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[flang] Subnormal arguments to and results from SPACING #108861

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Sep 16, 2024
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[flang] Subnormal arguments to and results from SPACING #108861

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6 changes: 4 additions & 2 deletions flang/lib/Evaluate/real.cpp
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Expand Up @@ -770,11 +770,13 @@ template <typename W, int P> Real<W, P> Real<W, P>::SPACING() const {
} else if (IsInfinite()) {
return NotANumber();
} else if (IsZero() || IsSubnormal()) {
return TINY(); // mandated by standard
return TINY(); // standard & 100% portable
} else {
Real result;
result.Normalize(false, Exponent(), Fraction::MASKR(1));
return result.IsZero() ? TINY() : result;
// Can the result be less than TINY()? No, with five commonly
// used compilers; yes, with two less commonly used ones.
return result.IsZero() || result.IsSubnormal() ? TINY() : result;
}
}

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27 changes: 14 additions & 13 deletions flang/runtime/numeric-templates.h
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Expand Up @@ -88,12 +88,16 @@ struct MaxOrMinIdentity<TypeCategory::Real, 16, IS_MAXVAL,

// Minimum finite representable value.
// For floating-point types, returns minimum positive normalized value.
template <typename T> struct MinValue {
template <int PREC, typename T> struct MinValue {
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The template argument "PREC" isn't used. Did you mean to do something with it?

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Yes, and I do below, where it allows me to define a template specialization for real(2).

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Thanks the the explanation!

static RT_API_ATTRS T get() { return std::numeric_limits<T>::min(); }
};
template <typename T> struct MinValue<11, T> {
// TINY(0._2)
static constexpr RT_API_ATTRS T get() { return 0.00006103515625E-04; }
};

#if HAS_FLOAT128
template <> struct MinValue<CppTypeFor<TypeCategory::Real, 16>> {
template <> struct MinValue<113, CppTypeFor<TypeCategory::Real, 16>> {
using Type = CppTypeFor<TypeCategory::Real, 16>;
static RT_API_ATTRS Type get() {
// Create a buffer to store binary representation of __float128 constant.
Expand Down Expand Up @@ -167,8 +171,8 @@ template <> struct MAXTy<CppTypeFor<TypeCategory::Real, 16>> {
};
#endif

template <typename T> struct MINTy {
static constexpr RT_API_ATTRS T compute() { return MinValue<T>::get(); }
template <int PREC, typename T> struct MINTy {
static constexpr RT_API_ATTRS T compute() { return MinValue<PREC, T>::get(); }
};

template <typename T> struct QNANTy {
Expand Down Expand Up @@ -339,23 +343,20 @@ template <int PREC, typename T> inline RT_API_ATTRS T RRSpacing(T x) {

// SPACING (16.9.180)
template <int PREC, typename T> inline RT_API_ATTRS T Spacing(T x) {
T tiny{MINTy<PREC, T>::compute()};
if (ISNANTy<T>::compute(x)) {
return x; // NaN -> same NaN
} else if (ISINFTy<T>::compute(x)) {
return QNANTy<T>::compute(); // +/-Inf -> NaN
} else if (x == 0) { // 0 -> TINY(x)
// The standard-mandated behavior seems broken, since TINY() can't be
// subnormal.
if constexpr (PREC == 11) { // REAL(2)
return 0.00006103515625E-04; // TINY(0._2)
} else {
// N.B. TINY(0._3) == TINY(0._4) so this works even if no std::bfloat16_t.
return MINTy<T>::compute();
}
return tiny;
} else {
T result{LDEXPTy<T>::compute(
static_cast<T>(1.0), ILOGBTy<T>::compute(x) + 1 - PREC)}; // 2**(e-p)
return result == 0 ? /*TINY(x)*/ MINTy<T>::compute() : result;
// All compilers return TINY(x) for |x| <= TINY(x), but differ over whether
// SPACING(x) can be < TINY(x) for |x| > TINY(x). The most common precedent
// is to never return a value < TINY(x).
return result <= tiny ? tiny : result;
}
}

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4 changes: 3 additions & 1 deletion flang/test/Evaluate/fold-spacing.f90
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Expand Up @@ -5,7 +5,9 @@ module m
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_5a = spacing(tiny(0.)) == tiny(0.)
logical, parameter :: test_5b = spacing(tiny(0.)/2) == tiny(0.)
logical, parameter :: test_5c = spacing(tiny(0.)*2) == tiny(0.)
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)
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