# Aliasing structs breaks type stability

**URL:** <https://discourse.julialang.org/t/aliasing-structs-breaks-type-stability/44206>\
**Category:** General Usage\
**Tags:** question, type, struct, type-stability\
**Created:** [August 3, 2020, 4:58pm UTC](https://discourse.julialang.org/t/aliasing-structs-breaks-type-stability/44206 "2020-08-03T16:58:03Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![louie4825](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/louie4825/32/27359_2.png) [@louie4825](https://discourse.julialang.org/u/louie4825)\
**Post date:** [August 3, 2020, 4:58pm UTC](https://discourse.julialang.org/t/aliasing-structs-breaks-type-stability/44206/1 "2020-08-03T16:58:03Z")

</div>

Hi. I’ve defined some parametric structs like those defined in the [Performance Tips](https://docs.julialang.org/en/latest/manual/performance-tips/index.html) 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?

```julia
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

```

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<div class="post-metadata">

**Author:** ![odow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/odow/32/28685_2.png) [@odow](https://discourse.julialang.org/u/odow)\
**Post date:** [August 3, 2020, 5:01pm UTC](https://discourse.julialang.org/t/aliasing-structs-breaks-type-stability/44206/2 "2020-08-03T17:01:26Z")

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> [@louie4825](#):
>
> `ShStruct = LongStructName`

You need to make the short name a `const`.

```Julia
const ShStruct = LongStructName

```

This is the first performance tip: [Performance Tips · The Julia Language](https://docs.julialang.org/en/v1/manual/performance-tips/index.html#Avoid-global-variables-1).

---

<div class="post-metadata">

**Author:** ![louie4825](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/louie4825/32/27359_2.png) [@louie4825](https://discourse.julialang.org/u/louie4825)\
**Post date:** [August 3, 2020, 5:02pm UTC](https://discourse.julialang.org/t/aliasing-structs-breaks-type-stability/44206/4 "2020-08-03T17:02:33Z")

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