code_warntype shows unused slots

[simeonschaub]

Is it intentional that we display the unused slots z@_8, z@_9 and z@_10 and their NewvarNodes here? Or should that have been cleaned up before printing?

julia> function foo(x, y)
           z = x + y
           while z < 4
               z += 1
           end
           u = (x -> x + z)(x)
           v = Ref{Union{Int, Missing}}(x)[] + y
           return z * 7
       end
foo (generic function with 1 method)

julia> @code_warntype foo(1, 2)
MethodInstance for foo(::Int64, ::Int64)
  from foo(x, y) in Main at REPL[10]:1
Arguments
  #self#::Core.Const(foo)
  x::Int64
  y::Int64
Locals
  #3::var"#3#4"
  v::Union{Missing, Int64}
  u::Any
  z@_7::Core.Box
  z@_8::Union{}
  z@_9::Union{}
  z@_10::Union{}
Body::Any
1 ──       Core.NewvarNode(:(#3))
│          Core.NewvarNode(:(v))
│          Core.NewvarNode(:(u))
│          (z@_7 = Core.Box())
│    %5  = (x + y)::Int64
└───       Core.setfield!(z@_7, :contents, %5)
2 ┄─ %7  = Core.isdefined(z@_7, :contents)::Bool
└───       goto #4 if not %7
3 ──       goto #5
4 ──       Core.NewvarNode(:(z@_8))
└───       z@_8
5 ┄─ %12 = Core.getfield(z@_7, :contents)::Any
│    %13 = (%12 < 4)::Any
└───       goto #10 if not %13
6 ── %15 = Core.isdefined(z@_7, :contents)::Bool
└───       goto #8 if not %15
7 ──       goto #9
8 ──       Core.NewvarNode(:(z@_9))
└───       z@_9
9 ┄─ %20 = Core.getfield(z@_7, :contents)::Any
│    %21 = (%20 + 1)::Any
│          Core.setfield!(z@_7, :contents, %21)
└───       goto #2
10 ─       (#3 = %new(Main.:(var"#3#4"), z@_7))
│    %25 = #3::var"#3#4"
│          (u = (%25)(x))
│    %27 = Core.apply_type(Main.Union, Main.Int, Main.Missing)::Core.Const(Union{Missing, Int64})
│    %28 = Core.apply_type(Main.Ref, %27)::Core.Const(Ref{Union{Missing, Int64}})
│    %29 = (%28)(x)::Base.RefValue{Union{Missing, Int64}}
│    %30 = Base.getindex(%29)::Union{Missing, Int64}
│          (v = %30 + y)
│    %32 = Core.isdefined(z@_7, :contents)::Bool
└───       goto #12 if not %32
11 ─       goto #13
12 ─       Core.NewvarNode(:(z@_10))
└───       z@_10
13 ┄ %37 = Core.getfield(z@_7, :contents)::Any
│    %38 = (%37 * 7)::Any
└───       return %38

[KristofferC]

#23735 used to clean up some of them at least.

[vtjnash]

We currently show pre-optimized code (so that it more closely corresponds to the original, pre-inlining)

[simeonschaub]

Ah, I didn't realize these slots are in the lowered code already. In that case, maybe the better question is why lowering emitted these unused slots in the first place.

[vtjnash]

It is used to throw an UndefVarError(:z)