Comparison of direct generated functions vs. `unroll_tuple` as a utility function to unroll expressions

test.jl

# "bespoke" generated functions
Pkg.checkout("StaticArrays","master")
using StaticArrays
f(v) = @inbounds return v[v]
v = SVector{3,Int}((1,2,3))

# results:
julia> @code_native f(v)
	.text
Filename: REPL[3]
	pushq	%rbp
	movq	%rsp, %rbp
Source line: 25
	movq	(%rsi), %rax
	movq	8(%rsi), %rcx
	movq	-8(%rsi,%rax,8), %rax
	movq	-8(%rsi,%rcx,8), %rcx
	movq	16(%rsi), %rdx
	movq	-8(%rsi,%rdx,8), %rdx
Source line: 1
	movq	%rax, (%rdi)
	movq	%rcx, 8(%rdi)
	movq	%rdx, 16(%rdi)
	movq	%rdi, %rax
	popq	%rbp
	retq
	nop

julia> @code_warntype f(v)
Variables:
  #self#::#f
  v::StaticArrays.SVector{3,Int64}

Body:
  begin 
      $(Expr(:inbounds, true))
      # meta: location /home/ferris/.julia/v0.6/StaticArrays/src/indexing.jl getindex 20
      # meta: location /home/ferris/.julia/v0.6/StaticArrays/src/indexing.jl # line 25:
      SSAValue(0) = $(Expr(:new, :($(QuoteNode(StaticArrays.SVector{3,Int64}))), :((Core.tuple)((Base.getfield)((Core.getfield)(v, :data)::Tuple{Int64,Int64,Int64}, (Base.getfield)((Core.getfield)(v, :data)::Tuple{Int64,Int64,Int64}, 1)::Int64)::Int64, (Base.getfield)((Core.getfield)(v, :data)::Tuple{Int64,Int64,Int64}, (Base.getfield)((Core.getfield)(v, :data)::Tuple{Int64,Int64,Int64}, 2)::Int64)::Int64, (Base.getfield)((Core.getfield)(v, :data)::Tuple{Int64,Int64,Int64}, (Base.getfield)((Core.getfield)(v, :data)::Tuple{Int64,Int64,Int64}, 3)::Int64)::Int64)::Tuple{Int64,Int64,Int64})))
      # meta: pop location
      # meta: pop location
      return SSAValue(0)
      $(Expr(:inbounds, :pop))
  end::StaticArrays.SVector{3,Int64}



# "centralized" generated functions
Pkg.checkout("StaticArrays","julia-0.6")
using StaticArrays
f(v) = @inbounds return v[v]
v = SVector{3,Int}((1,2,3))

# results:
julia> @code_native f(v)
	.text
Filename: REPL[3]
	pushq	%rbp
	movq	%rsp, %rbp
Source line: 7
	movq	16(%rsi), %rax
	movq	%rax, -80(%rbp)
	vmovups	(%rsi), %xmm0
	vmovaps	%xmm0, -96(%rbp)
	movq	16(%rsi), %rax
	movq	%rax, -56(%rbp)
	vmovups	(%rsi), %xmm0
	vmovups	%xmm0, -72(%rbp)
	vmovups	-96(%rbp), %ymm0
	vmovups	-80(%rbp), %ymm1
	vmovups	%ymm1, -32(%rbp)
	vmovups	%ymm0, -48(%rbp)
Source line: 13
	movq	-24(%rbp), %rax
	movq	-16(%rbp), %rcx
	movq	-56(%rbp,%rax,8), %rax
	movq	-56(%rbp,%rcx,8), %rcx
	movq	-8(%rbp), %rdx
	movq	-56(%rbp,%rdx,8), %rdx
Source line: 1
	movq	%rax, (%rdi)
	movq	%rcx, 8(%rdi)
	movq	%rdx, 16(%rdi)
	movq	%rdi, %rax
	popq	%rbp
	vzeroupper
	retq
	nopl	(%rax,%rax)

julia> @code_warntype f(v)
Variables:
  #self#::#f
  v::StaticArrays.SVector{3,Int64}
  #151::StaticArrays.##151#152{StaticArrays.SVector{3,Int64},StaticArrays.SVector{3,Int64}}

Body:
  begin 
      $(Expr(:inbounds, true))
      # meta: location /home/ferris/.julia/v0.6/StaticArrays/src/indexing.jl getindex 7
      SSAValue(0) = $(QuoteNode(StaticArrays.SVector{3,Int64}))
      #151::StaticArrays.##151#152{StaticArrays.SVector{3,Int64},StaticArrays.SVector{3,Int64}} = $(Expr(:new, :($(QuoteNode(StaticArrays.##151#152{StaticArrays.SVector{3,Int64},StaticArrays.SVector{3,Int64}}))), :(v), :(v)))
      SSAValue(1) = #151::StaticArrays.##151#152{StaticArrays.SVector{3,Int64},StaticArrays.SVector{3,Int64}}
      SSAValue(2) = $(QuoteNode(Length(3)))
      # meta: location /home/ferris/.julia/v0.6/StaticArrays/src/traits.jl unroll_tuple 105
      # meta: location /home/ferris/.julia/v0.6/StaticArrays/src/generated.jl unroll_tuple 9
      # meta: location /home/ferris/.julia/v0.6/StaticArrays/src/generated.jl # line 13:
      SSAValue(4) = (Core.tuple)((Base.getfield)((Core.getfield)((Core.getfield)(SSAValue(1), :a)::StaticArrays.SVector{3,Int64}, :data)::Tuple{Int64,Int64,Int64}, (Base.getfield)((Core.getfield)((Core.getfield)(SSAValue(1), :inds)::StaticArrays.SVector{3,Int64}, :data)::Tuple{Int64,Int64,Int64}, 1)::Int64)::Int64, (Base.getfield)((Core.getfield)((Core.getfield)(SSAValue(1), :a)::StaticArrays.SVector{3,Int64}, :data)::Tuple{Int64,Int64,Int64}, (Base.getfield)((Core.getfield)((Core.getfield)(SSAValue(1), :inds)::StaticArrays.SVector{3,Int64}, :data)::Tuple{Int64,Int64,Int64}, 2)::Int64)::Int64, (Base.getfield)((Core.getfield)((Core.getfield)(SSAValue(1), :a)::StaticArrays.SVector{3,Int64}, :data)::Tuple{Int64,Int64,Int64}, (Base.getfield)((Core.getfield)((Core.getfield)(SSAValue(1), :inds)::StaticArrays.SVector{3,Int64}, :data)::Tuple{Int64,Int64,Int64}, 3)::Int64)::Int64)::Tuple{Int64,Int64,Int64}
      # meta: pop location
      # meta: pop location
      # meta: pop location
      SSAValue(3) = SSAValue(4)
      # meta: pop location
      return $(Expr(:new, :($(QuoteNode(StaticArrays.SVector{3,Int64}))), SSAValue(3)))
      $(Expr(:inbounds, :pop))
  end::StaticArrays.SVector{3,Int64}

For reference, this is the "thunk-based" system that does generate good code:

@propagate_inbounds function getindex(a::AbstractArray, inds::StaticVector{<:Integer})
    _getindex(Size(inds), a, inds)
end

@generated function _getindex(s::Size{S}, a::AbstractArray, inds::StaticVector{<:Integer}) where S
    @assert(S isa Tuple{Int})
    exprs = [:(a[inds[$i]]) for i = 1:S[1]]
    return quote
        @_propagate_inbounds_meta
        return similar_type(a, eltype(a), s)($(Expr(:tuple, exprs...)))
    end
end

This just takes what is on master and moves the Size computation to a thunk and moves similar_type to the generated body.