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Faster optimized frozen dictionary creation (6/6) (dotnet#88093)
* don't use custom ToArray for small frozen collections, up to 50% gain for creation time for collections with <= 4 items * for these types GetHashCode returns their value casted to int, so when we receive a Dictionary/HashSet where there are key we know that all hash codes are unique and we can avoid some work later 10-15% CPU time gain and 15-20% allocation reduction for FrozenDictionary and FrozenHashSet where TKey is uint, short, ushort, byte, sbyte * move Length Buckets code to a dedicated helper type to reduce code duplication and decrease code size * add tests for Frozen Dictionaries with key being uint, short, ushort, byte, sbyte, nint and nuint * fix discovered bug: IntPtr started implementing IComparable<IntPtr> in .NET 5
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102 changes: 102 additions & 0 deletions
102
...raries/System.Collections.Immutable/src/System/Collections/Frozen/String/LengthBuckets.cs
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,102 @@ | ||
| // Licensed to the .NET Foundation under one or more agreements. | ||
| // The .NET Foundation licenses this file to you under the MIT license. | ||
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| using System.Buffers; | ||
| using System.Collections.Generic; | ||
| using System.Diagnostics; | ||
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| namespace System.Collections.Frozen | ||
| { | ||
| internal static class LengthBuckets | ||
| { | ||
| /// <summary>The maximum number of items allowed per bucket. The larger the value, the longer it can take to search a bucket, which is sequentially examined.</summary> | ||
| internal const int MaxPerLength = 5; | ||
| /// <summary>Allowed ratio between buckets with values and total buckets. Under this ratio, this implementation won't be used due to too much wasted space.</summary> | ||
| private const double EmptyLengthsRatio = 0.2; | ||
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| internal static int[]? CreateLengthBucketsArrayIfAppropriate(string[] keys, IEqualityComparer<string> comparer, int minLength, int maxLength) | ||
| { | ||
| Debug.Assert(comparer == EqualityComparer<string>.Default || comparer == StringComparer.Ordinal || comparer == StringComparer.OrdinalIgnoreCase); | ||
| Debug.Assert(minLength >= 0 && maxLength >= minLength); | ||
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| // If without even looking at the keys we know that some bucket will exceed the max per-bucket | ||
| // limit (pigeon hole principle), we can early-exit out without doing any further work. | ||
| int spread = maxLength - minLength + 1; | ||
| if (keys.Length / spread > MaxPerLength) | ||
| { | ||
| return null; | ||
| } | ||
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| int arraySize = spread * MaxPerLength; | ||
| #if NET6_0_OR_GREATER | ||
| if (arraySize > Array.MaxLength) | ||
| #else | ||
| if (arraySize > 0X7FFFFFC7) | ||
| #endif | ||
| { | ||
| // In the future we may lower the value, as it may be quite unlikely | ||
| // to have a LOT of strings of different sizes. | ||
| return null; | ||
| } | ||
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| // Instead of creating a dictionary of lists or a multi-dimensional array | ||
| // we rent a single dimension array, where every bucket has five slots. | ||
| // The bucket starts at (key.Length - minLength) * 5. | ||
| // Each value is an index of the key from _keys array | ||
| // or just -1, which represents "null". | ||
| int[] buckets = ArrayPool<int>.Shared.Rent(arraySize); | ||
| buckets.AsSpan(0, arraySize).Fill(-1); | ||
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| int nonEmptyCount = 0; | ||
| for (int i = 0; i < keys.Length; i++) | ||
| { | ||
| string key = keys[i]; | ||
| int startIndex = (key.Length - minLength) * MaxPerLength; | ||
| int endIndex = startIndex + MaxPerLength; | ||
| int index = startIndex; | ||
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| while (index < endIndex) | ||
| { | ||
| ref int bucket = ref buckets[index]; | ||
| if (bucket < 0) | ||
| { | ||
| if (index == startIndex) | ||
| { | ||
| nonEmptyCount++; | ||
| } | ||
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| bucket = i; | ||
| break; | ||
| } | ||
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| index++; | ||
| } | ||
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| if (index == endIndex) | ||
| { | ||
| // If we've already hit the max per-bucket limit, bail. | ||
| ArrayPool<int>.Shared.Return(buckets); | ||
| return null; | ||
| } | ||
| } | ||
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| // If there would be too much empty space in the lookup array, bail. | ||
| if (nonEmptyCount / (double)spread < EmptyLengthsRatio) | ||
| { | ||
| ArrayPool<int>.Shared.Return(buckets); | ||
| return null; | ||
| } | ||
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| #if NET6_0_OR_GREATER | ||
| // We don't need an array with every value initialized to zero if we are just about to overwrite every value anyway. | ||
| int[] copy = GC.AllocateUninitializedArray<int>(arraySize); | ||
| Array.Copy(buckets, copy, arraySize); | ||
| #else | ||
| int[] copy = buckets.AsSpan(0, arraySize).ToArray(); | ||
| #endif | ||
| ArrayPool<int>.Shared.Return(buckets); | ||
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| return copy; | ||
| } | ||
| } | ||
| } |
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