[PERF] Labels: faster varint for dedupelabels

Including tests.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

model/labels/labels_dedupelabels.go

@@ -104,25 +104,27 @@ func (t *nameTable) ToName(num int) string {
 	return t.byNum[num]
 }
 
+// "Varint" in this file is non-standard: we encode small numbers (up to 32767) in 2 bytes,
+// because we expect most Prometheus to have more than 127 unique strings.
+// And we don't encode numbers larger than 4 bytes because we don't expect more than 536,870,912 unique strings.
 func decodeVarint(data string, index int) (int, int) {
-	// Fast-path for common case of a single byte, value 0..127.
-	b := data[index]
+	b := int(data[index]) + int(data[index+1])<<8
+	index += 2
+	if b < 0x8000 {
+		return b, index
+	}
+
+	value := int(b & 0x7FFF)
+	b = int(data[index])
 	index++
 	if b < 0x80 {
-		return int(b), index
-	}
-	value := int(b & 0x7F)
-	for shift := uint(7); ; shift += 7 {
-		// Just panic if we go of the end of data, since all Labels strings are constructed internally and
-		// malformed data indicates a bug, or memory corruption.
-		b := data[index]
-		index++
-		value |= int(b&0x7F) << shift
-		if b < 0x80 {
-			break
-		}
+		return value | (b << 15), index
 	}
-	return value, index
+
+	value |= (b & 0x7f) << 15
+	b = int(data[index])
+	index++
+	return value | (b << 22), index
 }
 
 func decodeString(t *nameTable, data string, index int) (string, int) {
@@ -641,29 +643,24 @@ func marshalNumbersToSizedBuffer(nums []int, data []byte) int {
 
 func sizeVarint(x uint64) (n int) {
 	// Most common case first
-	if x < 1<<7 {
-		return 1
-	}
-	if x >= 1<<56 {
-		return 9
+	if x < 1<<15 {
+		return 2
 	}
-	if x >= 1<<28 {
-		x >>= 28
-		n = 4
+	if x < 1<<22 {
+		return 3
 	}
-	if x >= 1<<14 {
-		x >>= 14
-		n += 2
+	if x >= 1<<29 {
+		panic("Number too large to represent")
 	}
-	if x >= 1<<7 {
-		n++
-	}
-	return n + 1
+	return 4
 }
 
 func encodeVarintSlow(data []byte, offset int, v uint64) int {
 	offset -= sizeVarint(v)
 	base := offset
+	data[offset] = uint8(v)
+	v >>= 8
+	offset++
 	for v >= 1<<7 {
 		data[offset] = uint8(v&0x7f | 0x80)
 		v >>= 7
@@ -673,11 +670,12 @@ func encodeVarintSlow(data []byte, offset int, v uint64) int {
 	return base
 }
 
-// Special code for the common case that a value is less than 128
+// Special code for the common case that a value is less than 32768
 func encodeVarint(data []byte, offset, v int) int {
-	if v < 1<<7 {
-		offset--
+	if v < 1<<15 {
+		offset -= 2
 		data[offset] = uint8(v)
+		data[offset+1] = uint8(v >> 8)
 		return offset
 	}
 	return encodeVarintSlow(data, offset, uint64(v))

model/labels/labels_dedupelabels_test.go

@@ -0,0 +1,50 @@
+// Copyright 2024 The Prometheus Authors
+// 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.
+
+//go:build dedupelabels
+
+package labels
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestVarint(t *testing.T) {
+	cases := []struct {
+		v        int
+		expected []byte
+	}{
+		{0, []byte{0, 0}},
+		{1, []byte{1, 0}},
+		{2, []byte{2, 0}},
+		{0x7FFF, []byte{0xFF, 0x7F}},
+		{0x8000, []byte{0x00, 0x80, 0x01}},
+		{0x8001, []byte{0x01, 0x80, 0x01}},
+		{0x3FFFFF, []byte{0xFF, 0xFF, 0x7F}},
+		{0x400000, []byte{0x00, 0x80, 0x80, 0x01}},
+		{0x400001, []byte{0x01, 0x80, 0x80, 0x01}},
+		{0x1FFFFFFF, []byte{0xFF, 0xFF, 0xFF, 0x7F}},
+	}
+	var buf [16]byte
+	for _, c := range cases {
+		n := encodeVarint(buf[:], len(buf), c.v)
+		require.Equal(t, len(c.expected), len(buf)-n)
+		require.Equal(t, c.expected, buf[n:])
+		got, m := decodeVarint(string(buf[:]), n)
+		require.Equal(t, c.v, got)
+		require.Equal(t, len(buf), m)
+	}
+	require.Panics(t, func() { encodeVarint(buf[:], len(buf), 1<<29) })
+}