# Type conversion for \`Tuple\` without \`memcpy\`

**URL:** <https://discourse.julialang.org/t/type-conversion-for-tuple-without-memcpy/5132>\
**Category:** Internals & Design\
**Tags:** question, performance, llvm, tuple\
**Created:** [July 30, 2017, 5:50am UTC](https://discourse.julialang.org/t/type-conversion-for-tuple-without-memcpy/5132 "2017-07-30T05:50:31Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [July 30, 2017, 5:50am UTC](https://discourse.julialang.org/t/type-conversion-for-tuple-without-memcpy/5132/1 "2017-07-30T05:50:31Z")

</div>

Consider the following:

```julia
t = (1, 2) # a Tuple{Int64, Int64}
@code_llvm identity(t)
@code_llvm Tuple{Int64, Int64}(t)
@code_llvm convert(Tuple{Int64, Int64}, t)

```

The LLVM for all of these is the same (apart from the definition name):

```julia
define void @jlsys_convert_61082([2 x i64] addrspace(11)* noalias nocapture sret, %jl_value_t addrspace(10)*, [2 x i64] addrspace(11)* nocapture readonly dereferenceable(16)) #0 !dbg !5 {
top:
  %3 = bitcast [2 x i64] addrspace(11)* %2 to i8 addrspace(11)*
  %4 = bitcast [2 x i64] addrspace(11)* %0 to i8 addrspace(11)*
  call void @llvm.memcpy.p11i8.p11i8.i32(i8 addrspace(11)* %4, i8 addrspace(11)* %3, i32 16, i32 1, i1 false)
  ret void
}

```

I would have expected this to be a no-op, as it is for `Float64`, `Array`, etc., but there’s an `llvm.memcpy`. Is this a performance bug, or is this copy necessary somehow? If the latter is the case (which would be weird to me, since `Tuple`s are immutable), is there a way to elide the copy in `convert` calls?

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [July 30, 2017, 9:00am UTC](https://discourse.julialang.org/t/type-conversion-for-tuple-without-memcpy/5132/2 "2017-07-30T09:00:36Z")

</div>

The native code (that actually is run) just does two moves. Also, if you use the tuple for something, these moves are elided:

```julia
julia> getfirst(t) = (a = identity(t); a[1])
getfirst (generic function with 1 method)

julia> @code_llvm getfirst((1,2))

define i64 @julia_getfirst_60706([2 x i64]* nocapture readonly dereferenceable(16)) #0 !dbg !5 {
top:
  %1 = getelementptr inbounds [2 x i64], [2 x i64]* %0, i64 0, i64 0
  %2 = load i64, i64* %1, align 8
  ret i64 %2

```

---

<div class="post-metadata">

**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [July 30, 2017, 2:03pm UTC](https://discourse.julialang.org/t/type-conversion-for-tuple-without-memcpy/5132/3 "2017-07-30T14:03:09Z")

</div>

> [@kristoffer.carlsson](#):
>
> Also, if you use the tuple for something, these moves are elided

I see, thank you. That explains these surprising benchmark results:

```julia
using StaticArrays, BenchmarkTools
@benchmark 1 * identity(x) setup = x = rand(SMatrix{3, 3})
@benchmark identity(x) setup = x = rand(SMatrix{3, 3})

```

which resulted in a time of 3.271 ns for the first and 9.493 ns for the second. So I guess it just doesn’t make sense to benchmark/analyze methods that are essentially `identity` by themselves (out of context).