inference regression against julia 1.4-dev

[shashi]

calling rand with a no-op context causes Anys. Might exacerbate #91

julia> using Cassette

julia> Cassette.@context Foo;

julia> @code_typed Cassette.overdub(Foo(), rand)
CodeInfo(
1 ─ %1 = $(Expr(:foreigncall, :(:jl_threadid), Int16, svec(), 0, :(:ccall)))::Int16
│   %2 = (Core.Intrinsics.sext_int)(Int64, %1)::Int64
│   %3 = (Core.Intrinsics.add_int)(%2, 1)::Int64
│   %4 = invoke Cassette.overdub(_2::Cassette.Context{nametype(Foo),Nothing,Nothing,Cassette.var"##PassType#427",Nothing,Nothing}, Random.default_rng::typeof(Random.default_rng), %3::Int64)::Any
│   %5 = Cassette.overdub(##overdub_context#425, rand, %4, Random.Float64)::Any
└──      return %5
) => Any

Works on Julia 1.1/1.2 + Cassette master

[vchuravy]

Nailed this down to a sequence of type-instable code within Radom.make_seed:

s = read(::Base.OrCmds,::Type{String})::Any
x = parse(UInt64, s, base=16)::UInt64

When this code gets overdubbed, we call a generator:

code_warntype(Cassette.overdub, Tuple{typeof(Foo()), typeof(parse), Type{UInt64}, Any})
ERROR: cannot call @generated function `overdub(overdub_context::Cassette.Context, overdub_arguments...) in Cassette at /home/vchuravy/src/Cassette/src/overdub.jl:524` with abstract argument types: Tuple{Cassette.Context{nametype(Foo),Nothing,Nothing,Cassette.var"##PassType#427",Nothing,Nothing},typeof(parse),Type{UInt64},Any}

:/ this unveils a pretty drastic limitation of Cassette (that at least to me, wasn't obvious).