diff -r 000000000000 -r 6474c204b198 layout/style/nsRuleData.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/style/nsRuleData.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,57 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsRuleData.h"
+
+#include "mozilla/Poison.h"
+#include <stdint.h>
+
+inline size_t
+nsRuleData::GetPoisonOffset()
+{
+  // Fill in mValueOffsets such that mValueStorage + mValueOffsets[i]
+  // will yield the frame poison value for all uninitialized value
+  // offsets.
+  static_assert(sizeof(uintptr_t) == sizeof(size_t),
+                "expect uintptr_t and size_t to be the same size");
+  static_assert(uintptr_t(-1) > uintptr_t(0),
+                "expect uintptr_t to be unsigned");
+  static_assert(size_t(-1) > size_t(0),
+                "expect size_t to be unsigned");
+  uintptr_t framePoisonValue = mozPoisonValue();
+  return size_t(framePoisonValue - uintptr_t(mValueStorage)) /
+         sizeof(nsCSSValue);
+}
+
+nsRuleData::nsRuleData(uint32_t aSIDs, nsCSSValue* aValueStorage,
+                       nsPresContext* aContext, nsStyleContext* aStyleContext)
+  : mSIDs(aSIDs),
+    mCanStoreInRuleTree(true),
+    mPresContext(aContext),
+    mStyleContext(aStyleContext),
+    mValueStorage(aValueStorage)
+{
+#ifndef MOZ_VALGRIND
+  size_t framePoisonOffset = GetPoisonOffset();
+  for (size_t i = 0; i < nsStyleStructID_Length; ++i) {
+    mValueOffsets[i] = framePoisonOffset;
+  }
+#endif
+}
+
+#ifdef DEBUG
+nsRuleData::~nsRuleData()
+{
+#ifndef MOZ_VALGRIND
+  // assert nothing in mSIDs has poison value
+  size_t framePoisonOffset = GetPoisonOffset();
+  for (size_t i = 0; i < nsStyleStructID_Length; ++i) {
+    NS_ABORT_IF_FALSE(!(mSIDs & (1 << i)) ||
+                      mValueOffsets[i] != framePoisonOffset,
+                      "value in SIDs was left with poison offset");
+  }
+#endif
+}
+#endif