Temporary variable inferred as Union{Bool, Int64} when calling rand()

[pkofod]

Witnesses this when trying to hunt down another inference related issue. Return type is inferred, but what is the #temp#@_2::union{Bool, Int64} about? Does not happen for rand(1) (see bottom). On v0.5.0 release.

julia> foo() = rand()
foo (generic function with 1 method)

julia> @code_warntype foo()
Variables:
  #self#::#foo
  #temp#@_2::Union{Bool,Int64}
  #temp#@_3::Float64

Body:
  begin 
      $(Expr(:inbounds, false))
      # meta: location random.jl rand 227
      $(Expr(:inbounds, false))
      # meta: location random.jl rand 122
      $(Expr(:inbounds, false))
      # meta: location random.jl reserve_1 111
      unless ((Core.getfield)(Base.Random.GLOBAL_RNG,:idx)::Int64 === (Base.box)(Int64,(Base.sext_int)(Int64,Base.Random.MTCacheLength)))::Bool goto 19
      # meta: location random.jl gen_rand 107
      SSAValue(1) = (Core.getfield)(Base.Random.GLOBAL_RNG,:state)::Base.dSFMT.DSFMT_state
      SSAValue(0) = (Core.getfield)(Base.Random.GLOBAL_RNG,:vals)::Array{Float64,1}
      $(Expr(:invoke, LambdaInfo for dsfmt_fill_array_close1_open2!(::Base.dSFMT.DSFMT_state, ::Ptr{Float64}, ::Int64), :(Base.Random.dsfmt_fill_array_close1_open2!), SSAValue(1), :((Core.ccall)(:jl_array_ptr,(Core.apply_type)(Base.Ptr,Float64)::Type{Ptr{Float64}},(Core.svec)(Base.Any)::SimpleVector,SSAValue(0),0)::Ptr{Float64}), :((Base.arraylen)((Core.getfield)(Base.Random.GLOBAL_RNG,:vals)::Array{Float64,1})::Int64))) # line 108:
      # meta: location random.jl mt_setfull! 102
      (Core.setfield!)(Base.Random.GLOBAL_RNG,:idx,0)::Int64
      # meta: pop location
      # meta: pop location
      #temp#@_2::Union{Bool,Int64} = 0
      goto 21
      19: 
      #temp#@_2::Union{Bool,Int64} = false
      21: 
      # meta: pop location
      $(Expr(:inbounds, :pop))
      #temp#@_2::Union{Bool,Int64}
      $(Expr(:inbounds, false))
      # meta: location random.jl rand_inbounds 118
      $(Expr(:inbounds, false))
      # meta: location random.jl rand_inbounds 117
      $(Expr(:inbounds, false))
      # meta: location random.jl mt_pop! 104
      $(Expr(:inbounds, true))
      SSAValue(2) = (Core.getfield)(Base.Random.GLOBAL_RNG,:vals)::Array{Float64,1}
      SSAValue(3) = (Base.box)(Int64,(Base.add_int)((Core.getfield)(Base.Random.GLOBAL_RNG,:idx)::Int64,1))
      (Core.setfield!)(Base.Random.GLOBAL_RNG,:idx,SSAValue(3))::Int64
      #temp#@_3::Float64 = (Base.arrayref)(SSAValue(2),SSAValue(3))::Float64
      goto 38
      $(Expr(:inbounds, :pop))
      38: 
      # meta: pop location
      $(Expr(:inbounds, :pop))
      # meta: pop location
      $(Expr(:inbounds, :pop))
      # meta: pop location
      $(Expr(:inbounds, :pop))
      # meta: pop location
      $(Expr(:inbounds, :pop))
      # meta: pop location
      $(Expr(:inbounds, :pop))
      return (Base.box)(Base.Float64,(Base.sub_float)(#temp#@_3::Float64,1.0))
  end::Float64

julia> workspace()
julia> foo() = rand(1)
foo (generic function with 1 method)

julia> @code_warntype foo()
Variables:
  #self#::#foo

Body:
  begin 
      # meta: location random.jl rand 230
      # meta: location random.jl rand 229
      # meta: location random.jl rand 297
      # meta: location random.jl rand 300
      SSAValue(0) = (Core.ccall)(:jl_new_array,(Core.apply_type)(Core.Array,Float64,1)::Type{Array{Float64,1}},(Core.svec)(Core.Any,Core.Any)::SimpleVector,Array{Float64,1},0,(Base.tuple)(1)::Tuple{Int64},0)::Array{Float64,1}
      # meta: pop location
      # meta: pop location
      # meta: pop location
      # meta: pop location
      return $(Expr(:invoke, LambdaInfo for rand!(::MersenneTwister, ::Array{Float64,1}, ::Int64, ::Type{Base.Random.CloseOpen}), :(Base.Random.rand!), :(Base.Random.GLOBAL_RNG), SSAValue(0), :((Base.arraylen)(SSAValue(0))::Int64), :(Base.Random.CloseOpen)))
  end::Array{Float64,1}

[yuyichao]

It's in rand(1) too, just not inlined. I thought we already had an issue / pr for it but couldn't find it. This should not affect performance.

[pkofod]

This should not affect performance.

sure, I just found it odd

[GregPlowman]

I too have found that this pollutes @code_warntype output with red.
On v0.5.0, I think the offending code is in random.jl (lines 111-114)

@inline reserve_1(r::MersenneTwister) = mt_empty(r) && gen_rand(r)
# `reserve` allows one to call `rand_inbounds` n times
# precondition: n <= MTCacheLength
@inline reserve(r::MersenneTwister, n::Int) = mt_avail(r) < n && gen_rand(r)

gen_rand(r) always returns 0, so reserve() returns Union{Bool,Int}

Overwriting these seemed to fix the inference for me:

@inline reserve_1(r::MersenneTwister) = mt_empty(r) ? gen_rand(r) : 0
@inline reserve(r::MersenneTwister, n::Int) = (mt_avail(r) < n) ? gen_rand(r) : 0