[coverage] Add --trace-block-coverage flag

Tracing block coverage prints raw generated slots in the format:
{start_source_position,end_source_position}

Slots are printed before being mutated during coverage collection.

Bug: v8:6000
Change-Id: I3423e226a124e00c6b13ccd8dddb13d00e4989c7
Reviewed-on: https://chromium-review.googlesource.com/579374
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46807}

src/flag-definitions.h

@@ -290,6 +290,9 @@ DEFINE_IMPLICATION(track_field_types, track_fields)
 DEFINE_IMPLICATION(track_field_types, track_heap_object_fields)
 DEFINE_BOOL(type_profile, false, "collect type information")
 DEFINE_BOOL(block_coverage, false, "collect block coverage information")
+DEFINE_BOOL(trace_block_coverage, false,
+            "trace collected block coverage information")
+DEFINE_IMPLICATION(trace_block_coverage, block_coverage)
 DEFINE_BOOL(feedback_normalization, false,
             "feed back normalization to constructors")
 // TODO(jkummerow): This currently adds too much load on the stub cache.

src/interpreter/bytecode-generator.cc

@@ -786,6 +786,9 @@ Handle<BytecodeArray> BytecodeGenerator::FinalizeBytecode(Isolate* isolate) {
   if (info()->is_block_coverage_enabled()) {
     info()->set_coverage_info(
         isolate->factory()->NewCoverageInfo(block_coverage_builder_->slots()));
+    if (FLAG_trace_block_coverage) {
+      info()->coverage_info()->Print(info()->shared_info()->name());
+    }
   }
 
   if (HasStackOverflow()) return Handle<BytecodeArray>();

src/objects/debug-objects.cc

@@ -333,5 +333,25 @@ void CoverageInfo::ResetBlockCount(int slot_index) {
   set(slot_start + kSlotBlockCountIndex, Smi::kZero);
 }
 
+void CoverageInfo::Print(String* function_name) {
+  DCHECK(FLAG_trace_block_coverage);
+  DisallowHeapAllocation no_gc;
+
+  OFStream os(stdout);
+  os << "Coverage info (";
+  if (function_name->length() > 0) {
+    auto function_name_cstr = function_name->ToCString();
+    os << function_name_cstr.get();
+  } else {
+    os << "{anonymous}";
+  }
+  os << "):" << std::endl;
+
+  for (int i = 0; i < SlotCount(); i++) {
+    os << "{" << StartSourcePosition(i) << "," << EndSourcePosition(i) << "}"
+       << std::endl;
+  }
+}
+
 }  // namespace internal
 }  // namespace v8

src/objects/debug-objects.h

@@ -162,6 +162,9 @@ class CoverageInfo : public FixedArray {
 
   DECL_CAST(CoverageInfo)
 
+  // Print debug info.
+  void Print(String* function_name);
+
  private:
   static int FirstIndexForSlot(int slot_index) {
     return kFirstSlotIndex + slot_index * kSlotIndexCount;