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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161929 91177308-0d34-0410-b5e6-96231b3b80d8

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;
 }