[ADT] Fix PR20728 - Incorrect APFloat::fusedMultiplyAdd results for x86_fp80.

As detailed at http://llvm.org/PR20728, due to an internal overflow in
APFloat::multiplySignificand the APFloat::fusedMultiplyAdd method can return
incorrect results for x87DoubleExtended (x86_fp80) values. This commonly
manifests as incorrect constant folding of libm fmal calls on x86. E.g.

fmal(1.0L, 1.0L, 3.0L) == 0.0L      (should be 4.0L)

This patch fixes PR20728 by adding an extra bit to the significand for
intermediate results of APFloat::multiplySignificand, avoiding the overflow.

llvm-svn: 222374

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/APFloatTest.cpp b/llvm/unittests/ADT/APFloatTest.cpp
index a86be5af375..c7ec16ba803 100644
--- a/llvm/unittests/ADT/APFloatTest.cpp
+++ b/llvm/unittests/ADT/APFloatTest.cpp
@@ -474,6 +474,18 @@ TEST(APFloatTest, FMA) {
     f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
     EXPECT_EQ(12.0f, f1.convertToFloat());
   }
+
+  {
+    APFloat M1(APFloat::x87DoubleExtended, 1.0);
+    APFloat M2(APFloat::x87DoubleExtended, 1.0);
+    APFloat A(APFloat::x87DoubleExtended, 3.0);
+
+    bool losesInfo = false;
+    M1.fusedMultiplyAdd(M1, A, APFloat::rmNearestTiesToEven);
+    M1.convert(APFloat::IEEEsingle, APFloat::rmNearestTiesToEven, &losesInfo);
+    EXPECT_FALSE(losesInfo);
+    EXPECT_EQ(4.0f, M1.convertToFloat());
+  }
 }
 
 TEST(APFloatTest, MinNum) {