WIP: improve inference for getproperty

This has regressed quite a bit due to #848. With this PR, we should be able to get back the same performance as before, assuming there is no custom implementation or pullback for `getproperty`. Still need to add tests.

src/Zygote.jl

@@ -33,6 +33,7 @@ include("compiler/show.jl")
 
 include("lib/grad.jl")
 include("lib/lib.jl")
+include("lib/literal_getproperty.jl")
 include("lib/number.jl")
 include("lib/base.jl")
 include("lib/array.jl")

src/lib/literal_getproperty.jl

@@ -0,0 +1,82 @@
+# Mostly copied over from Cassette in `src/overdub.jl`
+# Return `Reflection` for signature `sigtypes` and `world`, if possible. Otherwise, return `nothing`.
+function reflect(@nospecialize(sigtypes::Tuple), world::UInt = typemax(UInt))
+    if length(sigtypes) > 2 && sigtypes[1] === typeof(invoke)
+        @assert sigtypes[3] <: Type{<:Tuple}
+        sigtypes = (sigtypes[2], sigtypes[3].parameters[1].parameters...)
+    end
+    # This works around a subtyping bug. Basically, callers can deconstruct upstream
+    # `UnionAll` types in such a way that results in a type with free type variables, in
+    # which case subtyping can just break.
+    #
+    # God help you if you try to use a type parameter here (e.g. `::Type{S} where S<:Tuple`)
+    # instead of this nutty workaround, because the compiler can just rewrite `S` into
+    # whatever it thinks is "type equal" to the actual provided value. In other words, if
+    # `S` is defined as e.g. `f(::Type{S}) where S`, and you call `f(T)`, you should NOT
+    # assume that `S === T`. If you did, SHAME ON YOU. It doesn't matter that such an
+    # assumption holds true for essentially all other kinds of values. I haven't counted in
+    # a while, but I'm pretty sure I have ~40+ hellish years of Julia experience, and this
+    # still catches me every time. Who even uses this crazy language?
+    S = Tuple{map(s -> Core.Compiler.has_free_typevars(s) ? typeof(s.parameters[1]) : s, sigtypes)...}
+    (S.parameters[1]::DataType).name.module === Core.Compiler && return nothing
+    _methods = Base._methods_by_ftype(S, -1, world)
+    method_index = 0
+    for i in 1:length(_methods)
+        if _methods[i][1] === S
+            method_index = i
+            break
+        end
+    end
+    method_index === 0 && return nothing
+    type_signature, raw_static_params, method = _methods[method_index]
+    method_instance = Core.Compiler.specialize_method(method, type_signature, raw_static_params, false)
+    method_signature = method.sig
+    static_params = Any[raw_static_params...]
+    return method_instance, method_signature, static_params
+end
+
+
+# ugly hack to make differentiating `getproperty` infer a lot better
+@generated function _pullback(cx::AContext, ::typeof(literal_getproperty), x, ::Val{f}) where f
+    sig(x) = Tuple{x, typeof(f)}
+    rrule_sig(x) = Tuple{typeof(getproperty), x, typeof(f)}
+    pb_sig(x) = Tuple{cx, typeof(getproperty), x, typeof(f)}
+
+    # either `getproperty` has a custom implementation or `_pullback(cx, getproperty, x, f)`
+    # / `rrule(getproperty, x, f) is overloaded directly
+    is_getfield_fallback = which(getproperty, sig(x)) == which(getproperty, sig(Any)) &&
+        which(_pullback, pb_sig(x)) == which(_pullback, pb_sig(Any)) &&
+        which(rrule, rrule_sig(x)) == which(rrule, rrule_sig(Any))
+
+    #ccall(:jl_safe_printf, Cvoid, (Cstring,), "$is_getfield_fallback: $x\n")
+
+    if is_getfield_fallback
+        # just copy pullback of `literal_getfield`
+        mi, _sig, sparams = reflect((typeof(_pullback), cx, typeof(literal_getfield), x, Val{f}))
+        ci = copy(Core.Compiler.retrieve_code_info(mi))
+
+        # we need to change the second arg to `_pullback` from `literal_getproperty` to
+        # `literal_getfield`
+        Meta.partially_inline!(
+            ci.code, Any[_pullback, Core.SlotNumber(2), literal_getfield],
+            _sig, sparams, 0, 0, :propagate,
+        )
+        ci.inlineable = true
+
+        # backedge for `_pullback`, see https://docs.julialang.org/en/v1/devdocs/ast/#MethodInstance
+        # this will cause a backedge to this particular MethodInstance to be attached to
+        # `_pullback(cx, getproperty, x, f)`
+        mi_pb_getproperty, _, _ = reflect((typeof(_pullback), pb_sig(x).parameters...))
+        mi_getproperty, _, _ = reflect((typeof(getproperty), sig(x).parameters...))
+        mi_rrule, _, _ = reflect((typeof(rrule), rrule_sig(x).parameters...))
+        ci.edges = Core.MethodInstance[mi, mi_pb_getproperty, mi_getproperty, mi_rrule]
+
+        return ci
+    else
+        # nothing to optimize here, need to recurse into `getproperty`
+        return quote
+            Base.@_inline_meta
+            _pullback(cx, getproperty, x, $(QuoteNode(f)))
+        end
+    end
+end

test/compiler.jl

@@ -144,5 +144,52 @@ end
   @test Zygote.gradient(sumall, ms) == ((a = 2, b = 2),)
 end
 
+using ChainRulesCore
+
+function _Gaussian(suffix::Symbol)
+    name = gensym(Symbol(:Gaussian_, suffix))
+    return @eval begin
+        struct $name{Tm, TP}
+            m::Tm
+            P::TP
+        end
+        $name
+    end
+end
+
+@testset "inference for `getproperty`" begin
+    Gaussian = _Gaussian(:getproperty)
+    g = Gaussian(randn(3), randn(3, 3))
+    y, back = @inferred pullback(x -> x.m, g)
+    @test y == getfield(g, :m)
+    @test Base.return_types(back, Tuple{Vector{Float64}}) == Any[Union{Tuple{Nothing}, typeof(((m = [1.0, 0.0, 0.0], P = nothing),))}]
+    @test back([1., 0, 0]) == ((m = [1.0, 0.0, 0.0], P = nothing),)
+
+    Base.getproperty(g::Gaussian, s::Symbol) = 2getfield(g, s)
+    y, back = pullback(x -> x.m, g)
+    @test y == 2getfield(g, :m)
+    @test back([1., 0, 0]) == ((m = [2.0, 0.0, 0.0], P = nothing),)
+
+
+    Gaussian = _Gaussian(:pullback)
+    g = Gaussian(randn(3), randn(3, 3))
+    y, back = @inferred pullback(x -> x.m, g)
+
+    Zygote._pullback(::typeof(getproperty), g::Gaussian, s::Symbol) = 3getfield(g, s), Δ -> (nothing, (; ((:m, :P) .=> nothing)..., s => 3Δ), nothing)
+    y, back = pullback(x -> x.m, g)
+    @test_broken y == 3getfield(g, :m)
+    @test_broken back([1., 0, 0]) == ((m = [3.0, 0.0, 0.0], P = nothing),)
+
+
+    Gaussian = _Gaussian(:rrule)
+    g = Gaussian(randn(3), randn(3, 3))
+    y, back = @inferred pullback(x -> x.m, g)
+
+    ChainRulesCore.rrule(::typeof(getproperty), g::Gaussian, s::Symbol) = 4getfield(g, s), Δ -> (NoTangent(), Tangent{typeof(g)}(; s => 4Δ), NoTangent())
+    y, back = pullback(x -> x.m, g)
+    @test y == 4getfield(g, :m)
+    @test back([1., 0, 0]) == ((m = [4.0, 0.0, 0.0], P = nothing),)
+end
+
 # issue 897
 @test gradient(x -> sum(norm, collect(eachcol(x))), ones(3, 400))[1] ≈ fill(0.5773502691896258, 3, 400)