From 3767574c7776d807e0ff0d67e5a3729ad765b97f Mon Sep 17 00:00:00 2001
From: Preston Van Loon <preston@prysmaticlabs.com>
Date: Fri, 3 Dec 2021 04:06:05 -0500
Subject: [PATCH] stateutil: Reduce allocations by specifying slice capacity
 eager (#9977)

* Reduce allocations by specifying slice capacity eager

* gofmt
---
 beacon-chain/state/stateutil/BUILD.bazel      |  1 +
 .../state/stateutil/validator_root.go         |  2 +-
 .../state/stateutil/validator_root_test.go    | 22 +++++++++++++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 beacon-chain/state/stateutil/validator_root_test.go

diff --git a/beacon-chain/state/stateutil/BUILD.bazel b/beacon-chain/state/stateutil/BUILD.bazel
index 67da4b457..24a9bf631 100644
--- a/beacon-chain/state/stateutil/BUILD.bazel
+++ b/beacon-chain/state/stateutil/BUILD.bazel
@@ -57,6 +57,7 @@ go_test(
         "state_root_test.go",
         "stateutil_test.go",
         "trie_helpers_test.go",
+        "validator_root_test.go",
     ],
     embed = [":go_default_library"],
     deps = [
diff --git a/beacon-chain/state/stateutil/validator_root.go b/beacon-chain/state/stateutil/validator_root.go
index 8c88d4a14..cf0d67fb9 100644
--- a/beacon-chain/state/stateutil/validator_root.go
+++ b/beacon-chain/state/stateutil/validator_root.go
@@ -58,7 +58,7 @@ func ValidatorRootWithHasher(hasher ssz.HashFn, validator *ethpb.Validator) ([32
 // a list of uint64 and mixed with registry limit.
 func Uint64ListRootWithRegistryLimit(balances []uint64) ([32]byte, error) {
 	hasher := hash.CustomSHA256Hasher()
-	balancesMarshaling := make([][]byte, 0)
+	balancesMarshaling := make([][]byte, 0, len(balances))
 	for i := 0; i < len(balances); i++ {
 		balanceBuf := make([]byte, 8)
 		binary.LittleEndian.PutUint64(balanceBuf, balances[i])
diff --git a/beacon-chain/state/stateutil/validator_root_test.go b/beacon-chain/state/stateutil/validator_root_test.go
new file mode 100644
index 000000000..7fc05e26e
--- /dev/null
+++ b/beacon-chain/state/stateutil/validator_root_test.go
@@ -0,0 +1,22 @@
+package stateutil_test
+
+import (
+	"testing"
+
+	"github.com/prysmaticlabs/prysm/beacon-chain/state/stateutil"
+)
+
+func BenchmarkUint64ListRootWithRegistryLimit(b *testing.B) {
+	balances := make([]uint64, 100000)
+	for i := 0; i < len(balances); i++ {
+		balances[i] = uint64(i)
+	}
+	b.Run("100k balances", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_, err := stateutil.Uint64ListRootWithRegistryLimit(balances)
+			if err != nil {
+				b.Fatal(err)
+			}
+		}
+	})
+}