Index: lib/builtins/fp_lib.h
===================================================================
--- lib/builtins/fp_lib.h
+++ lib/builtins/fp_lib.h
@@ -271,8 +271,18 @@
   // 1) +/- inf returns +inf; NaN returns NaN
   // 2) 0.0 returns -inf
   if (exp == maxExponent) {
-    if (((rep & signBit) == 0) || (x != x)) {
-      return x; // NaN or +inf: return x
+    if (x != x) {
+      // NaN
+#if defined(__APPLE__) && defined(__i386__)
+      // Darwin i386 libm when given a NaN with a raised
+      // sign bit returns a NaN without a sign bit.
+      if (rep & signBit) {
+        return fromRep((~signBit) & rep);
+      }
+#endif
+      return x;
+    } else if ((rep & signBit) == 0) {
+      return x; // +inf: return x
     } else {
       return -x; // -inf: return -x
     }