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Optimize Dictionary<>.TryInsert() by using ref var #108119

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Optimize Dictionary<>.TryInsert() by using ref var #108119

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1df42a2
Optimize `Dictionary<>.TryInsert()` by using `ref var`
SergeiPavlov Sep 23, 2024
0737e15
Merge branch 'main' into optimize_Dictionary
SergeiPavlov Sep 23, 2024
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46 changes: 26 additions & 20 deletions src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -526,20 +526,22 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
uint hashCode = (uint)((typeof(TKey).IsValueType && comparer == null) ? key.GetHashCode() : comparer!.GetHashCode(key));

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

if (typeof(TKey).IsValueType && // comparer can only be null for value types; enable JIT to eliminate entire if block for ref types
comparer == null)
{
// ValueType: Devirtualize with EqualityComparer<TKey>.Default intrinsic
while ((uint)i < (uint)entries.Length)
while ((uint)i < entriesLength)
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Note: (uint)i <= (uint)array.Length is already a trick for the JIT, when it can't prove i for non-negative. Extracting array length into a variable is an anti-pattern and will only increase the chance for JIT to mess things up.

{
if (entries[i].hashCode == hashCode && EqualityComparer<TKey>.Default.Equals(entries[i].key, key))
ref Entry e = ref entries[i];
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I don't think this really matters. Array accessing is a trivial expression and JIT is capable to do the CSE. Is there any assembly difference for this?

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Array accessing is a trivial expression and JIT is capable to do the CSE.

The Dictionary uses ref Entry already in other place:
https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs#L889

if (e.hashCode == hashCode && EqualityComparer<TKey>.Default.Equals(e.key, key))
{
if (behavior == InsertionBehavior.OverwriteExisting)
{
entries[i].value = value;
e.value = value;
return true;
}

Expand All @@ -551,10 +553,10 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
return false;
}

i = entries[i].next;
i = e.next;

collisionCount++;
if (collisionCount > (uint)entries.Length)
if (collisionCount > entriesLength)
{
// The chain of entries forms a loop; which means a concurrent update has happened.
// Break out of the loop and throw, rather than looping forever.
Expand All @@ -565,13 +567,14 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
else
{
Debug.Assert(comparer is not null);
while ((uint)i < (uint)entries.Length)
while ((uint)i < entriesLength)
{
if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key))
ref Entry e = ref entries[i];
if (e.hashCode == hashCode && comparer.Equals(e.key, key))
{
if (behavior == InsertionBehavior.OverwriteExisting)
{
entries[i].value = value;
e.value = value;
return true;
}

Expand All @@ -583,10 +586,10 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
return false;
}

i = entries[i].next;
i = e.next;

collisionCount++;
if (collisionCount > (uint)entries.Length)
if (collisionCount > entriesLength)
{
// The chain of entries forms a loop; which means a concurrent update has happened.
// Break out of the loop and throw, rather than looping forever.
Expand Down Expand Up @@ -1091,26 +1094,28 @@ internal static class CollectionsMarshalHelper
uint hashCode = (uint)((typeof(TKey).IsValueType && comparer == null) ? key.GetHashCode() : comparer!.GetHashCode(key));

uint collisionCount = 0;
uint entriesLength = (uint)entries.Length;
ref int bucket = ref dictionary.GetBucket(hashCode);
int i = bucket - 1; // Value in _buckets is 1-based

if (typeof(TKey).IsValueType && // comparer can only be null for value types; enable JIT to eliminate entire if block for ref types
comparer == null)
{
// ValueType: Devirtualize with EqualityComparer<TKey>.Default intrinsic
while ((uint)i < (uint)entries.Length)
while ((uint)i < entriesLength)
{
if (entries[i].hashCode == hashCode && EqualityComparer<TKey>.Default.Equals(entries[i].key, key))
ref Entry e = ref entries[i];
if (e.hashCode == hashCode && EqualityComparer<TKey>.Default.Equals(e.key, key))
{
exists = true;

return ref entries[i].value!;
return ref e.value!;
}

i = entries[i].next;
i = e.next;

collisionCount++;
if (collisionCount > (uint)entries.Length)
if (collisionCount > entriesLength)
{
// The chain of entries forms a loop; which means a concurrent update has happened.
// Break out of the loop and throw, rather than looping forever.
Expand All @@ -1121,19 +1126,20 @@ internal static class CollectionsMarshalHelper
else
{
Debug.Assert(comparer is not null);
while ((uint)i < (uint)entries.Length)
while ((uint)i < entriesLength)
{
if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key))
ref Entry e = ref entries[i];
if (e.hashCode == hashCode && comparer.Equals(e.key, key))
{
exists = true;

return ref entries[i].value!;
return ref e.value!;
}

i = entries[i].next;
i = e.next;

collisionCount++;
if (collisionCount > (uint)entries.Length)
if (collisionCount > entriesLength)
{
// The chain of entries forms a loop; which means a concurrent update has happened.
// Break out of the loop and throw, rather than looping forever.
Expand Down
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