Add VerifiedMemory helper for write-barrier verification.

runtime/vm/verified_memory.cc

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "vm/verified_memory.h"
+
+namespace dart {
+
+#if defined(DEBUG)
+
+DEFINE_FLAG(bool, verified_mem, false,
+            "Enable write-barrier verification mode (slow, DEBUG only).");
+DEFINE_FLAG(int, verified_mem_max_reserve_mb, 16,
+            "When verified_mem is true, largest supported reservation (MB).");
+
+
+VirtualMemory* VerifiedMemory::ReserveInternal(intptr_t size) {
+  if (size > offset()) {
+    FATAL1("Requested reservation of %" Pd " bytes exceeds the limit. "
+           "Use --verified_mem_max_reserve_mb to increase it.", size);
+  }
+  VirtualMemory* result = VirtualMemory::Reserve(size + offset());
+  if (result != NULL) {
+    // Commit the offset part of the reservation (writable, not executable).

[Ivan Posva, 2014/10/29 05:06:07]

This is a little light on comments given what is going on here: You basically
rely on the VirtualMemory class to remember the original reservation size and
thus by committing before truncating you create the shadow are at the
preselected offset.

[koda, 2014/10/29 19:47:31]

Commented here, and also documented Truncate's 'try_unmap' parameter in
virtual_memory.h.

+    result->Commit(result->start() + offset(), size, /* executable = */ false);
+    result->Truncate(size, false);
+  }
+  return result;
+}
+
+#endif  // DEBUG
+
+}  // namespace dart