Decouple root visitors from object visitors.

src/visitors.h

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_VISITORS_H_
+#define V8_VISITORS_H_
+
+#include "src/allocation.h"
+
+namespace v8 {
+namespace internal {
+
+class Object;
+
+#define ROOT_ID_LIST(V)                                                    \
+  V(kStringTable, "string_table", "(Internalized strings)")                \
+  V(kExternalStringsTable, "external_strings_table", "(External strings)") \
+  V(kStrongRootList, "strong_root_list", "(Strong roots)")                 \
+  V(kSmiRootList, "smi_root_list", "(Smi roots)")                          \
+  V(kBootstrapper, "bootstrapper", "(Bootstrapper)")                       \
+  V(kTop, "top", "(Isolate)")                                              \
+  V(kRelocatable, "relocatable", "(Relocatable)")                          \
+  V(kDebug, "debug", "(Debugger)")                                         \
+  V(kCompilationCache, "compilationcache", "(Compilation cache)")          \
+  V(kHandleScope, "handlescope", "(Handle scope)")                         \
+  V(kDispatchTable, "dispatchtable", "(Dispatch table)")                   \
+  V(kBuiltins, "builtins", "(Builtins)")                                   \
+  V(kGlobalHandles, "globalhandles", "(Global handles)")                   \
+  V(kEternalHandles, "eternalhandles", "(Eternal handles)")                \
+  V(kThreadManager, "threadmanager", "(Thread manager)")                   \
+  V(kStrongRoots, "strong roots", "(Strong roots)")                        \
+  V(kExtensions, "Extensions", "(Extensions)")
+
+class VisitorSynchronization : public AllStatic {
+ public:
+#define DECLARE_ENUM(enum_item, ignore1, ignore2) enum_item,
+  enum SyncTag { ROOT_ID_LIST(DECLARE_ENUM) kNumberOfSyncTags };
+#undef DECLARE_ENUM
+
+  static const char* const kTags[kNumberOfSyncTags];
+  static const char* const kTagNames[kNumberOfSyncTags];
+};
+
+enum class Root {
+#define DECLARE_ENUM(enum_item, ignore1, ignore2) enum_item,
+  ROOT_ID_LIST(DECLARE_ENUM)
+#undef DECLARE_ENUM
+  // TODO(ulan): Merge with the ROOT_ID_LIST.
+  kPartialSnapshotCache,
+  kWeakCollections
+};
+
+// Abstract base class for visiting, and optionally modifying, the
+// pointers contained in roots. Used in GC and serialization/deserialization.
+class RootVisitor BASE_EMBEDDED {
+ public:
+  virtual ~RootVisitor() {}
+
+  // Visits a contiguous arrays of pointers in the half-open range
+  // [start, end). Any or all of the values may be modified on return.
+  virtual void VisitRootPointers(Root root, Object** start, Object** end) = 0;
+
+  // Handy shorthand for visiting a single pointer.
+  virtual void VisitRootPointer(Root root, Object** p) {
+    VisitRootPointers(root, p, p + 1);
+  }
+
+  // Intended for serialization/deserialization checking: insert, or
+  // check for the presence of, a tag at this position in the stream.
+  // Also used for marking up GC roots in heap snapshots.
+  // TODO(ulan): Remove this.
+  virtual void Synchronize(VisitorSynchronization::SyncTag tag) {}
+};
+
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_VISITORS_H_