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Prototype EmptyArray optimization for Dictionary #114792

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@kg kg commented Apr 17, 2025

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@kg kg added NO-MERGE The PR is not ready for merge yet (see discussion for detailed reasons) NO-REVIEW Experimental/testing PR, do NOT review it labels Apr 17, 2025
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kg commented Apr 17, 2025

We have a few tests that measure how many times GetHashCode or Equals get called on the comparer, which requires a check for a count of zero before doing a search. That might shed the performance benefits of this optimization...

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kg commented Apr 17, 2025

@EgorBot -intel -amd

using System.Collections.Generic;
using BenchmarkDotNet.Attributes;

public class Bench
{
    const int Count = 40960;
    private static readonly Dictionary<int, int> ints = new ();

    static Bench () {
        for (int i = 0; i < Count; i++)
            ints[i] = i;
    }

    [Benchmark]
    public bool FindPresent() {
        for (int i = 0; i < Count; i++)
            if (!ints.TryGetValue(i, out _))
                return false;

        return true;
    }

    [Benchmark]
    public bool FindMissing() {
        for (int i = 0; i < Count; i++)
            if (ints.TryGetValue(i + Count, out _))
                return false;

        return true;
    }
}

Remove the _count check to see if we can claw back the small perf win
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kg commented Apr 18, 2025

@EgorBot -intel -amd

using System;
using System.Collections.Generic;
using BenchmarkDotNet.Attributes;

public class Bench
{
    const int Count = 40960;
    private static readonly Dictionary<int, int> ints = new (Count * 2);
    private static readonly List<int> keys = new ();

    static Bench () {
        var rng = new Random(1);
        for (int i = 0; i < Count; i++) {
            var key = rng.Next();
            keys.Add(key);
            ints[keys[i]] = i;
        }
    }

    [Benchmark]
    public bool FindPresent() {
        for (int i = 0; i < Count; i++)
            if (!ints.TryGetValue(keys[i], out _))
                return false;

        return true;
    }
}

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kg commented Apr 18, 2025

Looks like this change doesn't make SCG faster.

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