Fix a problem with APFloat::roundToIntegral where it would return incorrect results for negative inputs to trunc. Add unit tests to verify this behavior.

llvm-svn: 161929

2 files changed, 42 insertions, 0 deletions

diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index 5ea75a621ad..84021de0bf4 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -1774,19 +1774,31 @@ APFloat::opStatus APFloat::roundToIntegral(roundingMode rounding_mode) {
   // precision of our format, and then subtract it back off again.  The choice
   // of rounding modes for the addition/subtraction determines the rounding mode
   // for our integral rounding as well.
+  // NOTE: When the input value is negative, we do subtractation followed by
+  // addition instead.
   APInt IntegerConstant(NextPowerOf2(semanticsPrecision(*semantics)), 1);
   IntegerConstant <<= semanticsPrecision(*semantics)-1;
   APFloat MagicConstant(*semantics);
   fs = MagicConstant.convertFromAPInt(IntegerConstant, false,
                                       rmNearestTiesToEven);
+  MagicConstant.copySign(*this);
+
   if (fs != opOK)
     return fs;
 
+  // Preserve the input sign so that we can handle 0.0/-0.0 cases correctly.
+  bool inputSign = isNegative();
+
   fs = add(MagicConstant, rounding_mode);
   if (fs != opOK && fs != opInexact)
     return fs;
 
   fs = subtract(MagicConstant, rounding_mode);
+
+  // Restore the input sign.
+  if (inputSign != isNegative())
+    changeSign();
+
   return fs;
 }
 
diff --git a/llvm/unittests/ADT/APFloatTest.cpp b/llvm/unittests/ADT/APFloatTest.cpp
index cc207f764da..caa288afaf5 100644
--- a/llvm/unittests/ADT/APFloatTest.cpp
+++ b/llvm/unittests/ADT/APFloatTest.cpp
@@ -648,6 +648,36 @@ TEST(APFloatTest, exactInverse) {
   EXPECT_FALSE(APFloat(1.40129846e-45f).getExactInverse(0));
 }
 
+TEST(APFloatTest, roundToIntegral) {
+  APFloat T(-0.5), S(3.14), P(0.0);
+
+  P = T;
+  P.roundToIntegral(APFloat::rmTowardZero);
+  EXPECT_EQ(-0.0, P.convertToDouble());
+  P = T;
+  P.roundToIntegral(APFloat::rmTowardNegative);
+  EXPECT_EQ(-1.0, P.convertToDouble());
+  P = T;
+  P.roundToIntegral(APFloat::rmTowardPositive);
+  EXPECT_EQ(-0.0, P.convertToDouble());
+  P = T;
+  P.roundToIntegral(APFloat::rmNearestTiesToEven);
+  EXPECT_EQ(-0.0, P.convertToDouble());
+
+  P = S;
+  P.roundToIntegral(APFloat::rmTowardZero);
+  EXPECT_EQ(3.0, P.convertToDouble());
+  P = S;
+  P.roundToIntegral(APFloat::rmTowardNegative);
+  EXPECT_EQ(3.0, P.convertToDouble());
+  P = S;
+  P.roundToIntegral(APFloat::rmTowardPositive);
+  EXPECT_EQ(4.0, P.convertToDouble());
+  P = S;
+  P.roundToIntegral(APFloat::rmNearestTiesToEven);
+  EXPECT_EQ(3.0, P.convertToDouble());
+}
+
 TEST(APFloatTest, getLargest) {
   EXPECT_EQ(3.402823466e+38f, APFloat::getLargest(APFloat::IEEEsingle).convertToFloat());
   EXPECT_EQ(1.7976931348623158e+308, APFloat::getLargest(APFloat::IEEEdouble).convertToDouble());