[flang][runtime] Fix NEAREST() when exponent decreases (#75368)

When the result of NEAREST() has an exponent less than that of the
argument (e.g., NEAREST(1.,-1.) and NEAREST(-1.,1.)), the result was
wrong, because the increment value uses the result of SPACING() in terms
of the argument. Fix by just calling into the C runtime routine
std::nextafter().

Author: klausler

flang/runtime/numeric.cpp

@@ -261,12 +261,10 @@ template <int PREC, typename T> inline RT_API_ATTRS T Spacing(T x) {
 // NEAREST (16.9.139)
 template <int PREC, typename T>
 inline RT_API_ATTRS T Nearest(T x, bool positive) {
-  auto spacing{Spacing<PREC>(x)};
-  if (x == 0) {
-    auto least{std::numeric_limits<T>::denorm_min()};
-    return positive ? least : -least;
+  if (positive) {
+    return std::nextafter(x, std::numeric_limits<T>::infinity());
   } else {
-    return positive ? x + spacing : x - spacing;
+    return std::nextafter(x, -std::numeric_limits<T>::infinity());
   }
 }
 

flang/unittests/Runtime/Numeric.cpp

@@ -86,7 +86,7 @@ TEST(Numeric, Nearest) {
   EXPECT_EQ(RTNAME(Nearest8)(Real<8>{1.0}, true),
       Real<8>{1.0} + std::ldexp(Real<8>{1.0}, -52));
   EXPECT_EQ(RTNAME(Nearest8)(Real<8>{1.0}, false),
-      Real<8>{1.0} - std::ldexp(Real<8>{1.0}, -52));
+      Real<8>{1.0} - 0.5 * std::ldexp(Real<8>{1.0}, -52));
 }
 
 TEST(Numeric, Nint) {