# Why is this simple code type unstable?

**URL:** <https://discourse.julialang.org/t/why-is-this-simple-code-type-unstable/110734>\
**Category:** General Usage\
**Created:** [February 25, 2024, 12:19pm UTC](https://discourse.julialang.org/t/why-is-this-simple-code-type-unstable/110734 "2024-02-25T12:19:47Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![photor](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/photor/32/14343_2.png) [@photor](https://discourse.julialang.org/u/photor)\
**Post date:** [February 25, 2024, 12:19pm UTC](https://discourse.julialang.org/t/why-is-this-simple-code-type-unstable/110734/1 "2024-02-25T12:19:47Z")

</div>

```julia
julia> function test()
       for i=1:1 end
       end
test (generic function with 1 method)

julia> @code_warntype test()
MethodInstance for test()
  from test() in Main at REPL[1]:1
Arguments
  #self#::Core.Const(test)
Locals
  @_2::Union{Nothing, Tuple{Int64, Int64}}
  i::Int64
Body::Nothing
1 ─ %1 = (1:1)::Core.Const(1:1)
│ (@_2 = Base.iterate(%1))
│ %3 = (@_2::Core.Const((1, 1)) === nothing)::Core.Const(false)
│ %4 = Base.not_int(%3)::Core.Const(true)
└── goto #4 if not %4
2 ─ %6 = @_2::Core.Const((1, 1))
│ (i = Core.getfield(%6, 1))
│ %8 = Core.getfield(%6, 2)::Core.Const(1)
│ (@_2 = Base.iterate(%1, %8))
│ %10 = (@_2::Core.Const(nothing) === nothing)::Core.Const(true)
│ %11 = Base.not_int(%10)::Core.Const(false)
└── goto #4 if not %11
3 ─ Core.Const(:(goto %6))
4 ┄ return nothing

julia>

```

---

<div class="post-metadata">

**Author:** ![stabbles](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stabbles/32/946_2.png) [@stabbles](https://discourse.julialang.org/u/stabbles)\
**Post date:** [February 25, 2024, 12:31pm UTC](https://discourse.julialang.org/t/why-is-this-simple-code-type-unstable/110734/2 "2024-02-25T12:31:18Z")

</div>

The for loop turns into calls to `iterate(...)`, which returns the next value/state or nothing if the iterator is exhausted. It’s not problematic because of small union optimization – that’s why it’s not printed in red but orange.

See [Interfaces · The Julia Language](https://docs.julialang.org/en/v1/manual/interfaces/#man-interface-iteration)

---

<div class="post-metadata">

**Author:** ![mnemnion](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mnemnion/32/206596_2.png) [@mnemnion](https://discourse.julialang.org/u/mnemnion)\
**Post date:** [February 25, 2024, 5:29pm UTC](https://discourse.julialang.org/t/why-is-this-simple-code-type-unstable/110734/3 "2024-02-25T17:29:40Z")

</div>

It’s worth observing that a non-trivial version of this test is completely type stable:

```julia
julia> function test2()
       x = rand(1,10)
       y = rand(5,10)
       for i=1:y
          x += y
       end
       x
       end
test2 (generic function with 1 method)

julia> @code_warntype test2()
MethodInstance for test2()
  from test2() @ Main REPL[14]:1
Arguments
  #self#::Core.Const(test2)
Locals
  @_2::Union{}
  y::Matrix{Float64}
  x::Matrix{Float64}
  i::Union{}
Body::Union{}
1 ─ (x = Main.rand(1, 10))
│ (y = Main.rand(5, 10))
│ (1:y)
│ Core.Const(:(@_2 = Base.iterate(%3)))
│ Core.Const(:(@_2 === nothing))
│ Core.Const(:(Base.not_int(%5)))
│ Core.Const(:(goto %17 if not %6))
│ Core.Const(:(@_2))
│ Core.Const(:(i = Core.getfield(%8, 1)))
│ Core.Const(:(Core.getfield(%8, 2)))
│ Core.Const(:(x = x + y))
│ Core.Const(:(@_2 = Base.iterate(%3, %10)))
│ Core.Const(:(@_2 === nothing))
│ Core.Const(:(Base.not_int(%13)))
│ Core.Const(:(goto %17 if not %14))
│ Core.Const(:(goto %8))
└── Core.Const(:(return x))

```

The original is dead code:

```julia
julia> @code_llvm test()
; @ REPL[8]:1 within `test`
define void @julia_test_1195() #0 {
top:
; @ REPL[8]:2 within `test`
  ret void
}

```

With live code, the small union optimization is applied, making the body of the loop type-stable.