Constant-propagation / inference

[mcabbott]

The example of JuliaLang/julia#33435:

julia> a = rand(2, 2)
2×2 Array{Float64,2}:
 0.159947  0.956223
 0.712618  0.864652

julia> @code_warntype (x -> PermutedDimsArray(x, (2, 1)))(a)
Variables
  #self#::Core.Compiler.Const(var"##28#29"(), false)
  x::Array{Float64,2}

Body::PermutedDimsArray{Float64,2,_A,_B,Array{Float64,2}} where _B where _A
1 ─ %1 = Core.tuple(2, 1)::Core.Compiler.Const((2, 1), false)
│   %2 = Main.PermutedDimsArray(x, %1)::PermutedDimsArray{Float64,2,_A,_B,Array{Float64,2}} where _B where _A
└──      return %2

... works cleanly here:

julia> using TransmuteDims

julia> @code_warntype (x -> transmute(x, (2, 1)))(a)
Variables
  #self#::Core.Const(var"#17#18"())
  x::Matrix{Float64}

Body::LinearAlgebra.Transpose{Float64, Matrix{Float64}}
1 ─ %1 = Core.tuple(2, 1)::Core.Const((2, 1))
│   %2 = Main.transmute(x, %1)::LinearAlgebra.Transpose{Float64, Matrix{Float64}}
└──      return %2

... but makes a Transpose. Ones that make a TransmutedDimsArray aren't quite so clean:

julia> b = rand(2,2,2);

julia> @code_warntype (x -> transmute(x, (2, 3, 0, 1)))(b)
Variables
  #self#::Core.Const(var"#27#28"())
  x::Array{Float64, 3}

Body::Union{Array{Float64, 4}, TransmutedDimsArray{Float64, 4, (2, 3, 0, 1), (4, 1, 2), Array{Float64, 3}}}
1 ─ %1 = Core.tuple(2, 3, 0, 1)::Core.Const((2, 3, 0, 1))
│   %2 = Main.transmute(x, %1)::Union{Array{Float64, 4}, TransmutedDimsArray{Float64, 4, (2, 3, 0, 1), (4, 1, 2), Array{Float64, 3}}}
└──      return %2

julia> @code_warntype (x -> transmute(x, Val((2, 3, 0, 1))))(b)
MethodInstance for (::var"#29#30")(::Array{Float64, 3})
  from (::var"#29#30")(x) in Main at REPL[200]:1
Arguments
  #self#::Core.Const(var"#29#30"())
  x::Array{Float64, 3}
Body::TransmutedDimsArray{Float64, 4, (2, 3, 0, 1), (4, 1, 2), Array{Float64, 3}}
1 ─ %1 = Core.tuple(2, 3, 0, 1)::Core.Const((2, 3, 0, 1))
│   %2 = Main.Val(%1)::Core.Const(Val{(2, 3, 0, 1)}())
│   %3 = Main.transmute(x, %2)::TransmutedDimsArray{Float64, 4, (2, 3, 0, 1), (4, 1, 2), Array{Float64, 3}}
└──      return %3

Although still quite fast:

julia> @btime (x -> PermutedDimsArray(x, (2, 1)))($a);
  315.051 ns (4 allocations: 176 bytes)

julia> @btime (x -> transmute(x, (2, 1)))($a);
  1.500 ns (0 allocations: 0 bytes)

julia> @btime (x -> transmute(x, (2, 3, 0, 1)))($b);
  11.845 ns (1 allocation: 16 bytes)

julia> @btime (x -> transmute(x, Val((2, 3, 0, 1))))($b);
  0.875 ns (0 allocations: 0 bytes)