db: store pointer to *Comparer on Iterator

Previously the base.Comparer struct was copied into the Iterator. As the
Comparer and its fields have grown, the cost of this copy and the eventual
zeroing of this structure have grown too. This commit switches to storing just
a pointer to the DB's Comparer.

```
goos: darwin
goarch: arm64
pkg: github.com/cockroachdb/pebble
cpu: Apple M1 Pro
                    │  prev.txt   │              head.txt              │
                    │   sec/op    │   sec/op     vs base               │
NewIterClose/1-10     1.251µ ± 2%   1.213µ ± 0%  -3.04% (p=0.000 n=10)
NewIterClose/10-10    6.591µ ± 2%   6.466µ ± 1%  -1.90% (p=0.004 n=10)
NewIterClose/100-10   95.87µ ± 0%   93.27µ ± 2%  -2.71% (p=0.001 n=10)
geomean               9.246µ        9.010µ       -2.55%
```

Author: jbowens

db.go

@@ -1012,7 +1012,7 @@ func (d *DB) newIter(
 		ctx:                 ctx,
 		alloc:               buf,
 		merge:               d.merge,
-		comparer:            *d.opts.Comparer,
+		comparer:            d.opts.Comparer,
 		readState:           readState,
 		version:             newIterOpts.snapshot.vers,
 		keyBuf:              buf.keyBuf,
@@ -1076,7 +1076,7 @@ func finishInitializingIter(ctx context.Context, buf *iterAlloc) *Iterator {
 	}
 
 	if dbi.opts.rangeKeys() {
-		dbi.rangeKeyMasking.init(dbi, &dbi.comparer)
+		dbi.rangeKeyMasking.init(dbi, dbi.comparer)
 
 		// When iterating over both point and range keys, don't create the
 		// range-key iterator stack immediately if we can avoid it. This
@@ -1131,7 +1131,7 @@ func finishInitializingIter(ctx context.Context, buf *iterAlloc) *Iterator {
 			// NB: The interleaving iterator is always reinitialized, even if
 			// dbi already had an initialized range key iterator, in case the point
 			// iterator changed or the range key masking suffix changed.
-			dbi.rangeKey.iiter.Init(&dbi.comparer, dbi.iter, dbi.rangeKey.rangeKeyIter,
+			dbi.rangeKey.iiter.Init(dbi.comparer, dbi.iter, dbi.rangeKey.rangeKeyIter,
 				keyspan.InterleavingIterOpts{
 					Mask:       &dbi.rangeKeyMasking,
 					LowerBound: dbi.opts.LowerBound,
@@ -1262,7 +1262,7 @@ func (i *Iterator) constructPointIter(
 		addLevelIterForFiles := func(files manifest.LevelIterator, level manifest.Layer) {
 			li := &levels[levelsIndex]
 
-			li.init(ctx, i.opts, &i.comparer, i.newIters, files, level, internalOpts)
+			li.init(ctx, i.opts, i.comparer, i.newIters, files, level, internalOpts)
 			li.initRangeDel(&mlevels[mlevelsIndex])
 			li.initCombinedIterState(&i.lazyCombinedIter.combinedIterState)
 			mlevels[mlevelsIndex].levelIter = li

external_iterator.go

@@ -71,7 +71,7 @@ func NewExternalIterWithContext(
 		ctx:                 ctx,
 		alloc:               buf,
 		merge:               o.Merger.Merge,
-		comparer:            *o.Comparer,
+		comparer:            o.Comparer,
 		readState:           nil,
 		keyBuf:              buf.keyBuf,
 		prefixOrFullSeekKey: buf.prefixOrFullSeekKey,
@@ -236,7 +236,7 @@ func finishInitializingExternal(ctx context.Context, it *Iterator) error {
 	it.iter = it.pointIter
 
 	if it.opts.rangeKeys() {
-		it.rangeKeyMasking.init(it, &it.comparer)
+		it.rangeKeyMasking.init(it, it.comparer)
 		var rangeKeyIters []keyspan.FragmentIterator
 		if it.rangeKey == nil {
 			// We could take advantage of the lack of overlaps in range keys within
@@ -272,7 +272,7 @@ func finishInitializingExternal(ctx context.Context, it *Iterator) error {
 			if len(rangeKeyIters) > 0 {
 				it.rangeKey = iterRangeKeyStateAllocPool.Get().(*iteratorRangeKeyState)
 				it.rangeKey.rangeKeyIter = it.rangeKey.iterConfig.Init(
-					&it.comparer,
+					it.comparer,
 					base.SeqNumMax,
 					it.opts.LowerBound, it.opts.UpperBound,
 					&it.hasPrefix, &it.prefixOrFullSeekKey,
@@ -284,7 +284,7 @@ func finishInitializingExternal(ctx context.Context, it *Iterator) error {
 			}
 		}
 		if it.rangeKey != nil {
-			it.rangeKey.iiter.Init(&it.comparer, it.iter, it.rangeKey.rangeKeyIter,
+			it.rangeKey.iiter.Init(it.comparer, it.iter, it.rangeKey.rangeKeyIter,
 				keyspan.InterleavingIterOpts{
 					Mask:       &it.rangeKeyMasking,
 					LowerBound: it.opts.LowerBound,

get.go

@@ -115,7 +115,7 @@ func (d *DB) getInternal(key []byte, b *Batch, s *Snapshot) ([]byte, io.Closer,
 		iter:      pointIter,
 		pointIter: pointIter,
 		merge:     d.merge,
-		comparer:  *d.opts.Comparer,
+		comparer:  d.opts.Comparer,
 		readState: readState,
 		keyBuf:    buf.keyBuf,
 	}

get_test.go

@@ -462,7 +462,7 @@ func TestGetIter(t *testing.T) {
 			}
 
 			i := &buf.dbi
-			i.comparer = *testkeys.Comparer
+			i.comparer = testkeys.Comparer
 			i.merge = DefaultMerger.Merge
 			i.iter = get
 

iterator.go

@@ -201,7 +201,7 @@ type Iterator struct {
 	ctx       context.Context
 	opts      IterOptions
 	merge     Merge
-	comparer  base.Comparer
+	comparer  *base.Comparer
 	iter      internalIterator
 	pointIter topLevelIterator
 	// Either readState or version is set, but not both.

iterator_test.go

@@ -182,7 +182,7 @@ func TestIterator(t *testing.T) {
 		}
 		it := &Iterator{
 			opts:     opts,
-			comparer: *testkeys.Comparer,
+			comparer: testkeys.Comparer,
 			merge:    wrappedMerge,
 		}
 		// NB: Use a mergingIter to filter entries newer than seqNum.
@@ -854,7 +854,7 @@ func TestIteratorSeekOptErrors(t *testing.T) {
 		// with a readState. It suffices for this test.
 		return &Iterator{
 			opts:     opts,
-			comparer: *testkeys.Comparer,
+			comparer: testkeys.Comparer,
 			merge:    DefaultMerger.Merge,
 			iter:     &errorIter,
 		}
@@ -1644,7 +1644,7 @@ func newPointTestkeysDatabase(t *testing.T, ks testkeys.Keyspace) *DB {
 func BenchmarkIteratorSeekGE(b *testing.B) {
 	m, keys := buildMemTable(b)
 	iter := &Iterator{
-		comparer: *DefaultComparer,
+		comparer: DefaultComparer,
 		iter:     m.newIter(nil),
 	}
 	rng := rand.New(rand.NewPCG(0, uint64(time.Now().UnixNano())))
@@ -1659,7 +1659,7 @@ func BenchmarkIteratorSeekGE(b *testing.B) {
 func BenchmarkIteratorNext(b *testing.B) {
 	m, _ := buildMemTable(b)
 	iter := &Iterator{
-		comparer: *DefaultComparer,
+		comparer: DefaultComparer,
 		iter:     m.newIter(nil),
 	}
 
@@ -1675,7 +1675,7 @@ func BenchmarkIteratorNext(b *testing.B) {
 func BenchmarkIteratorPrev(b *testing.B) {
 	m, _ := buildMemTable(b)
 	iter := &Iterator{
-		comparer: *DefaultComparer,
+		comparer: DefaultComparer,
 		iter:     m.newIter(nil),
 	}
 
@@ -1793,7 +1793,7 @@ func BenchmarkIteratorSeqSeekPrefixGENotFound(b *testing.B) {
 							levelSlices := levelSlices[index]
 							m := buildMergingIter(readers, levelSlices)
 							iter := Iterator{
-								comparer: *testkeys.Comparer,
+								comparer: testkeys.Comparer,
 								merge:    DefaultMerger.Merge,
 								iter:     m,
 							}
@@ -1858,7 +1858,7 @@ func BenchmarkIteratorSeqSeekPrefixGEFound(b *testing.B) {
 							levelSlices := levelSlices[index]
 							m := buildMergingIter(readers, levelSlices)
 							iter := Iterator{
-								comparer: *testkeys.Comparer,
+								comparer: testkeys.Comparer,
 								merge:    DefaultMerger.Merge,
 								iter:     m,
 							}
@@ -1911,7 +1911,7 @@ func BenchmarkIteratorSeqSeekGEWithBounds(b *testing.B) {
 					false, false, twoLevelIndex)
 				m := buildMergingIter(readers, levelSlices)
 				iter := Iterator{
-					comparer: *testkeys.Comparer,
+					comparer: testkeys.Comparer,
 					merge:    DefaultMerger.Merge,
 					iter:     m,
 				}
@@ -1959,7 +1959,7 @@ func BenchmarkIteratorSeekGENoop(b *testing.B) {
 		b.Run(fmt.Sprintf("withLimit=%t", withLimit), func(b *testing.B) {
 			m := buildMergingIter(readers, levelSlices)
 			iter := Iterator{
-				comparer: *testkeys.Comparer,
+				comparer: testkeys.Comparer,
 				merge:    DefaultMerger.Merge,
 				iter:     m,
 			}

range_keys.go

@@ -20,7 +20,7 @@ import (
 // i.rangeKey.rangeKeyIter with the resulting iterator.
 func (i *Iterator) constructRangeKeyIter() {
 	i.rangeKey.rangeKeyIter = i.rangeKey.iterConfig.Init(
-		&i.comparer, i.seqNum, i.opts.LowerBound, i.opts.UpperBound,
+		i.comparer, i.seqNum, i.opts.LowerBound, i.opts.UpperBound,
 		&i.hasPrefix, &i.prefixOrFullSeekKey, false /* internalKeys */, &i.rangeKey.rangeKeyBuffers.internal)
 
 	if i.opts.DebugRangeKeyStack {
@@ -562,7 +562,7 @@ func (i *lazyCombinedIter) initCombinedIteration(
 
 	// Initialize the Iterator's interleaving iterator.
 	i.parent.rangeKey.iiter.Init(
-		&i.parent.comparer, i.parent.pointIter, i.parent.rangeKey.rangeKeyIter,
+		i.parent.comparer, i.parent.pointIter, i.parent.rangeKey.rangeKeyIter,
 		keyspan.InterleavingIterOpts{
 			Mask:       &i.parent.rangeKeyMasking,
 			LowerBound: i.parent.opts.LowerBound,