[llvm] r225123 - [APFloat][ADT] Fix sign handling logic for FMA results that truncate to zero.

[Lang Hames]

Author: lhames
Date: Sat Jan  3 19:20:55 2015
New Revision: 225123

URL: http://llvm.org/viewvc/llvm-project?rev=225123&view=rev
Log:
[APFloat][ADT] Fix sign handling logic for FMA results that truncate to zero.

This patch adds a check for underflow when truncating results back to lower
precision at the end of an FMA. The additional sign handling logic in
APFloat::fusedMultiplyAdd should only be performed when the result of the
addition step of the FMA (in full precision) is exactly zero, not when the
result underflows to zero.

Unit tests for this case and related signed zero FMA results are included.

Fixes <rdar://problem/18925551>.


Modified:
    llvm/trunk/lib/Support/APFloat.cpp
    llvm/trunk/unittests/ADT/APFloatTest.cpp

Modified: llvm/trunk/lib/Support/APFloat.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/APFloat.cpp?rev=225123&r1=225122&r2=225123&view=diff
==============================================================================
--- llvm/trunk/lib/Support/APFloat.cpp (original)
+++ llvm/trunk/lib/Support/APFloat.cpp Sat Jan  3 19:20:55 2015
@@ -1823,7 +1823,7 @@ APFloat::fusedMultiplyAdd(const APFloat
     /* If two numbers add (exactly) to zero, IEEE 754 decrees it is a
        positive zero unless rounding to minus infinity, except that
        adding two like-signed zeroes gives that zero.  */
-    if (category == fcZero && sign != addend.sign)
+    if (category == fcZero && !(fs & opUnderflow) && sign != addend.sign)
       sign = (rounding_mode == rmTowardNegative);
   } else {
     fs = multiplySpecials(multiplicand);

Modified: llvm/trunk/unittests/ADT/APFloatTest.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/ADT/APFloatTest.cpp?rev=225123&r1=225122&r2=225123&view=diff
==============================================================================
--- llvm/trunk/unittests/ADT/APFloatTest.cpp (original)
+++ llvm/trunk/unittests/ADT/APFloatTest.cpp Sat Jan  3 19:20:55 2015
@@ -475,6 +475,47 @@ TEST(APFloatTest, FMA) {
     EXPECT_EQ(12.0f, f1.convertToFloat());
   }
 
+  // Test for correct zero sign when answer is exactly zero.
+  // fma(1.0, -1.0, 1.0) -> +ve 0.
+  {
+    APFloat f1(1.0);
+    APFloat f2(-1.0);
+    APFloat f3(1.0);
+    f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+    EXPECT_TRUE(!f1.isNegative() && f1.isZero());
+  }
+
+  // Test for correct zero sign when answer is exactly zero and rounding towards
+  // negative.
+  // fma(1.0, -1.0, 1.0) -> +ve 0.
+  {
+    APFloat f1(1.0);
+    APFloat f2(-1.0);
+    APFloat f3(1.0);
+    f1.fusedMultiplyAdd(f2, f3, APFloat::rmTowardNegative);
+    EXPECT_TRUE(f1.isNegative() && f1.isZero());
+  }
+
+  // Test for correct (in this case -ve) sign when adding like signed zeros.
+  // Test fma(0.0, -0.0, -0.0) -> -ve 0.
+  {
+    APFloat f1(0.0);
+    APFloat f2(-0.0);
+    APFloat f3(-0.0);
+    f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+    EXPECT_TRUE(f1.isNegative() && f1.isZero());
+  }
+
+  // Test -ve sign preservation when small negative results underflow.
+  {
+    APFloat f1(APFloat::IEEEdouble,  "-0x1p-1074");
+    APFloat f2(APFloat::IEEEdouble, "+0x1p-1074");
+    APFloat f3(0.0);
+    f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+    EXPECT_TRUE(f1.isNegative() && f1.isZero());
+  }
+
+  // Test x87 extended precision case from http://llvm.org/PR20728.
   {
     APFloat M1(APFloat::x87DoubleExtended, 1.0);
     APFloat M2(APFloat::x87DoubleExtended, 1.0);