new_effects!(𝕃ₒ, compact, idx, stmt)
             end

base/dict.jl

@@ -889,10 +889,35 @@ _similar_for(c::AbstractDict, ::Type{T}, itr, isz, len) where {T} =
 
 include("hamt.jl")
 using .HashArrayMappedTries
+using Core.OptimizedGenerics: KeyValue
 const HAMT = HashArrayMappedTries
 
 struct PersistentDict{K,V} <: AbstractDict{K,V}
     trie::HAMT.HAMT{K,V}
+    # Serves as a marker for an empty initialization
+    @noinline function KeyValue.set(::Type{PersistentDict{K, V}}) where {K, V}
+        new{K, V}(HAMT.HAMT{K,V}())
+    end
+    @noinline function KeyValue.set(::Type{PersistentDict{K, V}}, ::Nothing, key, val) where {K, V}
+        new{K, V}(HAMT.HAMT{K, V}(key, val))
+    end
+    @noinline function KeyValue.set(dict::PersistentDict{K, V}, key, val) where {K, V}
+        trie = dict.trie
+        h = hash(key)
+        found, present, trie, i, bi, top, hs = HAMT.path(trie, key, h, #=persistent=# true)
+        HAMT.insert!(found, present, trie, i, bi, hs, val)
+        return new{K, V}(top)
+    end
+    @noinline function KeyValue.set(dict::PersistentDict{K, V}, key) where {K, V}
+        trie = dict.trie
+        h = hash(key)
+        found, present, trie, i, bi, top, _ = HAMT.path(trie, key, h, #=persistent=# true)
+        if found && present
+            deleteat!(trie.data, i)
+            HAMT.unset!(trie, bi)
+        end
+        return new{K, V}(top)
+    end
 end
 
 """
@@ -923,19 +948,27 @@ Base.PersistentDict{Symbol, Int64} with 1 entry:
 """
 PersistentDict
 
-PersistentDict{K,V}() where {K,V} = PersistentDict(HAMT.HAMT{K,V}())
-PersistentDict{K,V}(KV::Pair) where {K,V} = PersistentDict(HAMT.HAMT{K,V}(KV...))
-PersistentDict(KV::Pair{K,V}) where {K,V} = PersistentDict(HAMT.HAMT{K,V}(KV...))
+PersistentDict{K,V}() where {K, V} = KeyValue.set(PersistentDict{K,V})
+function PersistentDict{K,V}(KV::Pair) where {K,V}
+    KeyValue.set(
+        PersistentDict{K, V},
+        nothing,
+        KV...)
+end
+function PersistentDict(KV::Pair{K,V}) where {K,V}
+    KeyValue.set(
+        PersistentDict{K, V},
+        nothing,
+        KV...)
+end
 PersistentDict(dict::PersistentDict, pair::Pair) = PersistentDict(dict, pair...)
 PersistentDict{K,V}(dict::PersistentDict{K,V}, pair::Pair) where {K,V} = PersistentDict(dict, pair...)
+
+
 function PersistentDict(dict::PersistentDict{K,V}, key, val) where {K,V}
     key = convert(K, key)
     val = convert(V, val)
-    trie = dict.trie
-    h = hash(key)
-    found, present, trie, i, bi, top, hs = HAMT.path(trie, key, h, #=persistent=# true)
-    HAMT.insert!(found, present, trie, i, bi, hs, val)
-    return PersistentDict(top)
+    return KeyValue.set(dict, key, val)
 end
 
 function PersistentDict(kv::Pair, rest::Pair...)
@@ -950,84 +983,54 @@ end
 eltype(::PersistentDict{K,V}) where {K,V} = Pair{K,V}
 
 function in(key_val::Pair{K,V}, dict::PersistentDict{K,V}, valcmp=(==)) where {K,V}
-    trie = dict.trie
-    if HAMT.islevel_empty(trie)
-        return false
-    end
-
     key, val = key_val
-
-    h = hash(key)
-    found, present, trie, i, _, _, _ = HAMT.path(trie, key, h)
-    if found && present
-        leaf = @inbounds trie.data[i]::HAMT.Leaf{K,V}
-        return valcmp(val, leaf.val) && return true
-    end
-    return false
+    return KeyValue.get(found->valcmp(val, found), ()->false, dict, key)
 end
 
 function haskey(dict::PersistentDict{K}, key::K) where K
-    trie = dict.trie
-    h = hash(key)
-    found, present, _, _, _, _, _ = HAMT.path(trie, key, h)
-    return found && present
+    return KeyValue.get(_->true, ()->false, dict, key)
 end
 
 function getindex(dict::PersistentDict{K,V}, key::K) where {K,V}
-    trie = dict.trie
-    if HAMT.islevel_empty(trie)
-        throw(KeyError(key))
+    return KeyValue.get(dict, key) do
+        return throw(KeyError(key))
     end
-    h = hash(key)
-    found, present, trie, i, _, _, _ = HAMT.path(trie, key, h)
-    if found && present
-        leaf = @inbounds trie.data[i]::HAMT.Leaf{K,V}
-        return leaf.val
-    end
-    throw(KeyError(key))
 end
 
 function get(dict::PersistentDict{K,V}, key::K, default) where {K,V}
-    trie = dict.trie
-    if HAMT.islevel_empty(trie)
+    return KeyValue.get(dict, key) do
         return default
     end
-    h = hash(key)
-    found, present, trie, i, _, _, _ = HAMT.path(trie, key, h)
-    if found && present
-        leaf = @inbounds trie.data[i]::HAMT.Leaf{K,V}
-        return leaf.val
-    end
-    return default
 end
 
-function get(default::Callable, dict::PersistentDict{K,V}, key::K) where {K,V}
+@noinline function KeyValue.get(transform, default, dict::PersistentDict{K, V}, key) where {K, V}
     trie = dict.trie
     if HAMT.islevel_empty(trie)
-        return default
+        return default()
     end
     h = hash(key)
     found, present, trie, i, _, _, _ = HAMT.path(trie, key, h)
     if found && present
         leaf = @inbounds trie.data[i]::HAMT.Leaf{K,V}
-        return leaf.val
+        return transform(leaf.val)
     end
     return default()
 end
 
-iterate(dict::PersistentDict, state=nothing) = HAMT.iterate(dict.trie, state)
+@noinline function KeyValue.get(default, dict::PersistentDict, key)
+    KeyValue.get(identity, default, dict, key)
+end
+
+function get(default::Callable, dict::PersistentDict{K,V}, key::K) where {K,V}
+    return KeyValue.get(default, dict, key)
+end
 
 function delete(dict::PersistentDict{K}, key::K) where K
-    trie = dict.trie
-    h = hash(key)
-    found, present, trie, i, bi, top, _ = HAMT.path(trie, key, h, #=persistent=# true)
-    if found && present
-        deleteat!(trie.data, i)
-        HAMT.unset!(trie, bi)
-    end
-    return PersistentDict(top)
+    return KeyValue.set(dict, key)
 end
 
+iterate(dict::PersistentDict, state=nothing) = HAMT.iterate(dict.trie, state)
+
 length(dict::PersistentDict) = HAMT.length(dict.trie)
 isempty(dict::PersistentDict) = HAMT.isempty(dict.trie)
 empty(::PersistentDict, ::Type{K}, ::Type{V}) where {K, V} = PersistentDict{K, V}()

base/hamt.jl

@@ -64,12 +64,18 @@ mutable struct HAMT{K, V}
     bitmap::BITMAP
 end
 HAMT{K, V}() where {K, V} = HAMT(Vector{Union{Leaf{K, V}, HAMT{K, V}}}(undef, 0), zero(BITMAP))
-function HAMT{K,V}(k::K, v) where {K, V}
-    v = convert(V, v)
+
+@Base.assume_effects :nothrow function init_hamt(K, V, k, v)
     # For a single element we can't have a hash-collision
     trie = HAMT(Vector{Union{Leaf{K, V}, HAMT{K, V}}}(undef, 1), zero(BITMAP))
-    trie.data[1] = Leaf{K,V}(k,v)
+    @inbounds trie.data[1] = Leaf{K,V}(k,v)
+    return trie
+end
+
+function HAMT{K,V}(k::K, v) where {K, V}
+    v = convert(V, v)
     bi = BitmapIndex(HashState(k))
+    trie = init_hamt(K, V, k, v)
     set!(trie, bi)
     return trie
 end

base/optimized_generics.jl

@@ -0,0 +1,58 @@
+# This file is a part of Julia. License is MIT: https://julialang.org/license
+
+module OptimizedGenerics
+
+# This file defines interfaces that are recognized and optimized by the compiler
+# They are intended to be used by data structure implementations that wish to
+# opt into some level of compiler optimizations. These interfaces are
+# EXPERIMENTAL and currently intended for use by Base only. They are subject
+# to change or removal without notice. It is undefined behavior to add methods
+# to these generics that do not conform to the specified interface.
+#
+# The intended way to use these generics is that data structures will provide
+# appropriate implementations for a generic. In the absence of compiler
+# optimizations, these behave like regular methods. However, the compiler is
+# semantically allowed to perform certain structural optimizations on
+# appropriate combinations of these intrinsics without proving correctness.
+
+# Compiler-recognized generics for immutable key-value stores (dicts, etc.)
+"""
+    module KeyValue
+
+Implements a key-value like interface where the compiler has liberty to perform
+the following transformations. The core optimization semantically allowed for
+the compiler is:
+
+    get(set(x, key, val), key) -> val
+
+where the compiler will recursively look through `x`. Keys are compared by
+egality.
+
+Implementations must observe the following constraints:
+
+1. It is undefined behavior for `get` not to return the exact (by egality) val
+   stored for a given `key`.
+"""
+module KeyValue
+    """
+        set(collection, [key [, val]])
+        set(T, collection, key, val)
+
+    Set the `key` in `collection` to `val`. If `val` is omitted, deletes the
+    value from the collection. If `key` is omitted as well, deletes all elements
+    of the collection.
+    """
+    function set end
+
+    """
+        get([[transform,] default,] collection, key, val)
+
+    Retrieve the value corresponding to `key` in `collection`. Optionally takes
+    a `default` callback that is executed if `key` is not found and a `transform`
+    callback that is executed only if the value is found (but not on the return)
+    value of `default`.
+    """
+    function get end
+end
+
+end

base/reflection.jl

@@ -728,7 +728,7 @@ function objectid(x)
     return _objectid(x)
 end
 function _foldable_objectid(@nospecialize(x))
-    @_foldable_meta
+    @_total_meta
     _objectid(x)
 end
 _objectid(@nospecialize(x)) = ccall(:jl_object_id, UInt, (Any,), x)

test/compiler/irpasses.jl

@@ -1568,3 +1568,10 @@ let m = Meta.@lower 1 + 1
 
     Core.Compiler.verify_ir(ir)
 end
+
+# Test support for Core.OptimizedGenerics.KeyValue protocol
+function persistent_dict_elim()
+    a = Base.PersistentDict(:a => 1)
+    return a[:a]
+end
+@test fully_eliminated(persistent_dict_elim)