[flang] Fix SPACING() of very small values

[klausler]

Summary

SPACING() must return TINY() for zero arguments (which we do)
and also for subnormal values smaller than TINY() in absolute value,
which we get wrong. Fix folding and the runtime.

flang/lib/Evaluate/real.cpp

 }
 
 // 16.9.180
 template <typename W, int P> Real<W, P> Real<W, P>::SPACING() const {
   if (IsNotANumber()) {
     return *this;
   } else if (IsInfinite()) {
     return NotANumber();
-  } else if (IsZero()) {
-    return TINY();
+  } else if (IsZero() || IsSubnormal()) {
+    return TINY(); // mandated by standard
   } else {
     Real result;
     result.Normalize(false, Exponent(), Fraction::MASKR(1));
-    return result;
+    return result.IsZero() ? TINY() : result;
   }
 }
 
 // 16.9.171
 template <typename W, int P>
 Real<W, P> Real<W, P>::SET_EXPONENT(std::int64_t expo) const {
   if (IsNotANumber()) {
     return *this;

flang/runtime/numeric.cpp

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) {
     // The standard-mandated behavior seems broken, since TINY() can't be
     // subnormal.
     return std::numeric_limits<T>::min(); // 0 -> TINY(x)
   } else {
-    return std::ldexp(
-        static_cast<T>(1.0), std::ilogb(x) + 1 - PREC); // 2**(e-p)
+    T result{
+        std::ldexp(static_cast<T>(1.0), std::ilogb(x) + 1 - PREC)}; // 2**(e-p)
+    return result == 0 ? /*TINY(x)*/ std::numeric_limits<T>::min() : result;
   }
 }
 
 // NEAREST (16.9.139)
 template <int PREC, typename T> inline T Nearest(T x, bool positive) {
   auto spacing{Spacing<PREC>(x)};
   if (x == 0) {
     auto least{std::numeric_limits<T>::denorm_min()};

flang/test/Evaluate/fold-spacing.f90

 ! RUN: %python %S/test_folding.py %s %flang_fc1
 ! Tests folding of SPACING() and RRSPACING
 module m
   logical, parameter :: test_1 = spacing(3.0) == scale(1.0, -22)
   logical, parameter :: test_2 = spacing(-3.0) == scale(1.0, -22)
   logical, parameter :: test_3 = spacing(3.0d0) == scale(1.0, -51)
   logical, parameter :: test_4 = spacing(0.) == tiny(0.)
   logical, parameter :: test_5 = spacing(tiny(0.)) == 1.e-45
   logical, parameter :: test_6 = spacing(8388608.) == 1.
+  logical, parameter :: test_7 = spacing(spacing(tiny(.0))) == tiny(0.)
   logical, parameter :: test_11 = rrspacing(3.0) == scale(0.75, 24)
   logical, parameter :: test_12 = rrspacing(-3.0) == scale(0.75, 24)
   logical, parameter :: test_13 = rrspacing(3.0d0) == scale(0.75, 53)
   logical, parameter :: test_14 = rrspacing(0.) == 0.
 end module