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[libc++] Add a thread-safe version of std::lgamma in the dylib (#153631)

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Libc++ currently redeclares ::lgamma_r on platforms that provide it.
This causes issues when building with modules, and redeclaring functions
provided by another library (here the C library) is bad hygiene.

Instead, use an asm declaration to call the right function without
having to redeclare it.
This commit is contained in:
Louis Dionne 2026-02-25 11:46:51 -07:00 committed by GitHub
parent a224ba0689
commit 09d7b890e0
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
2 changed files with 30 additions and 23 deletions
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26
libcxx/include/__math/gamma.h
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@ -55,6 +55,32 @@ inline _LIBCPP_HIDE_FROM_ABI double tgamma(_A1 __x) _NOEXCEPT {
return __builtin_tgamma((double)__x);
}
// __lgamma_r
//
// POSIX systems provide a function named lgamma_r which is a reentrant version of lgamma. Use that
// whenever possible. However, we avoid re-declaring the actual function since different platforms
// declare it differently in the first place: instead use `asm` to get the compiler to call the right
// function.
#if defined(_LIBCPP_MSVCRT_LIKE) // reentrant version is not available on Windows
inline _LIBCPP_HIDE_FROM_ABI double __lgamma_r(double __d) _NOEXCEPT { return __builtin_lgamma(__d); }
#else
# if defined(_LIBCPP_OBJECT_FORMAT_MACHO)
double __lgamma_r_shim(double, int*) _NOEXCEPT __asm__("_lgamma_r");
# else
double __lgamma_r_shim(double, int*) _NOEXCEPT __asm__("lgamma_r");
# endif
inline _LIBCPP_HIDE_FROM_ABI double __lgamma_r(double __d) _NOEXCEPT {
int __sign;
return __math::__lgamma_r_shim(__d, &__sign);
}
#endif
} // namespace __math
_LIBCPP_END_NAMESPACE_STD

27
libcxx/include/__random/binomial_distribution.h
View File

@ -10,6 +10,7 @@
#define _LIBCPP___RANDOM_BINOMIAL_DISTRIBUTION_H
#include <__config>
#include <__math/gamma.h>
#include <__random/is_valid.h>
#include <__random/uniform_real_distribution.h>
#include <cmath>
@ -97,33 +98,13 @@ public:
}
};
// Some libc declares the math functions to be `noexcept`.
#if _LIBCPP_GLIBC_PREREQ(2, 8) || _LIBCPP_LIBC_LLVM_LIBC
# define _LIBCPP_LGAMMA_R_NOEXCEPT _NOEXCEPT
#else
# define _LIBCPP_LGAMMA_R_NOEXCEPT
#endif
#if !defined(_LIBCPP_MSVCRT_LIKE)
extern "C" double lgamma_r(double, int*) _LIBCPP_LGAMMA_R_NOEXCEPT;
#endif
inline _LIBCPP_HIDE_FROM_ABI double __libcpp_lgamma(double __d) {
#if defined(_LIBCPP_MSVCRT_LIKE)
return lgamma(__d);
#else
int __sign;
return lgamma_r(__d, &__sign);
#endif
}
template <class _IntType>
binomial_distribution<_IntType>::param_type::param_type(result_type __t, double __p) : __t_(__t), __p_(__p) {
if (0 < __p_ && __p_ < 1) {
__r0_ = static_cast<result_type>((__t_ + 1) * __p_);
__pr_ = std::exp(
std::__libcpp_lgamma(__t_ + 1.) - std::__libcpp_lgamma(__r0_ + 1.) - std::__libcpp_lgamma(__t_ - __r0_ + 1.) +
__r0_ * std::log(__p_) + (__t_ - __r0_) * std::log(1 - __p_));
__pr_ =
std::exp(__math::__lgamma_r(__t_ + 1.) - __math::__lgamma_r(__r0_ + 1.) -
__math::__lgamma_r(__t_ - __r0_ + 1.) + __r0_ * std::log(__p_) + (__t_ - __r0_) * std::log(1 - __p_));
__odds_ratio_ = __p_ / (1 - __p_);
}
}