Index: lib/Support/APFloat.cpp
===================================================================
--- lib/Support/APFloat.cpp
+++ lib/Support/APFloat.cpp
@@ -3045,7 +3045,9 @@
   assert(partCount()==2);
 
   sign = static_cast<unsigned int>(i2>>15);
-  if (myexponent==0 && mysignificand==0) {
+  if (myexponent != 0x7fff && mysignificand == 0) {
+    // if myexponent==0 this is a true zero
+    // if myexponent!=0 this is an unnormal value that happens to be zero
     // exponent, significand meaningless
     category = fcZero;
   } else if (myexponent==0x7fff && mysignificand==0x8000000000000000ULL) {
Index: unittests/ADT/APFloatTest.cpp
===================================================================
--- unittests/ADT/APFloatTest.cpp
+++ unittests/ADT/APFloatTest.cpp
@@ -983,6 +983,13 @@
   ASSERT_EQ("8.73183400000000010e+02", convertToString(873.1834, 0, 0, false));
   ASSERT_EQ("1.79769313486231570e+308",
             convertToString(1.7976931348623157E+308, 0, 0, false));
+
+  {
+    SmallString<64> Str;
+    APFloat UnnormalZero(APFloat::x87DoubleExtended(), APInt(80, {0, 1}));
+    UnnormalZero.toString(Str);
+    ASSERT_EQ("0", Str);
+  }
 }
 
 TEST(APFloatTest, toInteger) {