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Avoid mod operator when fast alternative available #27299

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Oct 26, 2019
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Avoid mod operator when fast alternative available #27299

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3bdf194
Avoid mod operator when fast alternative available
benaadams Oct 19, 2019
79e2529
Use fastmod
benaadams Oct 23, 2019
bc1818d
x64 only
benaadams Oct 24, 2019
3c19899
Feedback
benaadams Oct 25, 2019
e40200a
Feedback 2
benaadams Oct 25, 2019
a55391d
Feedback 3
benaadams Oct 25, 2019
8698984
Remove unsafe
benaadams Oct 25, 2019
9c9c7f6
nits
benaadams Oct 26, 2019
7a008ef
Guard against corruption from OOM
benaadams Oct 26, 2019
9db1979
Feedback
benaadams Oct 26, 2019
c6c9001
nit
benaadams Oct 26, 2019
06aacfd
Apply suggestions from code review
benaadams Oct 26, 2019
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17 changes: 17 additions & 0 deletions THIRD-PARTY-NOTICES.TXT
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Expand Up @@ -341,3 +341,20 @@ License notice for Xorshift (Wikipedia)

https://en.wikipedia.org/wiki/Xorshift
License: https://en.wikipedia.org/wiki/Wikipedia:Text_of_Creative_Commons_Attribution-ShareAlike_3.0_Unported_License

License for fastmod (https://github.com/lemire/fastmod)
--------------------------------------

Copyright 2018 Daniel Lemire

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
84 changes: 52 additions & 32 deletions src/System.Private.CoreLib/shared/System/Collections/Generic/Dictionary.cs
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Expand Up @@ -51,6 +51,9 @@ private struct Entry

private int[]? _buckets;
private Entry[]? _entries;
#if BIT64
private ulong _fastModMultiplier;
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#endif
private int _count;
private int _freeList;
private int _freeCount;
Expand Down Expand Up @@ -330,16 +333,15 @@ private ref TValue FindValue(TKey key)
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
}

int[]? buckets = _buckets;
ref Entry entry = ref Unsafe.NullRef<Entry>();
if (buckets != null)
if (_buckets != null)
{
Debug.Assert(_entries != null, "expected entries to be != null");
IEqualityComparer<TKey>? comparer = _comparer;
if (comparer == null)
{
uint hashCode = (uint)key.GetHashCode();
int i = buckets[hashCode % (uint)buckets.Length];
int i = GetBucket(hashCode);
Entry[]? entries = _entries;
uint collisionCount = 0;
if (default(TKey)! != null) // TODO-NULLABLE: default(T) == null warning (https://github.com/dotnet/roslyn/issues/34757)
Expand Down Expand Up @@ -407,7 +409,7 @@ private ref TValue FindValue(TKey key)
else
{
uint hashCode = (uint)comparer.GetHashCode(key);
int i = buckets[hashCode % (uint)buckets.Length];
int i = GetBucket(hashCode);
Entry[]? entries = _entries;
uint collisionCount = 0;
// Value in _buckets is 1-based; subtract 1 from i. We do it here so it fuses with the following conditional.
Expand Down Expand Up @@ -453,10 +455,16 @@ private ref TValue FindValue(TKey key)
private int Initialize(int capacity)
{
int size = HashHelpers.GetPrime(capacity);
int[] buckets = new int[size];
Entry[] entries = new Entry[size];

// Assign member varables after both arrays allocated to guard against corruption from OOM if second fails
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_freeList = -1;
_buckets = new int[size];
_entries = new Entry[size];
#if BIT64
_fastModMultiplier = HashHelpers.GetFastModMultiplier((uint)size);
#endif
_buckets = buckets;
_entries = entries;

return size;
}
Expand All @@ -481,7 +489,7 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
uint hashCode = (uint)((comparer == null) ? key.GetHashCode() : comparer.GetHashCode(key));

uint collisionCount = 0;
ref int bucket = ref _buckets[hashCode % (uint)_buckets.Length];
ref int bucket = ref GetBucket(hashCode);
// Value in _buckets is 1-based
int i = bucket - 1;

Expand Down Expand Up @@ -625,7 +633,7 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
if (count == entries.Length)
{
Resize();
bucket = ref _buckets[hashCode % (uint)_buckets.Length];
bucket = ref GetBucket(hashCode);
}
index = count;
_count = count + 1;
Expand Down Expand Up @@ -716,7 +724,6 @@ private void Resize(int newSize, bool forceNewHashCodes)
Debug.Assert(_entries != null, "_entries should be non-null");
Debug.Assert(newSize >= _entries.Length);

int[] buckets = new int[newSize];
Entry[] entries = new Entry[newSize];

int count = _count;
Expand All @@ -734,19 +741,23 @@ private void Resize(int newSize, bool forceNewHashCodes)
}
}

// Assign member varables after both arrays allocated to guard against corruption from OOM if second fails
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_buckets = new int[newSize];
#if BIT64
_fastModMultiplier = HashHelpers.GetFastModMultiplier((uint)newSize);
#endif
for (int i = 0; i < count; i++)
{
if (entries[i].next >= -1)
{
uint bucket = entries[i].hashCode % (uint)newSize;
ref int bucket = ref GetBucket(entries[i].hashCode);
// Value in _buckets is 1-based
entries[i].next = buckets[bucket] - 1;
entries[i].next = bucket - 1;
// Value in _buckets is 1-based
buckets[bucket] = i + 1;
bucket = i + 1;
}
}

_buckets = buckets;
_entries = entries;
}

Expand All @@ -760,17 +771,16 @@ public bool Remove(TKey key)
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
}

int[]? buckets = _buckets;
Entry[]? entries = _entries;
if (buckets != null)
if (_buckets != null)
{
Debug.Assert(entries != null, "entries should be non-null");
Debug.Assert(_entries != null, "entries should be non-null");
uint collisionCount = 0;
uint hashCode = (uint)(_comparer?.GetHashCode(key) ?? key.GetHashCode());
uint bucket = hashCode % (uint)buckets.Length;
ref int bucket = ref GetBucket(hashCode);
Entry[]? entries = _entries;
int last = -1;
// Value in buckets is 1-based
int i = buckets[bucket] - 1;
int i = bucket - 1;
while (i >= 0)
{
ref Entry entry = ref entries[i];
Expand All @@ -780,7 +790,7 @@ public bool Remove(TKey key)
if (last < 0)
{
// Value in buckets is 1-based
buckets[bucket] = entry.next + 1;
bucket = entry.next + 1;
}
else
{
Expand Down Expand Up @@ -829,17 +839,16 @@ public bool Remove(TKey key, [MaybeNullWhen(false)] out TValue value)
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
}

int[]? buckets = _buckets;
Entry[]? entries = _entries;
if (buckets != null)
if (_buckets != null)
{
Debug.Assert(entries != null, "entries should be non-null");
Debug.Assert(_entries != null, "entries should be non-null");
uint collisionCount = 0;
uint hashCode = (uint)(_comparer?.GetHashCode(key) ?? key.GetHashCode());
uint bucket = hashCode % (uint)buckets.Length;
ref int bucket = ref GetBucket(hashCode);
Entry[]? entries = _entries;
int last = -1;
// Value in buckets is 1-based
int i = buckets[bucket] - 1;
int i = bucket - 1;
while (i >= 0)
{
ref Entry entry = ref entries[i];
Expand All @@ -849,7 +858,7 @@ public bool Remove(TKey key, [MaybeNullWhen(false)] out TValue value)
if (last < 0)
{
// Value in buckets is 1-based
buckets[bucket] = entry.next + 1;
bucket = entry.next + 1;
}
else
{
Expand Down Expand Up @@ -982,6 +991,7 @@ public int EnsureCapacity(int capacity)
_version++;
if (_buckets == null)
return Initialize(capacity);

int newSize = HashHelpers.GetPrime(capacity);
Resize(newSize, forceNewHashCodes: false);
return newSize;
Expand Down Expand Up @@ -1011,8 +1021,8 @@ public void TrimExcess(int capacity)
{
if (capacity < Count)
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity);
int newSize = HashHelpers.GetPrime(capacity);

int newSize = HashHelpers.GetPrime(capacity);
Entry[]? oldEntries = _entries;
int currentCapacity = oldEntries == null ? 0 : oldEntries.Length;
if (newSize >= currentCapacity)
Expand All @@ -1022,7 +1032,6 @@ public void TrimExcess(int capacity)
_version++;
Initialize(newSize);
Entry[]? entries = _entries;
int[]? buckets = _buckets;
int count = 0;
for (int i = 0; i < oldCount; i++)
{
Expand All @@ -1031,11 +1040,11 @@ public void TrimExcess(int capacity)
{
ref Entry entry = ref entries![count];
entry = oldEntries[i];
uint bucket = hashCode % (uint)newSize;
ref int bucket = ref GetBucket(hashCode);
// Value in _buckets is 1-based
entry.next = buckets![bucket] - 1; // If we get here, we have entries, therefore buckets is not null.
entry.next = bucket - 1;
// Value in _buckets is 1-based
buckets[bucket] = count + 1;
bucket = count + 1;
count++;
}
}
Expand Down Expand Up @@ -1153,6 +1162,17 @@ void IDictionary.Remove(object key)
}
}

[MethodImpl(MethodImplOptions.AggressiveInlining)]
private ref int GetBucket(uint hashCode)
{
int[] buckets = _buckets!;
#if BIT64
return ref buckets[HashHelpers.FastMod(hashCode, (uint)buckets.Length, _fastModMultiplier)];
#else
return ref buckets[hashCode % (uint)buckets.Length];
#endif
}

public struct Enumerator : IEnumerator<KeyValuePair<TKey, TValue>>,
IDictionaryEnumerator
{
Expand Down
29 changes: 25 additions & 4 deletions src/System.Private.CoreLib/shared/System/Collections/HashHelpers.cs
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Expand Up @@ -3,6 +3,7 @@
// See the LICENSE file in the project root for more information.

using System.Diagnostics;
using System.Runtime.CompilerServices;

namespace System.Collections
{
Expand All @@ -28,12 +29,14 @@ internal static partial class HashHelpers
// h1(key) + i*h2(key), 0 <= i < size. h2 and the size must be relatively prime.
// We prefer the low computation costs of higher prime numbers over the increased
// memory allocation of a fixed prime number i.e. when right sizing a HashSet.
public static readonly int[] primes = {
private static readonly int[] s_primes =
{
3, 7, 11, 17, 23, 29, 37, 47, 59, 71, 89, 107, 131, 163, 197, 239, 293, 353, 431, 521, 631, 761, 919,
1103, 1327, 1597, 1931, 2333, 2801, 3371, 4049, 4861, 5839, 7013, 8419, 10103, 12143, 14591,
17519, 21023, 25229, 30293, 36353, 43627, 52361, 62851, 75431, 90523, 108631, 130363, 156437,
187751, 225307, 270371, 324449, 389357, 467237, 560689, 672827, 807403, 968897, 1162687, 1395263,
1674319, 2009191, 2411033, 2893249, 3471899, 4166287, 4999559, 5999471, 7199369 };
1674319, 2009191, 2411033, 2893249, 3471899, 4166287, 4999559, 5999471, 7199369
};

public static bool IsPrime(int candidate)
{
Expand All @@ -55,9 +58,8 @@ public static int GetPrime(int min)
if (min < 0)
throw new ArgumentException(SR.Arg_HTCapacityOverflow);

for (int i = 0; i < primes.Length; i++)
foreach (int prime in s_primes)
{
int prime = primes[i];
if (prime >= min)
return prime;
}
Expand Down Expand Up @@ -86,5 +88,24 @@ public static int ExpandPrime(int oldSize)

return GetPrime(newSize);
}

#if BIT64
public static ulong GetFastModMultiplier(uint divisor)
=> ulong.MaxValue / divisor + 1;

[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static uint FastMod(uint value, uint divisor, ulong multiplier)
{
// Using fastmod from Daniel Lemire https://lemire.me/blog/2019/02/08/faster-remainders-when-the-divisor-is-a-constant-beating-compilers-and-libdivide/

ulong lowbits = multiplier * value;
// 64bit * 64bit => 128bit isn't currently supported by Math https://github.com/dotnet/corefx/issues/41822
// otherwise we'd want this to be (uint)Math.MultiplyHigh(lowbits, divisor)
uint high = (uint)((((ulong)(uint)lowbits * divisor >> 32) + (lowbits >> 32) * divisor) >> 32);

Debug.Assert(high == value % divisor);
return high;
}
#endif
}
}