From 9b1999a5ffa92f9889d5c81d0ed62266a2d50709 Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@gmail.com>
Date: Wed, 14 Apr 2021 12:27:40 +0300
Subject: [PATCH] lib/uint64set: remove memory allocation in
 getOrCreateSmallPool()

This partially reverts fb82c4b9faa99fd8bcf843ac3e843a7fe90b7867

It has been appeared that the additional memory allocation may result in higher GC pauses.
It is better to spend CPU time on copying bigger bucket16 structs instead of increasing query latencies due to higher GC pauses
---
 lib/uint64set/uint64set.go | 39 +++++++-------------------------------
 1 file changed, 7 insertions(+), 32 deletions(-)

diff --git a/lib/uint64set/uint64set.go b/lib/uint64set/uint64set.go
index 4262a948d..4096c0aac 100644
--- a/lib/uint64set/uint64set.go
+++ b/lib/uint64set/uint64set.go
@@ -707,7 +707,7 @@ const (
 
 type bucket16 struct {
 	bits         *[wordsPerBucket]uint64
-	smallPool    *[smallPoolSize]uint16
+	smallPool    [smallPoolSize]uint16
 	smallPoolLen int
 }
 
@@ -789,9 +789,6 @@ func (b *bucket16) sizeBytes() uint64 {
 	if b.bits != nil {
 		n += unsafe.Sizeof(*b.bits)
 	}
-	if b.smallPool != nil {
-		n += unsafe.Sizeof(*b.smallPool)
-	}
 	return uint64(n)
 }
 
@@ -802,19 +799,8 @@ func (b *bucket16) copyTo(dst *bucket16) {
 		bits := *b.bits
 		dst.bits = &bits
 	}
+	dst.smallPool = b.smallPool
 	dst.smallPoolLen = b.smallPoolLen
-	if b.smallPool != nil {
-		sp := dst.getOrCreateSmallPool()
-		*sp = *b.smallPool
-	}
-}
-
-func (b *bucket16) getOrCreateSmallPool() *[smallPoolSize]uint16 {
-	if b.smallPool == nil {
-		var sp [smallPoolSize]uint16
-		b.smallPool = &sp
-	}
-	return b.smallPool
 }
 
 func (b *bucket16) add(x uint16) bool {
@@ -861,7 +847,7 @@ func (b *bucket16) addToSmallPool(x uint16) bool {
 	if b.hasInSmallPool(x) {
 		return false
 	}
-	sp := b.getOrCreateSmallPool()
+	sp := b.smallPool[:]
 	if b.smallPoolLen < len(sp) {
 		sp[b.smallPoolLen] = x
 		b.smallPoolLen++
@@ -886,11 +872,7 @@ func (b *bucket16) has(x uint16) bool {
 }
 
 func (b *bucket16) hasInSmallPool(x uint16) bool {
-	sp := b.smallPool
-	if sp == nil {
-		return false
-	}
-	for _, v := range sp[:b.smallPoolLen] {
+	for _, v := range b.smallPool[:b.smallPoolLen] {
 		if v == x {
 			return true
 		}
@@ -910,10 +892,7 @@ func (b *bucket16) del(x uint16) bool {
 }
 
 func (b *bucket16) delFromSmallPool(x uint16) bool {
-	sp := b.smallPool
-	if sp == nil {
-		return false
-	}
+	sp := b.smallPool[:]
 	for i, v := range sp[:b.smallPoolLen] {
 		if v == x {
 			copy(sp[i:], sp[i+1:])
@@ -927,15 +906,11 @@ func (b *bucket16) delFromSmallPool(x uint16) bool {
 func (b *bucket16) appendTo(dst []uint64, hi uint32, hi16 uint16) []uint64 {
 	hi64 := uint64(hi)<<32 | uint64(hi16)<<16
 	if b.bits == nil {
-		sp := b.smallPool
-		if sp == nil {
-			return dst
-		}
 		// Use smallPoolSorter instead of sort.Slice here in order to reduce memory allocations.
 		sps := smallPoolSorterPool.Get().(*smallPoolSorter)
-		// Sort a copy of sp, since b must be readonly in order to prevent from data races
+		// Sort a copy of b.smallPool, since b must be readonly in order to prevent from data races
 		// when b.appendTo is called from concurrent goroutines.
-		sps.smallPool = *sp
+		sps.smallPool = b.smallPool
 		sps.a = sps.smallPool[:b.smallPoolLen]
 		if len(sps.a) > 1 && !sort.IsSorted(sps) {
 			sort.Sort(sps)