[debugger] add utility to print break location.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index 42192b0b5340bebf36e94fadd38b1862c2241831..718bb2d097c4865e5aa99855726955306c5ed51c 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -1853,6 +1853,10 @@ void Debug::OnDebugBreak(Handle<Object> break_points_hit, bool auto_continue) {
   // Bail out if there is no listener for this event
   if (ignore_events()) return;
 
+#ifdef DEBUG
+  PrintBreakLocation();
+#endif  // DEBUG
+
   HandleScope scope(isolate_);
   // Create the event data object.
   Handle<Object> event_data;
@@ -2314,6 +2318,45 @@ void Debug::ProcessDebugMessages(bool debug_command_only) {
   OnDebugBreak(isolate_->factory()->undefined_value(), debug_command_only);
 }
 
+#ifdef DEBUG
+void Debug::PrintBreakLocation() {
+  if (!FLAG_print_break_location) return;
+  HandleScope scope(isolate_);
+  JavaScriptFrameIterator iterator(isolate_);
+  if (iterator.done()) return;
+  JavaScriptFrame* frame = iterator.frame();
+  FrameSummary summary = GetFirstFrameSummary(frame);
+  int source_position =
+      summary.abstract_code()->SourcePosition(summary.code_offset());
+  Handle<Object> script_obj(summary.function()->shared()->script(), isolate_);
+  PrintF("[debug] break in function '");
+  summary.function()->PrintName();
+  PrintF("'.\n");
+  if (script_obj->IsScript()) {
+    Handle<Script> script = Handle<Script>::cast(script_obj);
+    Handle<String> source(String::cast(script->source()));
+    Script::InitLineEnds(script);
+    int line = Script::GetLineNumber(script, source_position);
+    int column = Script::GetColumnNumber(script, source_position);
+    Handle<FixedArray> line_ends(FixedArray::cast(script->line_ends()));
+    int line_start =
+        line == 0 ? 0 : Smi::cast(line_ends->get(line - 1))->value();
+    int line_end = Smi::cast(line_ends->get(line))->value();
+    DisallowHeapAllocation no_gc;
+    String::FlatContent content = source->GetFlatContent();
+    if (content.IsOneByte()) {
+      PrintF("[debug] %.*s\n", line_end - line_start,
+             content.ToOneByteVector().start() + line_start);
+      PrintF("[debug] ");
+      for (int i = 0; i < column; i++) PrintF(" ");
+      PrintF("^\n");
+      return;

[vogelheim, 2016/02/25 10:42:47]

super nitpick: Maybe remove? return in this branch but not the other is a bit
inconsistent, since they're both at the end of the function execution.

+    } else {
+      PrintF("[debug] at line %d column %d\n", line, column);
+    }
+  }
+}
+#endif  // DEBUG
 
 DebugScope::DebugScope(Debug* debug)
     : debug_(debug),