[libc++] Avoid redeclaring lgamma_r

This causes issues when building with modules, and redeclaring functions
provided by another library (here the C library) is bad hygiene. Instead,
use <math.h> to access the declaration provided by the system.

As a drive-by, this also starts using ::lgamma_r instead of ::lgamma when
building on top of LLVM-libc. We didn't do that previously due to a
declration conflict, but using the _r version when we can should only
make things thread safe.

Author: ldionne

libcxx/include/__configuration/availability.h

@@ -84,6 +84,9 @@
 // in all versions of the library are available.
 #if !_LIBCPP_HAS_VENDOR_AVAILABILITY_ANNOTATIONS
 
+#  define _LIBCPP_INTRODUCED_IN_LLVM_22 1
+#  define _LIBCPP_INTRODUCED_IN_LLVM_22_ATTRIBUTE /* nothing */
+
 #  define _LIBCPP_INTRODUCED_IN_LLVM_21 1
 #  define _LIBCPP_INTRODUCED_IN_LLVM_21_ATTRIBUTE /* nothing */
 
@@ -120,6 +123,11 @@
 
 // clang-format off
 
+// LLVM 22
+// TODO: Fill this in
+#  define _LIBCPP_INTRODUCED_IN_LLVM_22 0
+#  define _LIBCPP_INTRODUCED_IN_LLVM_22_ATTRIBUTE __attribute__((unavailable))
+
 // LLVM 21
 // TODO: Fill this in
 #  define _LIBCPP_INTRODUCED_IN_LLVM_21 0
@@ -355,6 +363,11 @@
 #define _LIBCPP_AVAILABILITY_HAS_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE _LIBCPP_INTRODUCED_IN_LLVM_21
 // No attribute, since we've had bad_function_call::what() in the headers before
 
+// This controls whether we provide an implementation of a thread safe variant of the `std::lgamma`
+// function in the dylib.
+#define _LIBCPP_AVAILABILITY_HAS_THREAD_SAFE_LGAMMA _LIBCPP_INTRODUCED_IN_LLVM_22
+// No attribute, since we call ::lgamma_r instead
+
 // Define availability attributes that depend on both
 // _LIBCPP_HAS_EXCEPTIONS and _LIBCPP_HAS_RTTI.
 #if !_LIBCPP_HAS_EXCEPTIONS || !_LIBCPP_HAS_RTTI

libcxx/include/__math/gamma.h

@@ -55,6 +55,31 @@ inline _LIBCPP_HIDE_FROM_ABI double tgamma(_A1 __x) _NOEXCEPT {
   return __builtin_tgamma((double)__x);
 }
 
+// __lgamma_r
+
+struct __lgamma_result {
+  double __result;
+  int __sign;
+};
+
+#if _LIBCPP_AVAILABILITY_HAS_THREAD_SAFE_LGAMMA
+_LIBCPP_EXPORTED_FROM_ABI __lgamma_result __lgamma_thread_safe_impl(double) _NOEXCEPT;
+
+inline _LIBCPP_HIDE_FROM_ABI __lgamma_result __lgamma_thread_safe(double __d) _NOEXCEPT {
+  return __math::__lgamma_thread_safe_impl(__d);
+}
+#else
+// When deploying to older targets, call `lgamma_r` directly but avoid declaring the actual
+// function since different platforms declare the function slightly differently.
+double __lgamma_r_shim(double, int*) _NOEXCEPT __asm__("lgamma_r");
+
+inline _LIBCPP_HIDE_FROM_ABI __lgamma_result __lgamma_thread_safe(double __d) _NOEXCEPT {
+  int __sign;
+  double __res = __math::__lgamma_r_shim(__d, &__sign);
+  return __lgamma_result{__res, __sign};
+}
+#endif
+
 } // namespace __math
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/__random/binomial_distribution.h

@@ -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>
@@ -97,39 +98,14 @@ class binomial_distribution {
   }
 };
 
-// Some libc declares the math functions to be `noexcept`.
-#if defined(_LIBCPP_GLIBC_PREREQ)
-#  if _LIBCPP_GLIBC_PREREQ(2, 8)
-#    define _LIBCPP_LGAMMA_R_NOEXCEPT _NOEXCEPT
-#  else
-#    define _LIBCPP_LGAMMA_R_NOEXCEPT
-#  endif
-#elif defined(__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_thread_safe(__t_ + 1.).__result - __math::__lgamma_thread_safe(__r0_ + 1.).__result -
+                 __math::__lgamma_thread_safe(__t_ - __r0_ + 1.).__result + __r0_ * std::log(__p_) +
+                 (__t_ - __r0_) * std::log(1 - __p_));
     __odds_ratio_ = __p_ / (1 - __p_);
   }
 }

libcxx/src/CMakeLists.txt

@@ -30,6 +30,7 @@ set(LIBCXX_SOURCES
   include/ryu/ryu.h
   include/to_chars_floating_point.h
   include/from_chars_floating_point.h
+  math.cpp
   memory.cpp
   memory_resource.cpp
   new_handler.cpp

libcxx/src/math.cpp

@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifdef __APPLE__
+#  define _REENTRANT
+#endif
+
+#include <cmath>
+#include <math.h> // for lgamma_r
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+namespace __math {
+
+_LIBCPP_EXPORTED_FROM_ABI __lgamma_result __lgamma_thread_safe_impl(double __d) noexcept {
+  int __sign;
+  double __result = ::lgamma_r(__d, &__sign);
+  return __lgamma_result{__result, __sign};
+}
+
+} // namespace __math
+_LIBCPP_END_NAMESPACE_STD