Match compiler waiting entries directly

[preciz]

Assisted-by: Codex:GPT-6
Assisted-by: Antigravity:Gemini 3.8 Flash

Match compiler waiting entries directly in Kernel.ParallelCompiler.

│ Match the waiting list directly instead of popping its first element through
│ a generic helper. Preserve profile output while avoiding List.pop_at/2
│ dispatch and eliminating pop-result tuple allocations.

Bench:

# Run with: ./bin/elixir bench.exs
Mix.install([:benchee])

defmodule Bench do
  def via_pop_at(waiting) do
    case List.pop_at(waiting, 0) do
      {nil, []} -> :empty
      {{kind, on, time}, rest} -> {kind, on, time, rest}
    end
  end

  def via_match(waiting) do
    case waiting do
      [] -> :empty
      [{kind, on, time} | rest] -> {kind, on, time, rest}
    end
  end
end

inputs = %{
  "empty waiting list" => [],
  "1 waiting entry" => [{:module, Foo, 123}],
  "10 waiting entries" => for(i <- 1..10, do: {:module, Foo, i})
}

Benchee.run(
  %{
    "List.pop_at/2" => &Bench.via_pop_at/1,
    "pattern match" => &Bench.via_match/1
  },
  inputs: inputs,
  time: 2,
  warmup: 1,
  memory_time: 1
)

Results:

Operating System: Linux
CPU Information: AMD Ryzen 7 8845HS w
Number of Available Cores: 16
Available memory: 54.72 GB
Elixir 1.21.0-dev
Erlang 29.0.5
JIT enabled: true

##### With input empty waiting list #####
Name                    ips        average  deviation         median         99th %
pattern match       44.17 M       22.64 ns   ±642.31%          20 ns          31 ns
List.pop_at/2       20.95 M       47.74 ns ±11058.59%          30 ns          50 ns

Comparison: 
pattern match       44.17 M
List.pop_at/2       20.95 M - 2.11x slower +25.10 ns

Memory usage statistics:

Name             Memory usage
pattern match             0 B
List.pop_at/2            24 B - ∞ x memory usage +24 B

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

##### With input 1 waiting entry #####
Name                    ips        average  deviation         median         99th %
pattern match       33.06 M       30.25 ns   ±759.47%          30 ns          40 ns
List.pop_at/2       20.30 M       49.26 ns  ±8897.95%          40 ns          60 ns

Comparison: 
pattern match       33.06 M
List.pop_at/2       20.30 M - 1.63x slower +19.01 ns

Memory usage statistics:

Name             Memory usage
pattern match            40 B
List.pop_at/2            64 B - 1.60x memory usage +24 B

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

##### With input 10 waiting entries #####
Name                    ips        average  deviation         median         99th %
pattern match       32.02 M       31.23 ns  ±2635.49%          30 ns          41 ns
List.pop_at/2       23.18 M       43.14 ns  ±4781.26%          40 ns          60 ns

Comparison: 
pattern match       32.02 M
List.pop_at/2       23.18 M - 1.38x slower +11.91 ns

Memory usage statistics:

Name             Memory usage
pattern match            40 B
List.pop_at/2            64 B - 1.60x memory usage +24 B

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

[josevalim]

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