stateutil: Reduce allocations by specifying slice capacity eager (#9977)

* Reduce allocations by specifying slice capacity eager

* gofmt

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 = [

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])

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)
+			}
+		}
+	})
+}