Strange speed gain from eachindex and writing in a useless array?

General Usage Performance
11

Thanks everyone for your help. Seeing how most of you can’t reproduce this behavior, I thought of another experiment: installing Julia directly on Windows instead of WSL (on the same machine). The code runs a bit more slowly and, there seems to be a slight improvement from my_useless_func, but nowhere near as dramatic as before.

julia> @benchmark my_func!($useless, $x)
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  134.300 μs … 784.400 μs  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     145.200 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   147.662 μs ±  14.218 μs  ┊ GC (mean ± σ):  0.00% ± 0.00%

  █ ▁▂  █▅ ▁█ ▁▁▇ ▁▁█ ▁▂▆  ▁▇  ▁▆   ▁▅    ▅    ▁    ▁           ▃
  █▅███▇██▇██▇███████▇███▇███▇████▇▇██▆▆▇▇█▅▇▇▆█▄▂▆▅█▄▆▆▆▅▇▄▃▄▄ █
  134 μs        Histogram: log(frequency) by time        181 μs <

 Memory estimate: 0 bytes, allocs estimate: 0.

julia> @benchmark my_useless_func!($useless, $x)
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  125.300 μs … 452.600 μs  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     135.500 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   139.672 μs ±  22.715 μs  ┊ GC (mean ± σ):  0.00% ± 0.00%

    ▅█▃▄▄
  ▆▃█████▄▅▄▃▂▂▃▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▁▂▂▂▂▂▂▂▂▂▂▁▂▂▂▁▂▂▂▂▁▂▂▁▂ ▃
  125 μs           Histogram: frequency by time          269 μs <

 Memory estimate: 0 bytes, allocs estimate: 0.

julia> versioninfo()
Julia Version 1.11.7
Commit f2b3dbda30 (2025-09-08 12:10 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: 32 × 13th Gen Intel(R) Core(TM) i9-13900HX
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, alderlake)
Threads: 1 default, 0 interactive, 1 GC (on 32 virtual cores)

julia>

This strange outcome is highly platform specific.