Introduce --zap-cpp-pointers (off by default)

src/frames.cc

Index: src/frames.cc
diff --git a/src/frames.cc b/src/frames.cc
index 90610fafb67500e67ce868e13e21276212831ad8..f5b9e8551c2625a9cda21298aeebbe0e7f941b70 100644
--- a/src/frames.cc
+++ b/src/frames.cc
@@ -2269,6 +2269,168 @@ Vector<StackFrame*> CreateStackMap(Isolate* isolate, Zone* zone) {
   return list.ToVector();
 }
 
+#ifdef DEBUG
+
+class HeapPointerZapper : public RootVisitor {
+ public:
+  explicit HeapPointerZapper(Isolate* isolate)
+      : isolate_(isolate),
+        heap_(isolate->heap()),
+        next_relocatable_(isolate->relocatable_top()),
+        next_trycatch_(isolate->try_catch_handler()) {}
+
+  // Stack layout (in case there is an exit frame):
+  // - entry frame's FP     <-- frame->fp()
+  // - saved C++ PC
+  // - last C++ stack slot  <-- cpp_start
+  // [C++ stuff...]
+  // - saved FP             <-- cpp_limit (exclusive)
+  // - saved CEntry PC
+  // - exit frame's SP      <-- caller_sp
+  // [exit frame's contents...]
+  // - exit frame's FP      <-- caller_fp
+
+  void Zap(ThreadLocalTop* t) {
+    // Part One: the current bunch of C++ frames on top of the stack.
+    ZapRange(reinterpret_cast<Address>(GetCurrentStackPosition()),
+             isolate_->c_entry_fp(t));
+
+    // Part Two: any nested C++ stack sections buried under some JS.
+    for (StackFrameIterator it(isolate_, t); !it.done(); it.Advance()) {
+      if (it.frame()->type() != StackFrame::ENTRY) continue;
+
+      EntryFrame* frame = reinterpret_cast<EntryFrame*>(it.frame());
+      // Add 2 * kPointerSize for next FP and caller's PC.
+      Address cpp_start = frame->fp() + 2 * kPointerSize;
+      const int offset = EntryFrameConstants::kCallerFPOffset;
+      Address caller_fp = Memory::Address_at(frame->fp() + offset);
+      ZapRange(cpp_start, caller_fp);
+    }
+  }
+
+  // RootVisitor interface.
+  void VisitRootPointers(Root root, Object** start, Object** end) override {
+    // Support for nested constellations of Relocatables and their skipped
+    // ranges is currently unimplemented.
+    DCHECK_NULL(next_relocatable_start_);
+    // Remember that the range [start, end) should be skipped.
+    next_relocatable_start_ = start;
+    next_relocatable_end_ = end;
+  }
+
+ private:
+  void ZapRange(Address start, Address maybe_exit_frame_fp) {
+    Address limit;
+    if (maybe_exit_frame_fp == nullptr) {
+      // Walk until the end of the stack, which has been saved by
+      // StackGuard::ThreadLocal::Initialize().
+      limit = reinterpret_cast<Address>(
+          isolate_->stack_guard()->stack_base_position());
+    } else {
+      // If there's a previous JavaScript exit frame, stop iterating at its top.
+      Address exit_frame_sp =
+          ExitFrame::ComputeStackPointer(maybe_exit_frame_fp);
+      // Subtract 2 * kPointerSize for CEntry PC and next FP.
+      limit = exit_frame_sp - 2 * kPointerSize;
+    }
+
+    Object** start_obj = reinterpret_cast<Object**>(start);
+    Object** limit_obj = reinterpret_cast<Object**>(limit);
+    for (cursor_ = start_obj; cursor_ < limit_obj; ++cursor_) {
+      // Check if we've found a Relocatable (a GC safe on-stack object that
+      // must not be zapped).
+      if (reinterpret_cast<intptr_t>(cursor_) ==
+          reinterpret_cast<intptr_t>(next_relocatable_)) {
+        // FlatStringReader restores its fields in PostGarbageCollection()
+        // (after we may have zapped them).
+        if (prev_relocatable_ != nullptr) {
+          prev_relocatable_->PostGarbageCollection();
+        }
+        // CustomArgumentsBase can inform us about its skippable fields
+        // via IterateInstance().
+        next_relocatable_->IterateInstance(this);
+        prev_relocatable_ = next_relocatable_;
+        next_relocatable_ = next_relocatable_->prev_;
+        continue;
+      }
+
+      // Check if we're running into a Relocatable's skippable range.
+      if (cursor_ == next_relocatable_start_) {
+        cursor_ = next_relocatable_end_;
+        next_relocatable_start_ = nullptr;  // Mark as consumed.
+      }
+
+      // Check if we've found a v8::TryCatch.
+      if (reinterpret_cast<intptr_t>(cursor_) ==
+          reinterpret_cast<intptr_t>(next_trycatch_)) {
+        cursor_ += (sizeof(v8::TryCatch) / kPointerSize) - 1;
+        next_trycatch_ = Isolate::next_try_catch(next_trycatch_);
+        continue;
+      }
+
+      if (!(*cursor_)->IsHeapObject()) continue;
+      HeapObject* obj = HeapObject::cast(*cursor_);
+      // If something has a heap pointer tag but doesn't point into the heap,
+      // ignore it. (We'll crash anyway if we try to use it later.)
+      if (!heap_->ContainsSlow(obj->address())) continue;
+      // TODO(jkummerow): Maps don't move, but they can die.
+      // Should we be more strict here?
+      // The difficulty is the "Allocate(map, allocation_space)" pattern.
+      MemoryChunk* chunk = MemoryChunk::FromAddress(obj->address());
+      if (chunk->NeverEvacuate()) continue;
+      // Keeping raw pointers to immortal immovables is fine.
+      if (SlowIsImmortalImmovable(obj)) continue;
+      // PrintF("Found heap pointer: *%p = %p, zapping it!\n", cursor_, obj);
+
+      // Local variables of type bool present a difficulty. When they get
+      // allocated over a garbage heap pointer, they tend to change it so
+      // little that it still looks like a heap pointer; but if we zap it
+      // by overwriting it with kZapValue, that makes such bools invalid.
+      // To avoid this, zap the values in such a way that the values of any
+      // bytes used for bools are preserved.
+#if V8_HOST_ARCH_64_BIT
+      const uintptr_t kMask = 0x0101010101010101ul;
+#else
+      const uintptr_t kMask = 0x01010101u;
+#endif
+      *(reinterpret_cast<uintptr_t*>(cursor_)) &= kMask;
+    }
+    if (prev_relocatable_ != nullptr) {
+      prev_relocatable_->PostGarbageCollection();
+    }
+  }
+
+  bool SlowIsImmortalImmovable(HeapObject* obj) {
+    // Fast paths.
+    if (obj->IsUndefined(isolate_)) return true;
+    // Slow lookup.
+    Object** roots = heap_->roots_array_start();
+    for (int index = 0; index < Heap::kRootListLength; index++) {
+      if (obj == roots[index]) {
+        return Heap::RootIsImmortalImmovable(index);
+      }
+    }
+    return false;
+  }
+
+  Isolate* isolate_;
+  Heap* heap_;
+  Relocatable* next_relocatable_;
+  Relocatable* prev_relocatable_ = nullptr;
+  Object** cursor_ = nullptr;
+  Object** next_relocatable_start_ = nullptr;
+  Object** next_relocatable_end_ = nullptr;
+  v8::TryCatch* next_trycatch_;
+};
+
+void ZapHeapPointersInCppFrames(Isolate* isolate, ThreadLocalTop* t) {
+  DCHECK(FLAG_zap_cpp_pointers);
+  if (t == NULL) t = isolate->thread_local_top();
+  HeapPointerZapper zapper(isolate);
+  zapper.Zap(t);
+}
+
+#endif
 
 }  // namespace internal
 }  // namespace v8