primitives: Add merkle root benchmarks.

BenchmarkCalcMerkleRootInPlace
------------------------------
20_leaves      95551      12436 ns/op   0 B/op   0 allocs/op
1000_leaves     2047     591660 ns/op   0 B/op   0 allocs/op
2000_leaves     1018    1179846 ns/op   0 B/op   0 allocs/op
4000_leaves      507    2357246 ns/op   0 B/op   0 allocs/op
8000_leaves      254    4710341 ns/op   0 B/op   0 allocs/op
16000_leaves     127    9499098 ns/op   0 B/op   0 allocs/op
32000_leaves      62   18876558 ns/op   0 B/op   0 allocs/op

BenchmarkCalcMerkleRoot
-----------------------
20_leaves      92455      12840 ns/op       640 B/op   1 allocs/op
1000_leaves     1965     608127 ns/op     32769 B/op   1 allocs/op
2000_leaves      969    1261865 ns/op     65536 B/op   1 allocs/op
4000_leaves      493    2439323 ns/op    131073 B/op   1 allocs/op
8000_leaves      246    4850361 ns/op    262144 B/op   1 allocs/op
16000_leaves     122    9826700 ns/op    516101 B/op   1 allocs/op
32000_leaves      63   19420881 ns/op   1024008 B/op   1 allocs/op

internal/staging/primitives/merkle_bench_test.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2019-2021 The Decred developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package primitives
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/decred/dcrd/chaincfg/chainhash"
+)
+
+// BenchmarkCalcMerkleRootInPlace benchmarks merkle root calculation for various
+// numbers of leaves using the mutable in-place algorithm.
+func BenchmarkCalcMerkleRootInPlace(b *testing.B) {
+	// Create several slices of leaves of various sizes to benchmark.
+	numLeavesToBench := []int{20, 1000, 2000, 4000, 8000, 16000, 32000}
+	origLeaves := make([][]chainhash.Hash, len(numLeavesToBench))
+	for i, numLeaves := range numLeavesToBench {
+		origLeaves[i] = make([]chainhash.Hash, numLeaves)
+	}
+
+	for benchIdx := range origLeaves {
+		testLeaves := origLeaves[benchIdx]
+		benchName := fmt.Sprintf("%d leaves", len(testLeaves))
+		b.Run(benchName, func(b *testing.B) {
+			b.ResetTimer()
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				_ = CalcMerkleRootInPlace(testLeaves)
+			}
+		})
+	}
+}
+
+// BenchmarkCalcMerkleRoot benchmarks merkle root calculation for various
+// numbers of leaves using the non-mutable version.
+func BenchmarkCalcMerkleRoot(b *testing.B) {
+	// Create several slices of leaves of various sizes to benchmark.
+	numLeavesToBench := []int{20, 1000, 2000, 4000, 8000, 16000, 32000}
+	origLeaves := make([][]chainhash.Hash, len(numLeavesToBench))
+	for i, numLeaves := range numLeavesToBench {
+		origLeaves[i] = make([]chainhash.Hash, numLeaves)
+	}
+
+	for benchIdx := range origLeaves {
+		testLeaves := origLeaves[benchIdx]
+		benchName := fmt.Sprintf("%d leaves", len(testLeaves))
+		b.Run(benchName, func(b *testing.B) {
+			b.ResetTimer()
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				_ = CalcMerkleRoot(testLeaves)
+			}
+		})
+	}
+}