[test] add performance tests for String#startsWith

Add benchmarks for String.prototype.startsWith in order to keep track
of performance ahead of the torque port.

Bug: v8:8400
Change-Id: I0276b84b315024bba1a0d6f761ee2c6cf2c516a2
Reviewed-on: https://chromium-review.googlesource.com/c/1416070
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58870}

test/js-perf-test/JSTests.json

@@ -528,6 +528,21 @@
             {"name": "endsIndex"}
           ]
         },
+        {
+          "name": "StringStartsWith",
+          "main": "run.js",
+          "resources": [ "string-startswith.js" ],
+          "test_flags": [ "string-startswith" ],
+          "results_regexp": "^%s\\-Strings\\(Score\\): (.+)$",
+          "run_count": 1,
+          "flags": [ "--allow-natives-syntax" ],
+          "tests": [
+            {"name": "DirectStringsDirectSearch"},
+            {"name": "ConsStringsDirectSearch"},
+            {"name": "DirectStringsConsSearch"},
+            {"name": "ConsStringsConsSearch"}
+          ]
+        },
         {
           "name": "StringSubstring",
           "main": "run.js",

test/js-perf-test/Strings/string-startswith.js

+// Copyright 2019 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 createSuite(name, count, fn) {
+  new BenchmarkSuite(name, [count], [new Benchmark(name, true, false, 0, fn)]);
+}
+
+const inputs = [
+  'Lorem ipsum dolor sit amet, consectetur adipiscing elit.',
+  'Integer eu augue suscipit, accumsan ipsum nec, sagittis sem.',
+  'In vitae pellentesque dolor. Curabitur leo nunc, luctus vitae',
+  'risus eget, fermentum hendrerit justo.',
+  'hello'.repeat(1024),
+  'h',
+  ''
+];
+const firsts = ['I', 'Integer', 'Lorem', 'risus', 'hello'];
+
+function simpleHelper() {
+  let sum = 0;
+  for (const input of inputs) {
+    for (const first of firsts) {
+      sum += input.startsWith(first);
+    }
+  }
+  return sum;
+}
+
+function consInputHelper() {
+  let sum = 0;
+  for (const inputOne of inputs) {
+    for (const inputTwo of inputs) {
+      for (const first of firsts) {
+        // Skip if the length is too small for %ConstructConsString
+        if (inputOne.length + inputTwo.length < 13) break;
+        sum += %ConstructConsString(inputOne, inputTwo).startsWith(first);
+      }
+    }
+  }
+  return sum;
+}
+
+function consFirstHelper() {
+  let sum = 0;
+  for (const input of inputs) {
+    for (const firstOne of firsts) {
+      for (const firstTwo of firsts) {
+        // Skip if the length is too small for %ConstructConsString
+        if (firstOne.length + firstTwo.length < 13) break;
+        sum += input.startsWith(%ConstructConsString(firstOne, firstTwo));
+      }
+    }
+  }
+  return sum;
+}
+
+function doubleConsHelper() {
+  let sum = 0;
+  for (const inputOne of inputs) {
+    for (const inputTwo of inputs) {
+      for (const firstOne of firsts) {
+        for (const firstTwo of firsts) {
+          // Skip if the length is too small for %ConstructConsString
+          if (inputOne.length + inputTwo.length < 13 || firstOne.length + firstTwo.length) break;
+          sum += % ConstructConsString(inputOne, inputTwo).startsWith(
+            % ConstructConsString(firstOne, firstTwo)
+          );
+        }
+      }
+    }
+  }
+}
+
+createSuite('DirectStringsDirectSearch', 1000, simpleHelper);
+createSuite('ConsStringsDirectSearch', 1000, consInputHelper);
+createSuite('DirectStringsConsSearch', 1000, consFirstHelper);
+createSuite('ConsStringsConsSearch', 1000, doubleConsHelper);