[debugger] fix step-next for tail calls.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index 3a6e0ac0de86cc3ae4d9929eb8323da7bc9aa495..7c76742bb973cb3fa6fc605ea1fe4316ad608fde 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -108,6 +108,9 @@ int BreakLocation::CodeIterator::GetModeMask(BreakLocatorType type) {
   mask |= RelocInfo::ModeMask(RelocInfo::STATEMENT_POSITION);
   mask |= RelocInfo::ModeMask(RelocInfo::DEBUG_BREAK_SLOT_AT_RETURN);
   mask |= RelocInfo::ModeMask(RelocInfo::DEBUG_BREAK_SLOT_AT_CALL);
+  if (isolate()->is_tail_call_elimination_enabled()) {
+    mask |= RelocInfo::ModeMask(RelocInfo::DEBUG_BREAK_SLOT_AT_TAIL_CALL);
+  }
   if (type == ALL_BREAK_LOCATIONS) {
     mask |= RelocInfo::ModeMask(RelocInfo::DEBUG_BREAK_SLOT_AT_POSITION);
     mask |= RelocInfo::ModeMask(RelocInfo::DEBUGGER_STATEMENT);
@@ -161,6 +164,10 @@ BreakLocation BreakLocation::CodeIterator::GetBreakLocation() {
     type = DEBUG_BREAK_SLOT_AT_RETURN;
   } else if (RelocInfo::IsDebugBreakSlotAtCall(rmode())) {
     type = DEBUG_BREAK_SLOT_AT_CALL;
+  } else if (RelocInfo::IsDebugBreakSlotAtTailCall(rmode())) {
+    type = isolate()->is_tail_call_elimination_enabled()
+               ? DEBUG_BREAK_SLOT_AT_TAIL_CALL
+               : DEBUG_BREAK_SLOT_AT_CALL;
   } else if (RelocInfo::IsDebuggerStatement(rmode())) {
     type = DEBUGGER_STATEMENT;
   } else if (RelocInfo::IsDebugBreakSlot(rmode())) {
@@ -226,6 +233,10 @@ BreakLocation::BytecodeArrayIterator::GetDebugBreakType() {
     return DEBUGGER_STATEMENT;
   } else if (bytecode == interpreter::Bytecode::kReturn) {
     return DEBUG_BREAK_SLOT_AT_RETURN;
+  } else if (bytecode == interpreter::Bytecode::kTailCall) {
+    return isolate()->is_tail_call_elimination_enabled()
+               ? DEBUG_BREAK_SLOT_AT_TAIL_CALL
+               : DEBUG_BREAK_SLOT_AT_CALL;
   } else if (interpreter::Bytecodes::IsCallOrNew(bytecode)) {
     return DEBUG_BREAK_SLOT_AT_CALL;
   } else if (source_position_iterator_.is_statement()) {
@@ -604,22 +615,26 @@ void Debug::Break(JavaScriptFrame* frame) {
   Address target_fp = thread_local_.target_fp_;
   Address last_fp = thread_local_.last_fp_;
 
-  bool step_break = true;
+  bool step_break = false;
   switch (step_action) {
     case StepNone:
       return;
     case StepOut:
       // Step out has not reached the target frame yet.
       if (current_fp < target_fp) return;
+      step_break = true;
       break;
     case StepNext:
       // Step next should not break in a deeper frame.
       if (current_fp < target_fp) return;
+      // For step-next, a tail call is like a return and should break.
+      step_break = location.IsTailCall();
     // Fall through.
     case StepIn: {
       FrameSummary summary = GetFirstFrameSummary(frame);
       int offset = summary.code_offset();
-      step_break = location.IsReturn() || (current_fp != last_fp) ||
+      step_break = step_break || location.IsReturn() ||
+                   (current_fp != last_fp) ||
                    (thread_local_.last_statement_position_ !=
                     location.abstract_code()->SourceStatementPosition(offset));
       break;
@@ -1015,8 +1030,10 @@ void Debug::PrepareStep(StepAction step_action) {
   BreakLocation location =
       BreakLocation::FromCodeOffset(debug_info, call_offset);
 
-  // At a return statement we will step out either way.
+  // Any step at a return is a step-out.
   if (location.IsReturn()) step_action = StepOut;
+  // A step-next at a tail call is a step-out.
+  if (location.IsTailCall() && step_action == StepNext) step_action = StepOut;
 
   thread_local_.last_statement_position_ =
       debug_info->abstract_code()->SourceStatementPosition(
@@ -1592,7 +1609,7 @@ bool Debug::IsBreakAtReturn(JavaScriptFrame* frame) {
   Handle<DebugInfo> debug_info(shared->GetDebugInfo());
   BreakLocation location =
       BreakLocation::FromCodeOffset(debug_info, summary.code_offset());
-  return location.IsReturn();
+  return location.IsReturn() || location.IsTailCall();
 }