[debug] do not report unnecessary coverage data.

src/debug/debug-coverage.cc

Index: src/debug/debug-coverage.cc
diff --git a/src/debug/debug-coverage.cc b/src/debug/debug-coverage.cc
index 1489f4168b428f66986944634a08111b96f07a12..c60062ff3f2d8fec95485515f8d5b905b688a884 100644
--- a/src/debug/debug-coverage.cc
+++ b/src/debug/debug-coverage.cc
@@ -119,7 +119,7 @@ Coverage* Coverage::Collect(Isolate* isolate, bool reset_count) {
 
     // 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;
@@ -133,6 +133,9 @@ Coverage* Coverage::Collect(Isolate* isolate, bool reset_count) {
       std::sort(sorted.begin(), sorted.end(), CompareSharedFunctionInfo);
     }
 
+    // Stack to track nested functions, referring function by index.
+    std::vector<size_t> nesting;

[caseq, 2017/03/23 00:09:56]

nit: perhaps std::vector<const CoverageFunction*> for less indirection?

[Yang, 2017/03/23 07:43:16]

I considered that. However, std::vector reallocates and copies its content when
growing the backing store. Raw pointers into the backing store will then become
invalid.

+
     // Use sorted list to reconstruct function nesting.
     for (SharedFunctionInfo* info : sorted) {
       int start = StartPosition(info);
@@ -142,9 +145,22 @@ Coverage* Coverage::Collect(Isolate* isolate, bool reset_count) {
         count = info->has_reported_binary_coverage() ? 0 : 1;
         info->set_has_reported_binary_coverage(true);
       }
-      Handle<String> name(info->DebugName(), isolate);
-      functions->emplace_back(start, end, count, name);
+      // Find the correct outer function based on start position.
+      while (!nesting.empty() && functions->at(nesting.back()).end <= start) {
+        nesting.pop_back();
+      }
+      if (count > 0 ||
+          (!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.
+        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;
 }