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[flang] Handle subnormals while folding SCALE, SET_EXPONENT, & FRACTION 

The implementations of folding for the intrinsic functions SCALE and SET_EXPONENT
do not cope correctly with numbers in the subnormal range.  Fix SCALE,
then modify SET_EXPONENT to be a special case of SCALE.

Differential Revision: https://reviews.llvm.org/D131099
This commit is contained in:
Peter Klausler 2022-07-26 09:39:34 -07:00
parent d1bb3016dd
commit 1d1f5a5251
3 changed files with 10 additions and 7 deletions
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12
flang/include/flang/Evaluate/real.h
View File

@ -177,17 +177,21 @@ public:
template <typename INT>
ValueWithRealFlags<Real> SCALE(const INT &by,
Rounding rounding = TargetCharacteristics::defaultRounding) const {
auto expo{exponentBias + by.ToInt64()};
// Normalize a fraction with just its LSB set and then multiply.
// (Set the LSB, not the MSB, in case the scale factor needs to
// be subnormal.)
auto adjust{exponentBias + binaryPrecision - 1};
auto expo{adjust + by.ToInt64()};
if (IsZero()) {
expo = exponentBias; // ignore by, don't overflow
} else if (by > INT{maxExponent}) {
expo = maxExponent;
} else if (by < INT{-exponentBias}) {
expo = maxExponent + binaryPrecision - 1;
} else if (by < INT{-adjust}) {
expo = -1;
}
Real twoPow;
RealFlags flags{
twoPow.Normalize(false, static_cast<int>(expo), Fraction::MASKL(1))};
twoPow.Normalize(false, static_cast<int>(expo), Fraction::MASKR(1))};
ValueWithRealFlags<Real> result{Multiply(twoPow, rounding)};
result.flags |= flags;
return result;

4
flang/lib/Evaluate/real.cpp
View File

@ -765,9 +765,7 @@ Real<W, P> Real<W, P>::SET_EXPONENT(int expo) const {
} else if (IsZero()) {
return *this;
} else {
Real result;
result.Normalize(IsNegative(), expo + exponentBias - 1, GetFraction());
return result;
return SCALE(Integer<32>(expo - UnbiasedExponent() - 1)).value;
}
}

1
flang/test/Evaluate/folding07.f90
View File

@ -280,5 +280,6 @@ module m
logical, parameter :: test_set_exponent_0 = set_exponent(1., 0) == 0.5
logical, parameter :: test_set_exponent_1 = set_exponent(1., 1) == 1.
logical, parameter :: test_set_exponent_2 = set_exponent(1., 2) == 2.
logical, parameter :: test_set_exponent_min = set_exponent(1., -149) == 1.40129846432481707092372958328991613128026194187651577175706828388979108268586060148663818836212158203125e-45_4
end module