Views are unperformant

[penelopeysm]

Operations on SubArrays scale quite poorly with size (or to be precise, they scale linearly with size which is quite normal, but given that getindex is almost constant-time, one might hope that views could also be optimised?):

using ReverseDiff, Chairmarks
function f_getindex(x)
    b = x[2:end]
    return sum(b)
end
function f_view(x)
    b = @view x[2:end]
    return sum(b)
end
for d in [10, 20, 50, 100]
    x = randn(d)
    tape_gi = ReverseDiff.GradientTape(f_getindex, x)
    tape_v  = ReverseDiff.GradientTape(f_view, x)
    result = similar(x)
    println("d = $d:")
    print("  getindex: "); display(@b ReverseDiff.gradient!($result, $tape_gi, $x))
    print("  view:     "); display(@b ReverseDiff.gradient!($result, $tape_v, $x))
end

d = 10:
  getindex: 146.648 ns
  view:     284.160 ns
d = 20:
  getindex: 151.278 ns
  view:     512.643 ns
d = 50:
  getindex: 153.263 ns
  view:     1.188 μs
d = 100:
  getindex: 182.759 ns
  view:     2.306 μs

(ppl) pkg> st
Status `~/ppl/Project.toml`
  [0ca39b1e] Chairmarks v1.3.1
  [37e2e3b7] ReverseDiff v1.16.2

julia> versioninfo()
Julia Version 1.11.9
Commit 53a02c0720c (2026-02-06 00:27 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin24.0.0)
  CPU: 10 × Apple M1 Pro
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, apple-m1)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)