[debugger] implement per-function code coverage.

test/mjsunit/code-coverage-ad-hoc.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.
+
+// Flags: --allow-natives-syntax --no-always-opt
+
+// Test code coverage without explicitly activating it upfront.
+
+function GetCoverage(source) {
+  var scripts = %DebugGetLoadedScripts();
+  for (var script of scripts) {
+    if (script.source == source) {
+      var coverage = %DebugCollectCoverage();
+      for (var data of coverage) {
+        if (data.script_id == script.id) return data.entries;
+      }
+    }
+  }
+  return undefined;
+}
+
+function ApplyCoverageToSource(source, coverage) {
+  var result = "";
+  var cursor = 0;
+  for (var entry of coverage) {
+    var chunk = source.substring(cursor, entry.end_position);
+    cursor = entry.end_position;
+    result += `[${chunk}[${entry.count}]]`;
+  }
+  return result;
+}
+
+function TestCoverage(name, source, expectation) {
+  source = source.trim();
+  expectation = expectation.trim();
+  eval(source);
+  var coverage = GetCoverage(source);
+  var result = ApplyCoverageToSource(source, coverage);
+  print(result);
+  assertEquals(expectation, result, name + " failed");
+}
+
+TestCoverage(
+"call simple function twice",
+`
+function f() {}
+f();
+f();
+`,
+`
+[function f() {}[2]][
+f();
+f();[1]]
+`
+);
+
+TestCoverage(
+"call arrow function twice",
+`
+var f = () => 1;
+f();
+f();
+`,
+`
+[var f = [1]][() => 1[2]][;
+f();
+f();[1]]
+`
+);
+
+TestCoverage(
+"call nested function",
+`
+function f() {
+  function g() {}
+  g();
+  g();
+}
+f();
+f();
+`,
+`
+[function f() {
+  [2]][function g() {}[4]][
+  g();
+  g();
+}[2]][
+f();
+f();[1]]
+`
+);
+
+TestCoverage(
+"call recursive function",
+`
+function fib(x) {
+  if (x < 2) return 1;
+  return fib(x-1) + fib(x-2);
+}
+fib(5);
+`,
+`
+[function fib(x) {
+  if (x < 2) return 1;
+  return fib(x-1) + fib(x-2);
+}[15]][
+fib(5);[1]]
+`
+);