[julia-users] Question about type instability in simple function

[Kristoffer Carlsson]

I had a type instability in my program and I condensed it down to this 
minimal working example.

Why is sqrt2 here unstable? The variable a should be inferred to always be 
a Float64 and then the function should be stable just like the normal sqrt 
function is?


julia> function sqrt2(a::Float64)
       return sqrt(a)
       end
sqrt2 (generic function with 1 method)



julia> @code_warntype sqrt2(5.0)
Variables:
  a::Float64


Body:
  begin  # none, line 2:
      return (GetfieldNode(Base.Math,:box,Any))(Float64,(GetfieldNode(Base.
Math,:sqrt_llvm,Any))(a::Float64)::Any)::Float64
  end::Float64


julia> @code_warntype sqrt(5.0)
Variables:
  x::Float64


Body:
  begin  # math.jl, line 130:
      return box(Float64,sqrt_llvm(x::Float64))
  end::Float64



Best regards,
Kristoffer Carlsson