[flang] Retain the sign of the argument for the result of fraction(0)

[vdonaldson]

Summary

The f18 clause 16.9.80 description of the FRACTION(X) intrinsic states:

Result Value. The result has the value ....
If X has the value zero, the result is zero.
If X is an IEEE NaN, the result is that NaN.
If X is an IEEE infinity, the result is an IEEE NaN.

This clause does not specify whether fraction(-0.0) should be -0.0 or +0.0.
However, a folded result and a runtime result should be consistent, and
returning -0.0 is more in line with the result for fraction(NaN).

For this test:

print '(2f6.1)', 0.0, fraction(0.0)
call f(0.0)
print '(2f6.1)', -0.0, fraction(-0.0)
call f(-0.0)
end


subroutine f(x)
  print '(2f6.1)', x, fraction(x)
end

Current output is:

 0.0   0.0
 0.0   0.0
-0.0  -0.0
-0.0   0.0

Change that to:

 0.0   0.0
 0.0   0.0
-0.0  -0.0
-0.0  -0.0

[vdonaldson]

https://reviews.llvm.org/D151471

flang/runtime/numeric.cpp

 
 // FRACTION (16.9.80)
 template <typename T> inline T Fraction(T x) {
   if (std::isnan(x)) {
     return x; // NaN -> same NaN
   } else if (std::isinf(x)) {
     return std::numeric_limits<T>::quiet_NaN(); // +/-Inf -> NaN
   } else if (x == 0) {
-    return 0; // 0 -> 0
+    return x; // 0 -> same 0
   } else {
     int ignoredExp;
     return std::frexp(x, &ignoredExp);
   }
 }
 
 // MOD & MODULO (16.9.135, .136)
 template <bool IS_MODULO, typename T>