[js-perf-test] Add micro-benchmark for StringAt functions

This benchmark checks comparison with StringAt functions against
each other and and constants. The benchmarks will serve to measure
the effect of an optimization that will omit the implicit
String.fromCharCode in such cases.

Bug: v8:7531
Change-Id: I171df92301516c96beb6a4ed86f1dec8d10e34f5
Reviewed-on: https://chromium-review.googlesource.com/957086Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51839}

test/js-perf-test/JSTests.json

@@ -255,6 +255,22 @@
             {"name": "StringCodePointAtNonConstantInbounds"}
           ]
         },
+        {
+          "name": "StringAtComparison",
+          "main": "run.js",
+          "resources": [ "string-stringat-comp.js" ],
+          "test_flags": [ "string-stringat-comp" ],
+          "results_regexp": "^%s\\-Strings\\(Score\\): (.+)$",
+          "run_count": 1,
+          "tests": [
+            {"name": "charCodeAt_const"},
+            {"name": "charCodeAt_both"},
+            {"name": "charAt_const"},
+            {"name": "charAt_never"},
+            {"name": "charAt_both"},
+            {"name": "stringIndex_const"}
+          ]
+        },
         {
           "name": "StringSubstring",
           "main": "run.js",

test/js-perf-test/Strings/string-stringat-comp.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+function benchy(f, name) {
+  new BenchmarkSuite(name, [1], [
+    new Benchmark(name, true, false, 0, f),
+  ]);
+}
+
+const input = 'äϠ�𝌆 Lorem ipsum test test';
+
+function helper(fn) {
+  var sum = 0;
+  for (var i = 0; i < input.length; i++) {
+    sum += fn(input, i, i);
+  }
+  return sum;
+}
+
+function charCodeAt(str, i) {
+  return str.charCodeAt(i) === 116;
+}
+
+function charCodeAtBoth(str, i, j) {
+  return str.charCodeAt(j) == str.charCodeAt(i);
+}
+
+function charAt(str, i) {
+  return 't' == str.charAt(i);
+}
+
+function charAtNever(str, i) {
+  return '𝌆' == str.charAt(i);
+}
+
+function charAtBoth(str, i, j) {
+  return str.charAt(j) == str.charAt(i);
+}
+
+function stringIndex(str, i) {
+  return str[i] === 't';
+}
+
+benchy(() => helper(charCodeAt), "charCodeAt_const");
+benchy(() => helper(charCodeAtBoth), "charCodeAt_both");
+benchy(() => helper(charAt), "charAt_const");
+benchy(() => helper(charAtNever), "charAt_never");
+benchy(() => helper(charAtBoth), "charAt_both");
+benchy(() => helper(stringIndex), "stringIndex_const");