[debug] do not report unnecessary coverage data.

Only include function ranges for with non-0 counts or if the immediate outer function has non-0 count.

R=caseq@chromium.org, jgruber@chromium.org
BUG=v8:5808

Review-Url: https://codereview.chromium.org/2764073004
Cr-Commit-Position: refs/heads/master@{#44079}

src/debug/debug-coverage.cc

@@ -121,7 +121,7 @@ Coverage* Coverage::Collect(Isolate* isolate,
 
     // Create and add new script data.
     Handle<Script> script_handle(script, isolate);
-    result->emplace_back(isolate, script_handle);
+    result->emplace_back(script_handle);
     std::vector<CoverageFunction>* functions = &result->back().functions;
 
     std::vector<SharedFunctionInfo*> sorted;
@@ -135,11 +135,18 @@ Coverage* Coverage::Collect(Isolate* isolate,
       std::sort(sorted.begin(), sorted.end(), CompareSharedFunctionInfo);
     }
 
+    // Stack to track nested functions, referring function by index.
+    std::vector<size_t> nesting;
+
     // Use sorted list to reconstruct function nesting.
     for (SharedFunctionInfo* info : sorted) {
       int start = StartPosition(info);
       int end = info->end_position();
       uint32_t count = counter_map.Get(info);
+      // Find the correct outer function based on start position.
+      while (!nesting.empty() && functions->at(nesting.back()).end <= start) {
+        nesting.pop_back();
+      }
       if (count != 0) {
         switch (collectionMode) {
           case v8::debug::Coverage::kPreciseCount:
@@ -152,10 +159,18 @@ Coverage* Coverage::Collect(Isolate* isolate,
             count = 1;
             break;
         }
+      } else if (nesting.empty() || functions->at(nesting.back()).count == 0) {
+        // Only include a function range if it has a non-0 count, or
+        // if it is directly nested inside a function with non-0 count.
+        continue;
       }
       Handle<String> name(info->DebugName(), isolate);
+      nesting.push_back(functions->size());
       functions->emplace_back(start, end, count, name);
     }
+
+    // Remove entries for scripts that have no coverage.
+    if (functions->empty()) result->pop_back();
   }
   return result;
 }

src/debug/debug-coverage.h

@@ -27,7 +27,7 @@ struct CoverageFunction {
 
 struct CoverageScript {
   // Initialize top-level function in case it has been garbage-collected.
-  CoverageScript(Isolate* isolate, Handle<Script> s) : script(s) {}
+  explicit CoverageScript(Handle<Script> s) : script(s) {}
   Handle<Script> script;
   // Functions are sorted by start position, from outer to inner function.
   std::vector<CoverageFunction> functions;

test/inspector/cpu-profiler/coverage.js

@@ -34,6 +34,22 @@ function h() {
 f(3);
 `;
 
+var nested =
+`
+var f = (function outer() {
+  function nested_0() {
+    return function nested_1() {
+      return function nested_2() {
+        return function nested_3() {}
+      }
+    }
+  }
+  function nested_4() {}
+  return nested_0();
+})();
+f()()();
+`;
+
 InspectorTest.log("Test collecting code coverage data with Profiler.collectCoverage.");
 
 function ClearAndGC() {
@@ -78,8 +94,8 @@ InspectorTest.runTestSuite([
       .then(() => Protocol.Profiler.startPreciseCoverage({callCount: true}))
       .then(() => Protocol.Runtime.compileScript({ expression: source, sourceURL: arguments.callee.name, persistScript: true }))
       .then((result) => Protocol.Runtime.runScript({ scriptId: result.result.scriptId }))
-      .then(ClearAndGC)
       .then(InspectorTest.logMessage)
+      .then(ClearAndGC)
       .then(Protocol.Profiler.takePreciseCoverage)
       .then(LogSorted)
       .then(Protocol.Profiler.takePreciseCoverage)
@@ -208,4 +224,44 @@ InspectorTest.runTestSuite([
       .then(ClearAndGC)
       .then(next);
   },
+  function testPreciseEmptyScriptCoverageEntries(next)
+  {
+    // Enabling the debugger holds onto script objects even though its
+    // functions can be garbage collected. We would get empty ScriptCoverage
+    // entires unless we remove them.
+    Protocol.Debugger.enable()
+      .then(Protocol.Runtime.enable)
+      .then(() => Protocol.Runtime.compileScript({ expression: source, sourceURL: arguments.callee.name, persistScript: true }))
+      .then((result) => Protocol.Runtime.runScript({ scriptId: result.result.scriptId }))
+      .then(ClearAndGC)
+      .then(Protocol.Profiler.enable)
+      .then(Protocol.Profiler.startPreciseCoverage)
+      .then(Protocol.Profiler.takePreciseCoverage)
+      .then(LogSorted)
+      .then(Protocol.Profiler.stopPreciseCoverage)
+      .then(Protocol.Profiler.disable)
+      .then(Protocol.Runtime.disable)
+      .then(Protocol.Debugger.disable)
+      .then(ClearAndGC)
+      .then(next);
+  },
+  function testPreciseCountCoveragePartial(next)
+  {
+    Protocol.Runtime.enable()
+      .then(Protocol.Profiler.enable)
+      .then(() => Protocol.Profiler.startPreciseCoverage({callCount: true}))
+      .then(() => Protocol.Runtime.compileScript({ expression: nested, sourceURL: arguments.callee.name, persistScript: true }))
+      .then((result) => Protocol.Runtime.runScript({ scriptId: result.result.scriptId }))
+      .then(InspectorTest.logMessage)
+      .then(Protocol.Profiler.takePreciseCoverage)
+      .then(LogSorted)
+      .then(() => Protocol.Runtime.evaluate({ expression: "f()" }))
+      .then(Protocol.Profiler.takePreciseCoverage)
+      .then(LogSorted)
+      .then(Protocol.Profiler.stopPreciseCoverage)
+      .then(Protocol.Profiler.disable)
+      .then(Protocol.Runtime.disable)
+      .then(ClearAndGC)
+      .then(next);
+  },
 ]);