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libstdc++: Fix std::abs(__float128) for -NaN and -0.0 [PR109758]

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The current implementation of this non-standard overload of std::abs
incorrectly returns a negative value for negative NaNs and negative
zero, because x < 0 is false in both cases.

Use fabsl(long double) or fabsf128(_Float128) if those do the right
thing.  Otherwise, use __builtin_signbit(x) instead of x < 0 to detect
negative inputs. This assumes that __builtin_signbit handles __float128
correctly, but that seems to be true for all of GCC, clang and icc.

libstdc++-v3/ChangeLog:

	PR libstdc++/109758
	* include/bits/std_abs.h (abs(__float128)): Handle negative NaN
	and negative zero correctly.
	* testsuite/26_numerics/headers/cmath/109758.cc: New test.
This commit is contained in:
Jonathan Wakely 2023-05-10 12:20:58 +01:00
parent 0918360d70
commit af595613ac
2 changed files with 63 additions and 2 deletions
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13
libstdc++-v3/include/bits/std_abs.h
View file

@ -135,11 +135,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__extension__ inline _GLIBCXX_CONSTEXPR
__float128
abs(__float128 __x)
{ return __x < 0 ? -__x : __x; }
{
#if defined(_GLIBCXX_LDOUBLE_IS_IEEE_BINARY128)
return __builtin_fabsl(__x);
#elif defined(_GLIBCXX_HAVE_FLOAT128_MATH)
return __builtin_fabsf128(__x);
#else
// Assume that __builtin_signbit works for __float128.
return __builtin_signbit(__x) ? -__x : __x;
#endif
}
#endif
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
} // extern "C"++"
} // extern "C++"
#endif // _GLIBCXX_BITS_STD_ABS_H

52
libstdc++-v3/testsuite/26_numerics/headers/cmath/109758.cc Normal file
View file

@ -0,0 +1,52 @@
// { dg-do run }
// PR libstdc++/109758 std::abs(__float128) doesn't support NaN
#include <cmath>
#include <testsuite_hooks.h>
#if !defined(__STRICT_ANSI__) && defined(_GLIBCXX_USE_FLOAT128)
void
test_nan()
{
__float128 nan = __builtin_nanl("");
VERIFY( !__builtin_signbit(std::abs(nan)) );
VERIFY( !__builtin_signbit(std::abs(-nan)) );
}
void
test_zero()
{
__float128 zero = 0.0;
VERIFY( !__builtin_signbit(std::abs(zero)) );
VERIFY( !__builtin_signbit(std::abs(zero * -2.0)) );
}
void
test_neg()
{
VERIFY( std::abs((__float128)-1.0) == -1.0 );
VERIFY( std::abs((__float128)-2e9) == -2e9 );
VERIFY( std::abs((__float128)-3e-4) == 3e-4 );
}
void
test_inf()
{
__float128 inf = __builtin_huge_vall();
VERIFY( std::abs(inf) == inf );
VERIFY( std::abs(-inf) == inf );
}
#if __cplusplus >= 201103L
static_assert( std::abs((__float128)-1.0) == (__float128)1.0,
"std::abs(__float128) is usable in constant expressions" );
#endif
int main()
{
test_nan();
test_zero();
}
#else
int main() { }
#endif