diff --git a/gfx/src/nsCoord.h b/gfx/src/nsCoord.h
index ea4668347d3b..ff41a68a9533 100644
--- a/gfx/src/nsCoord.h
+++ b/gfx/src/nsCoord.h
@@ -59,7 +59,7 @@ inline void VERIFY_COORD(nscoord aCoord) {
 
 inline nscoord NSToCoordRound(float aValue)
 {
-#if defined(XP_WIN32) && defined(_M_IX86) && !defined(__GNUC__)
+#if defined(XP_WIN32) && defined(_M_IX86) && !defined(__GNUC__) && !defined(__clang__)
   return NS_lroundup30(aValue);
 #else
   return nscoord(floorf(aValue + 0.5f));
@@ -68,7 +68,7 @@ inline nscoord NSToCoordRound(float aValue)
 
 inline nscoord NSToCoordRound(double aValue)
 {
-#if defined(XP_WIN32) && defined(_M_IX86) && !defined(__GNUC__)
+#if defined(XP_WIN32) && defined(_M_IX86) && !defined(__GNUC__) && !defined(__clang__)
   return NS_lroundup30((float)aValue);
 #else
   return nscoord(floor(aValue + 0.5f));
diff --git a/xpcom/ds/nsMathUtils.h b/xpcom/ds/nsMathUtils.h
index 558bdae32153..aedd31eab77c 100644
--- a/xpcom/ds/nsMathUtils.h
+++ b/xpcom/ds/nsMathUtils.h
@@ -35,7 +35,7 @@ inline int32_t NS_lround(double x)
 /* NS_roundup30 rounds towards infinity for positive and       */
 /* negative numbers.                                           */
 
-#if defined(XP_WIN32) && defined(_M_IX86) && !defined(__GNUC__)
+#if defined(XP_WIN32) && defined(_M_IX86) && !defined(__GNUC__) && !defined(__clang__)
 inline int32_t NS_lroundup30(float x)
 {
     /* Code derived from Laurent de Soras' paper at             */