https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77776
g.peterh...@t-online.de changed:
What |Removed |Added
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CC| |g.peterh...@t-online.de
--- Comment #11 from g.peterh...@t-online.de ---
Would this be a good implementation for hypot3 in cmath?
#define GCC_UNLIKELY(x) __builtin_expect(x, 0)
#define GCC_LIKELY(x) __builtin_expect(x, 1)
namespace __detail
{
template <typename _Tp>
inline _GLIBCXX_CONSTEXPR typename
enable_if<is_floating_point<_Tp>::value, bool>::type __isinf3(const _Tp __x,
const _Tp __y, const _Tp __z) noexcept
{
return bool(int(std::isinf(__x)) | int(std::isinf(__y)) |
int(std::isinf(__z)));
}
template <typename _Tp>
inline _GLIBCXX_CONSTEXPR typename
enable_if<is_floating_point<_Tp>::value, _Tp>::type __hypot3(_Tp __x, _Tp __y,
_Tp __z) noexcept
{
__x = std::fabs(__x);
__y = std::fabs(__y);
__z = std::fabs(__z);
const _Tp
__max = std::fmax(std::fmax(__x, __y), __z);
if (GCC_UNLIKELY(__max == _Tp{}))
{
return __max;
}
else
{
__x /= __max;
__y /= __max;
__z /= __max;
return std::sqrt(__x*__x + __y*__y + __z*__z) * __max;
}
}
} // __detail
template <typename _Tp>
inline _GLIBCXX_CONSTEXPR typename
enable_if<is_floating_point<_Tp>::value, _Tp>::type __hypot3(const _Tp __x,
const _Tp __y, const _Tp __z) noexcept
{
return (GCC_UNLIKELY(__detail::__isinf3(__x, __y, __z))) ?
numeric_limits<_Tp>::infinity() : __detail::__hypot3(__x, __y, __z);
}
#undef GCC_UNLIKELY
#undef GCC_LIKELY
How does it work?
* Basically, I first pull out the special case INFINITY (see
https://en.cppreference.com/w/cpp/numeric/math/hypot).
* As an additional safety measure (to prevent misuse) the functions are defined
by enable_if.
constexpr
* The hypot3 functions can thus be defined as _GLIBCXX_CONSTEXPR.
Questions
* To get a better runtime behavior I define GCC_(UN)LIKELY. Are there already
such macros (which I have overlooked)?
* The functions are noexcept. Does that make sense? If yes: why are the math
functions not noexcept?
thx
Gero
