diff --git a/layout/base/nsCSSRendering.cpp b/layout/base/nsCSSRendering.cpp
index a66a9e429313..ba5c49ed7bae 100644
--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -2805,6 +2805,28 @@ nsCSSRendering::PaintBackground(nsPresContext* aPresContext,
                         aBorder, aPadding, aUsePrintSettings, aBGClipRect);
 }
 
+inline nscoord IntDivFloor(nscoord aDividend, nscoord aDivisor)
+{
+  NS_PRECONDITION(aDivisor > 0,
+                  "this function only works for positive divisors");
+  // ANSI C, ISO 9899:1999 section 6.5.5 defines integer division as
+  // truncation of the result towards zero.  Earlier C standards, as
+  // well as the C++ standards (1998 and 2003) do not, but we depend
+  // on it elsewhere.
+  return (aDividend < 0 ? (aDividend - aDivisor + 1) : aDividend) / aDivisor;
+}
+
+inline nscoord IntDivCeil(nscoord aDividend, nscoord aDivisor)
+{
+  NS_PRECONDITION(aDivisor > 0,
+                  "this function only works for positive divisors");
+  // ANSI C, ISO 9899:1999 section 6.5.5 defines integer division as
+  // truncation of the result towards zero.  Earlier C standards, as
+  // well as the C++ standards (1998 and 2003) do not, but we depend
+  // on it elsewhere.
+  return (aDividend > 0 ? (aDividend + aDivisor - 1) : aDividend) / aDivisor;
+}
+
 /**
  * Return the largest 'v' such that v = aTileOffset + N*aTileSize, for some
  * integer N, and v <= aDirtyStart.
@@ -2813,8 +2835,8 @@ static nscoord
 FindTileStart(nscoord aDirtyStart, nscoord aTileOffset, nscoord aTileSize)
 {
   // Find largest integer N such that aTileOffset + N*aTileSize <= aDirtyStart
-  PRInt32 n = NSToIntFloor((aDirtyStart*1.0f - aTileOffset)/aTileSize);
-  return aTileOffset + n*aTileSize;
+  return aTileOffset +
+         IntDivFloor(aDirtyStart - aTileOffset, aTileSize) * aTileSize;
 }
 
 /**
@@ -2825,8 +2847,8 @@ static nscoord
 FindTileEnd(nscoord aDirtyEnd, nscoord aTileOffset, nscoord aTileSize)
 {
   // Find smallest integer N such that aTileOffset + N*aTileSize >= aDirtyEnd
-  PRInt32 n = NSToIntCeil((aDirtyEnd*1.0f - aTileOffset)/aTileSize);
-  return aTileOffset + n*aTileSize;
+  return aTileOffset +
+         IntDivCeil(aDirtyEnd - aTileOffset, aTileSize) * aTileSize;
 }
 
 void