util: improve spliceOne perf

Do less variable allocations and reassignments inside spliceOne
since it's relied on by some performance sensitive code.

PR-URL: #20453
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>

benchmark/util/splice-one.js

@@ -0,0 +1,33 @@
+'use strict';
+
+const common = require('../common');
+
+const bench = common.createBenchmark(main, {
+  n: [1e7],
+  pos: ['start', 'middle', 'end'],
+  size: [10, 100, 500],
+}, { flags: ['--expose-internals'] });
+
+function main({ n, pos, size }) {
+  const { spliceOne } = require('internal/util');
+  const arr = new Array(size);
+  arr.fill('');
+  let index;
+  switch (pos) {
+    case 'end':
+      index = size - 1;
+      break;
+    case 'middle':
+      index = Math.floor(size / 2);
+      break;
+    default: // start
+      index = 0;
+  }
+
+  bench.start();
+  for (var i = 0; i < n; i++) {
+    spliceOne(arr, index);
+    arr.push('');
+  }
+  bench.end(n);
+}

lib/internal/util.js

@@ -322,10 +322,11 @@ function join(output, separator) {
   return str;
 }
 
-// About 1.5x faster than the two-arg version of Array#splice().
+// As of V8 6.6, depending on the size of the array, this is anywhere
+// between 1.5-10x faster than the two-arg version of Array#splice()
 function spliceOne(list, index) {
-  for (var i = index, k = i + 1, n = list.length; k < n; i += 1, k += 1)
-    list[i] = list[k];
+  for (; index + 1 < list.length; index++)
+    list[index] = list[index + 1];
   list.pop();
 }
 

test/parallel/test-benchmark-util.js

@@ -10,6 +10,8 @@ runBenchmark('util',
               'method=Array',
               'n=1',
               'option=none',
+              'pos=start',
+              'size=1',
               'type=',
               'version=native'],
              { NODEJS_BENCHMARK_ZERO_ALLOWED: 1 });