diff --git a/gfx/2d/BaseRect.h b/gfx/2d/BaseRect.h
index 593e7468ce2..c0719112a61 100644
--- a/gfx/2d/BaseRect.h
+++ b/gfx/2d/BaseRect.h
@@ -226,20 +226,6 @@ struct BaseRect {
   }
   void Inflate(const SizeT& aSize) { Inflate(aSize.width, aSize.height); }
 
-  void InflateToMultiple(const SizeT& aMultiple)
-  {
-    T xMost = XMost();
-    T yMost = YMost();
-
-    x = static_cast<T>(floor(x / aMultiple.width)) * aMultiple.width;
-    y = static_cast<T>(floor(y / aMultiple.height)) * aMultiple.height;
-    xMost = static_cast<T>(ceil(x / aMultiple.width)) * aMultiple.width;
-    yMost = static_cast<T>(ceil(y / aMultiple.height)) * aMultiple.height;
-
-    width = xMost - x;
-    height = yMost - y;
-  }
-
   void Deflate(T aD) { Deflate(aD, aD); }
   void Deflate(T aDx, T aDy)
   {
diff --git a/gfx/2d/NumericTools.h b/gfx/2d/NumericTools.h
new file mode 100644
index 00000000000..a1004e17bc3
--- /dev/null
+++ b/gfx/2d/NumericTools.h
@@ -0,0 +1,37 @@
+/* -*- Mode: C++; tab-width: 20; 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/. */
+
+#ifndef MOZILLA_GFX_NUMERICTOOLS_H_
+#define MOZILLA_GFX_NUMERICTOOLS_H_
+
+// Returns the largest multiple of aMultiplied that's <= x.
+// Same as int32_t(floor(double(x) / aMultiplier)) * aMultiplier,
+// but faster.
+static int32_t
+RoundDownToMultiple(int32_t x, int32_t aMultiplier)
+{
+  // We don't use float division + floor because that's hard for the compiler
+  // to optimize.
+  int mod = x % aMultiplier;
+  if (x > 0) {
+    return x - mod;
+  }
+  return mod ? x - aMultiplier - mod : x;
+}
+
+// Returns the smallest multiple of aMultiplied that's >= x.
+// Same as int32_t(ceil(double(x) / aMultiplier)) * aMultiplier,
+// but faster.
+static int32_t
+RoundUpToMultiple(int32_t x, int32_t aMultiplier)
+{
+  int mod = x % aMultiplier;
+  if (x > 0) {
+    return mod ? x + aMultiplier - mod : x;
+  }
+  return x - mod;
+}
+
+#endif /* MOZILLA_GFX_NUMERICTOOLS_H_ */
diff --git a/gfx/2d/moz.build b/gfx/2d/moz.build
index 48d90b25f3d..54fcafad87e 100644
--- a/gfx/2d/moz.build
+++ b/gfx/2d/moz.build
@@ -26,6 +26,7 @@ EXPORTS.mozilla.gfx += [
     'Helpers.h',
     'Logging.h',
     'Matrix.h',
+    'NumericTools.h',
     'PathHelpers.h',
     'PatternHelpers.h',
     'Point.h',
diff --git a/gfx/src/nsRect.h b/gfx/src/nsRect.h
index e02750ae1f9..e424cefd1da 100644
--- a/gfx/src/nsRect.h
+++ b/gfx/src/nsRect.h
@@ -14,6 +14,7 @@
 #include "gfxCore.h"                    // for NS_GFX
 #include "mozilla/Likely.h"             // for MOZ_UNLIKELY
 #include "mozilla/gfx/BaseRect.h"       // for BaseRect
+#include "mozilla/gfx/NumericTools.h"   // for RoundUpToMultiple, RoundDownToMultiple
 #include "nsCoord.h"                    // for nscoord, etc
 #include "nsISupportsImpl.h"            // for MOZ_COUNT_CTOR, etc
 #include "nsPoint.h"                    // for nsIntPoint, nsPoint
@@ -207,6 +208,20 @@ struct NS_GFX nsIntRect :
     return r;
   }
 
+  void InflateToMultiple(const nsIntSize& aTileSize)
+  {
+    int32_t xMost = XMost();
+    int32_t yMost = YMost();
+
+    x = RoundDownToMultiple(x, aTileSize.width);
+    y = RoundDownToMultiple(y, aTileSize.height);
+    xMost = RoundUpToMultiple(xMost, aTileSize.width);
+    yMost = RoundUpToMultiple(yMost, aTileSize.height);
+
+    width = xMost - x;
+    height = yMost - y;
+  }
+
   // This is here only to keep IPDL-generated code happy. DO NOT USE.
   bool operator==(const nsIntRect& aRect) const
   {