Index: llvm/trunk/lib/Support/APFloat.cpp
===================================================================
--- llvm/trunk/lib/Support/APFloat.cpp
+++ llvm/trunk/lib/Support/APFloat.cpp
@@ -1740,44 +1740,20 @@
   return fs;
 }
 
-/* Normalized llvm frem (C fmod).
-   This is not currently correct in all cases.  */
+/* Normalized llvm frem (C fmod). */
 IEEEFloat::opStatus IEEEFloat::mod(const IEEEFloat &rhs) {
   opStatus fs;
   fs = modSpecials(rhs);
 
-  if (isFiniteNonZero() && rhs.isFiniteNonZero()) {
-    IEEEFloat V = *this;
-    unsigned int origSign = sign;
-
-    fs = V.divide(rhs, rmNearestTiesToEven);
-    if (fs == opDivByZero)
-      return fs;
-
-    int parts = partCount();
-    integerPart *x = new integerPart[parts];
-    bool ignored;
-    fs = V.convertToInteger(makeMutableArrayRef(x, parts),
-                            parts * integerPartWidth, true, rmTowardZero,
-                            &ignored);
-    if (fs == opInvalidOp) {
-      delete[] x;
-      return fs;
-    }
-
-    fs = V.convertFromZeroExtendedInteger(x, parts * integerPartWidth, true,
-                                          rmNearestTiesToEven);
-    assert(fs==opOK);   // should always work
-
-    fs = V.multiply(rhs, rmNearestTiesToEven);
-    assert(fs==opOK || fs==opInexact);   // should not overflow or underflow
-
+  while (isFiniteNonZero() && rhs.isFiniteNonZero() &&
+         compareAbsoluteValue(rhs) != cmpLessThan) {
+    IEEEFloat V = scalbn(rhs, ilogb(*this) - ilogb(rhs), rmNearestTiesToEven);
+    if (compareAbsoluteValue(V) == cmpLessThan)
+      V = scalbn(V, -1, rmNearestTiesToEven);
+    V.sign = sign;
+  
     fs = subtract(V, rmNearestTiesToEven);
-    assert(fs==opOK || fs==opInexact);   // likewise
-
-    if (isZero())
-      sign = origSign;    // IEEE754 requires this
-    delete[] x;
+    assert(fs==opOK);
   }
   return fs;
 }
Index: llvm/trunk/unittests/ADT/APFloatTest.cpp
===================================================================
--- llvm/trunk/unittests/ADT/APFloatTest.cpp
+++ llvm/trunk/unittests/ADT/APFloatTest.cpp
@@ -3192,10 +3192,73 @@
   EXPECT_TRUE(APFloat(APFloat::IEEEdouble(), "0x1.c60f120d9f87cp-1").bitwiseIsEqual(Frac));
 }
 
+TEST(APFloatTest, mod) {
+  {
+    APFloat f1(APFloat::IEEEdouble(), "1.5");
+    APFloat f2(APFloat::IEEEdouble(), "1.0");
+    APFloat expected(APFloat::IEEEdouble(), "0.5");
+    EXPECT_EQ(f1.mod(f2), APFloat::opOK);
+    EXPECT_TRUE(f1.bitwiseIsEqual(expected));
+  }
+  {
+    APFloat f1(APFloat::IEEEdouble(), "0.5");
+    APFloat f2(APFloat::IEEEdouble(), "1.0");
+    APFloat expected(APFloat::IEEEdouble(), "0.5");
+    EXPECT_EQ(f1.mod(f2), APFloat::opOK);
+    EXPECT_TRUE(f1.bitwiseIsEqual(expected));
+  }
+  {
+    APFloat f1(APFloat::IEEEdouble(), "0x1.3333333333333p-2"); // 0.3
+    APFloat f2(APFloat::IEEEdouble(), "0x1.47ae147ae147bp-7"); // 0.01
+    APFloat expected(APFloat::IEEEdouble(),
+                     "0x1.47ae147ae1471p-7"); // 0.009999999999999983
+    EXPECT_EQ(f1.mod(f2), APFloat::opOK);
+    EXPECT_TRUE(f1.bitwiseIsEqual(expected));
+  }
+  {
+    APFloat f1(APFloat::IEEEdouble(), "0x1p64"); // 1.8446744073709552e19
+    APFloat f2(APFloat::IEEEdouble(), "1.5");
+    APFloat expected(APFloat::IEEEdouble(), "1.0");
+    EXPECT_EQ(f1.mod(f2), APFloat::opOK);
+    EXPECT_TRUE(f1.bitwiseIsEqual(expected));
+  }
+  {
+    APFloat f1(APFloat::IEEEdouble(), "0x1p1000");
+    APFloat f2(APFloat::IEEEdouble(), "0x1p-1000");
+    APFloat expected(APFloat::IEEEdouble(), "0.0");
+    EXPECT_EQ(f1.mod(f2), APFloat::opOK);
+    EXPECT_TRUE(f1.bitwiseIsEqual(expected));
+  }
+  {
+    APFloat f1(APFloat::IEEEdouble(), "0.0");
+    APFloat f2(APFloat::IEEEdouble(), "1.0");
+    APFloat expected(APFloat::IEEEdouble(), "0.0");
+    EXPECT_EQ(f1.mod(f2), APFloat::opOK);
+    EXPECT_TRUE(f1.bitwiseIsEqual(expected));
+  }
+  {
+    APFloat f1(APFloat::IEEEdouble(), "1.0");
+    APFloat f2(APFloat::IEEEdouble(), "0.0");
+    EXPECT_EQ(f1.mod(f2), APFloat::opInvalidOp);
+    EXPECT_TRUE(f1.isNaN());
+  }
+  {
+    APFloat f1(APFloat::IEEEdouble(), "0.0");
+    APFloat f2(APFloat::IEEEdouble(), "0.0");
+    EXPECT_EQ(f1.mod(f2), APFloat::opInvalidOp);
+    EXPECT_TRUE(f1.isNaN());
+  }
+  {
+    APFloat f1 = APFloat::getInf(APFloat::IEEEdouble(), false);
+    APFloat f2(APFloat::IEEEdouble(), "1.0");
+    EXPECT_EQ(f1.mod(f2), APFloat::opInvalidOp);
+    EXPECT_TRUE(f1.isNaN());
+  }
+}
+
 TEST(APFloatTest, PPCDoubleDoubleAddSpecial) {
   using DataType = std::tuple<uint64_t, uint64_t, uint64_t, uint64_t,
-                              APFloat::fltCategory, APFloat::roundingMode>;
-  DataType Data[] = {
+                              APFloat::fltCategory, APFloat::roundingMode>;  DataType Data[] = {
       // (1 + 0) + (-1 + 0) = fcZero
       std::make_tuple(0x3ff0000000000000ull, 0, 0xbff0000000000000ull, 0,
                       APFloat::fcZero, APFloat::rmNearestTiesToEven),