Macro.classify_atom - binary fast path by dkuku · Pull Request #15768 · elixir-lang/elixir

dkuku · 2026-08-18T21:42:54Z

Add a function to check if an atom is an identifier - it is very common but it was last fallback:
:identifier - the atom can be used as a variable or local function call (as well as be an unquoted atom)
Convert alias check to binary match as the binary name is already available.
Additional regression tests.

ids = for i <- 1..500, do: String.to_atom("field_name_#{i}")
short_ids = for i <- 1..500, do: String.to_atom("k#{rem(i, 26) + 97}")
bangs = for i <- 1..500, do: String.to_atom("valid_#{i}?")
aliases = for i <- 1..500, do: String.to_atom("Elixir.My.App.Schema#{i}.Nested")
quoted = for i <- 1..500, do: String.to_atom("has spaces #{i}")
unicode = for i <- 1..500, do: String.to_atom("héllo_wörld_#{i}")
upcased = for i <- 1..500, do: String.to_atom("NotAnElixirAlias#{i}")

inputs = %{
  "500 identifiers (long)" => ids,
  "500 identifiers (short)" => short_ids,
  "500 identifiers with ?" => bangs,
  "500 aliases" => aliases,
  "500 quoted atoms (slow path)" => quoted,
  "500 unicode atoms (slow path)" => unicode,
  "500 uppercase non-aliases (slow path)" => upcased
}


##### With input 500 aliases #####
Name                                  ips        average  deviation         median         99th %
inspect_atom(:key, _) (new)       19.24 K       51.98 μs    ±11.91%       51.57 μs       57.00 μs
inspect_atom(:key, _) (old)       10.28 K       97.31 μs    ±14.45%       96.14 μs      106.65 μs

Comparison:
inspect_atom(:key, _) (new)       19.24 K
inspect_atom(:key, _) (old)       10.28 K - 1.87x slower +45.33 μs

Memory usage statistics:

Name                           Memory usage
inspect_atom(:key, _) (new)        61.68 KB
inspect_atom(:key, _) (old)       258.95 KB - 4.20x memory usage +197.27 KB

**All measurements for memory usage were the same**

##### With input 500 identifiers (long) #####
Name                                  ips        average  deviation         median         99th %
inspect_atom(:key, _) (new)       27.77 K       36.01 μs    ±12.01%       35.70 μs       40.30 μs
inspect_atom(:key, _) (old)       11.01 K       90.79 μs    ±19.91%       88.22 μs      135.49 μs

Comparison:
inspect_atom(:key, _) (new)       27.77 K
inspect_atom(:key, _) (old)       11.01 K - 2.52x slower +54.78 μs

Memory usage statistics:

Name                           Memory usage
inspect_atom(:key, _) (new)        42.94 KB
inspect_atom(:key, _) (old)       399.78 KB - 9.31x memory usage +356.84 KB

**All measurements for memory usage were the same**

##### With input 500 identifiers (short) #####
Name                                  ips        average  deviation         median         99th %
inspect_atom(:key, _) (new)       35.85 K       27.90 μs    ±14.04%       27.60 μs       32.29 μs
inspect_atom(:key, _) (old)       16.17 K       61.84 μs    ±21.10%       60.11 μs      100.18 μs

Comparison:
inspect_atom(:key, _) (new)       35.85 K
inspect_atom(:key, _) (old)       16.17 K - 2.22x slower +33.95 μs

Memory usage statistics:

Name                           Memory usage
inspect_atom(:key, _) (new)        39.04 KB
inspect_atom(:key, _) (old)       165.14 KB - 4.23x memory usage +126.10 KB

**All measurements for memory usage were the same**

##### With input 500 identifiers with ? #####
Name                                  ips        average  deviation         median         99th %
inspect_atom(:key, _) (new)       30.14 K       33.18 μs    ±13.97%       32.82 μs       37.56 μs
inspect_atom(:key, _) (old)       12.91 K       77.44 μs    ±15.39%       76.26 μs       88.02 μs

Comparison:
inspect_atom(:key, _) (new)       30.14 K
inspect_atom(:key, _) (old)       12.91 K - 2.33x slower +44.26 μs

Memory usage statistics:

Name                           Memory usage
inspect_atom(:key, _) (new)        42.94 KB
inspect_atom(:key, _) (old)       314.38 KB - 7.32x memory usage +271.44 KB

**All measurements for memory usage were the same**

##### With input 500 quoted atoms (slow path) #####
Name                                  ips        average  deviation         median         99th %
inspect_atom(:key, _) (old)        4.65 K      214.92 μs    ±11.11%      212.50 μs      265.73 μs
inspect_atom(:key, _) (new)        4.52 K      221.11 μs     ±8.53%      219.96 μs      233.50 μs

Comparison:
inspect_atom(:key, _) (old)        4.65 K
inspect_atom(:key, _) (new)        4.52 K - 1.03x slower +6.18 μs

Memory usage statistics:

Name                           Memory usage
inspect_atom(:key, _) (old)       306.08 KB
inspect_atom(:key, _) (new)       324.84 KB - 1.06x memory usage +18.76 KB

**All measurements for memory usage were the same**

##### With input 500 unicode atoms (slow path) #####
Name                                  ips        average  deviation         median         99th %
inspect_atom(:key, _) (old)        5.67 K      176.37 μs    ±11.58%      174.73 μs      211.31 μs
inspect_atom(:key, _) (new)        5.52 K      181.06 μs    ±10.66%      179.50 μs      197.49 μs

Comparison:
inspect_atom(:key, _) (old)        5.67 K
inspect_atom(:key, _) (new)        5.52 K - 1.03x slower +4.69 μs

Memory usage statistics:

Name                           Memory usage
inspect_atom(:key, _) (old)       657.63 KB
inspect_atom(:key, _) (new)       668.48 KB - 1.02x memory usage +10.84 KB

**All measurements for memory usage were the same**

##### With input 500 uppercase non-aliases (slow path) #####
Name                                  ips        average  deviation         median         99th %
inspect_atom(:key, _) (old)       10.35 K       96.64 μs    ±19.16%       94.00 μs      168.90 μs
inspect_atom(:key, _) (new)        9.78 K      102.23 μs    ±12.61%      101.06 μs      119.90 μs

Comparison:
inspect_atom(:key, _) (old)       10.35 K
inspect_atom(:key, _) (new)        9.78 K - 1.06x slower +5.59 μs

Memory usage statistics:

Name                           Memory usage
inspect_atom(:key, _) (old)       520.28 KB
inspect_atom(:key, _) (new)       540.47 KB - 1.04x memory usage +20.19 KB

Assisted-by: Claude Opus 5

Atoms made exclusively of ASCII letters, numbers and underscores, beginning with a lowercase letter or underscore and optionally ending with ? or !, are always identifiers, and only atoms prefixed by "Elixir" are candidates for aliases. Recognizing both directly on the binary avoids building a charlist and running the unicode aware identifier tokenizer, which dominates the cost of classifying keyword list, map and struct keys when inspecting them. Both are matched at once because the first byte tells them apart and matching them separately would set up and throw away a second match context. For the same reason the alias scan branches between two states instead of returning the rest of the binary from a helper, which would build a sub binary per piece. The special form and quoted operator atoms move into guards so that no binary is built for atoms that are not callable. Inspecting a keyword list of 8 pairs goes from 10.4us to 8.9us (1.17x), a list of 8 module atoms from 4.0us to 3.5us (1.15x). Atoms that still fall through to the tokenizer pay ~5-10% for the extra scan. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

The classification of ASCII identifiers and aliases is boundary heavy: only one trailing ? or !, only at the end, and the "Elixir" prefix is an alias only when every following piece starts with an uppercase letter. None of it was covered, so add the cases that separate identifiers and aliases from atoms that merely look like them. These pass before and after the binary classification fast path, pinning it to the tokenizer's verdict. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

josevalim · 2026-08-19T09:57:10Z

-    cond do
-      atom in [:%, :%{}, :{}, :<<>>, :..., :.., :., :..//, :->] ->
-        :not_callable
+  defp inner_classify(atom)


Can you please keep it as a cond, as that change is unrelated to this pull request? Change only the code that affects the benchmark.

josevalim · 2026-08-19T09:57:42Z

    end
  end

+  describe "classify_atom/1" do


Those test cases seem excessive and I believe we already test this functionality by proxy through other places.

dkuku and others added 3 commits August 18, 2026 22:59

add edge cases to tests

8451b76

dkuku changed the title ~~Dk classify atom binary fast path~~ Macro.classify_atom - binary fast path Aug 19, 2026

josevalim reviewed Aug 19, 2026

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Macro.classify_atom - binary fast path - #15768

Macro.classify_atom - binary fast path#15768
dkuku wants to merge 3 commits into
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dkuku:dk_classify_atom_binary_fast_path

dkuku commented Aug 18, 2026 •

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dkuku commented Aug 18, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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dkuku commented Aug 18, 2026 •

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