Aliasing structs breaks type stability

General Usage
, , ,
1

Hi. I’ve defined some parametric structs like those defined in the Performance Tips section of the Julia manual. Since some of the structs had quite long names, I decided to alias them.

I was struggling to wrap my head around why @btime was so slow and why @code_warntype was giving me errors when I had the thought to try using the longer names for the structs. Lo and behold, type-stability was achieved.

Is this correct Julia behavior, and if so, is it documented anywhere? Or have I just not yet read the manual enough?

struct LongStructName{T<:Integer}
	i::T
end

ShStruct = LongStructName

# Check that they are totally equal
julia> ShStruct === LongStructName
true

# Use constructors in separate function
function create_objects(a::Integer, b::Integer)
	return LongStructName(a), LongStructName(b)
end
	
function create_objects_alias(a::Integer, b::Integer)
	return ShStruct(a), ShStruct(b)
end

julia> @code_warntype create_objects(1, 2)
Variables
  #self#::Core.Compiler.Const(create_objects, false)
  a::Int64
  b::Int64

Body::Tuple{LongStructName{Int64},LongStructName{Int64}}
1 ─ %1 = Main.LongStructName(a)::LongStructName{Int64}
│   %2 = Main.LongStructName(b)::LongStructName{Int64}
│   %3 = Core.tuple(%1, %2)::Tuple{LongStructName{Int64},LongStructName{Int64}}
└──      return %3


# Aliased name breaks type-stability
julia> @code_warntype create_objects_alias(1, 2)
Variables
  #self#::Core.Compiler.Const(create_objects_alias, false)
  a::Int64
  b::Int64

Body::Tuple{Any,Any}
1 ─ %1 = Main.ShStruct(a)::Any
│   %2 = Main.ShStruct(b)::Any
│   %3 = Core.tuple(%1, %2)::Tuple{Any,Any}
└──      return %

# Raw constructors do not give errors
julia> @code_warntype ShStruct(1)
Variables
  #self#::Type{LongStructName}
  i::Int64

Body::LongStructName{Int64}
1 ─ %1 = Core.apply_type(Main.LongStructName, $(Expr(:static_parameter, 1)))::Core.Compiler.Const(LongStructName{Int64}, false)
│   %2 = (%1)(i)::LongStructName{Int64}
└──      return %2

julia> @code_warntype ShStruct(1)
Variables
  #self#::Type{LongStructName}
  i::Int64

Body::LongStructName{Int64}
1 ─ %1 = Core.apply_type(Main.LongStructName, $(Expr(:static_parameter, 1)))::Core.Compiler.Const(LongStructName{Int64}, false)
│   %2 = (%1)(i)::LongStructName{Int64}
└──      return %2
2

You need to make the short name a const.

const ShStruct = LongStructName

This is the first performance tip: Performance Tips · The Julia Language.

5 Likes
4

Oh. Oops. Never connected “structs” to “global variables” before. Thanks for the info!

2 Likes