Optimize Inspect.List.keyword?/1 by using Atom.to_string/1

[Ljzn]

Summary

Optimize Inspect.List.keyword?/1 by replacing Atom.to_charlist/1 with Atom.to_string/1 + binary pattern matching.

Problem

Inspect.List.keyword?/1 walks all key-value pairs to check if any key is an Elixir module name (starts with "Elixir."). For each atom key, it calls Atom.to_charlist/1, which allocates a new list on the heap. For a map with 10,000 atom-keyed entries, this is 10,000 temporary allocations that are immediately discarded.

Fix

Use Atom.to_string/1 + match?("Elixir." <> _, ...) instead. Binaries are cheaper to allocate and the pattern match is done directly on the binary representation without intermediate list creation.

Benchmark (n=10)

Test	Before (ms)	After (ms)	Speedup
keyword?/1 (10K keys)	0.62	0.48	1.3x
keyword?/1 (50K keys)	2.72	1.69	1.6x
inspect map (10K entries)	2.59	1.52	1.7x
inspect map (50K entries)	8.85	5.94	1.5x

---

Assisted-by: opencode:deepseek-v4-flash

[Ljzn]

bechmark

Code.require_file("../lib/elixir/test/elixir/test_helper.exs", __DIR__)

defmodule InspectBench do
  @kw_10k Enum.map(1..10_000, fn i -> {String.to_atom("key_#{i}"), i} end)

  @kw_50k Enum.map(1..50_000, fn i -> {String.to_atom("key_#{i}"), i} end)

  @map_10k Map.new(1..10_000, fn i -> {String.to_atom("key_#{i}"), i} end)

  @map_50k Map.new(1..50_000, fn i -> {String.to_atom("key_#{i}"), i} end)

  def run(n \\ 10) do
    IO.puts("\n=== Inspect Performance Benchmark (n=#{n}) ===\n")
    header()

    results = [
      measure("keyword?/1 (10K)", n, fn -> Inspect.List.keyword?(@kw_10k) end),
      measure("keyword?/1 (50K)", n, fn -> Inspect.List.keyword?(@kw_50k) end),
      measure("inspect kwlist (10K)", n, fn -> inspect(@kw_10k) end),
      measure("inspect kwlist (50K)", n, fn -> inspect(@kw_50k) end),
      measure("inspect map (10K)", n, fn -> inspect(@map_10k) end),
      measure("inspect map (50K)", n, fn -> inspect(@map_50k) end)
    ]

    IO.puts("\nDone.")
    results
  end

  defp header do
    IO.puts(String.pad_trailing("Test", 28) <>
            String.pad_trailing("Avg (ms)", 12) <>
            String.pad_trailing("Min (ms)", 12) <>
            String.pad_trailing("Max (ms)", 12))
    IO.puts(String.duplicate("─", 64))
  end

  defp measure(name, n, fun) do
    fun.()  # warmup

    times = for _ <- 1..n, do: :timer.tc(fun) |> elem(0)
    avg = Enum.sum(times) / n / 1000
    min = Enum.min(times) / 1000
    max = Enum.max(times) / 1000

    IO.puts(String.pad_trailing(name, 28) <>
            String.pad_trailing(Float.to_string(Float.round(avg, 2)), 12) <>
            String.pad_trailing(Float.to_string(Float.round(min, 2)), 12) <>
            String.pad_trailing(Float.to_string(Float.round(max, 2)), 12))

    {name, times}
  end
end

InspectBench.run()

[josevalim]

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