[libc++] Conditionally declare lgamma_r as noexcept

[yuxuanchen1997]

An older PR #102036 suggested that LLVM libc declares lgamma_r as noexcept and is incompatible with this redeclaration. However, I recently discovered that glibc also declares the math functions to be noexcept under C++ mode.

This line usually don't cause issues because both the glibc and this file are included as "system headers". According to this godbolt, both GCC and clang ignore the different exception specification between multiple declarations if they are in system headers.

However, this seems not the case for NVCC/EDG, so a fix for this redeclaration is still desirable. This patch proposes that we should declare the function as noexcept under known libc integrations to keep the declared function consistent.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[philnik777]

I think #153631 (comment) is rather what we should go for.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Yuxuan Chen (yuxuanchen1997)

Changes

An older PR #102036 suggested that LLVM libc declares lgamma_r as noexcept and is incompatible with this redeclaration. However, I recently discovered that glibc also declares the math functions to be noexcept under C++ mode.

This line usually don't cause issues because both the glibc and this file are included as "system headers". According to this godbolt, both GCC and clang ignore the different exception specification between multiple declarations if they are in system headers.

However, this seems not the case for NVCC/EDG, so a fix for this redeclaration is still desirable. This patch proposes that we should declare the function as noexcept under known libc integrations to keep the declared function consistent.

---

Full diff: https://github.com/llvm/llvm-project/pull/156547.diff

1 Files Affected:

• (modified) libcxx/include/__random/binomial_distribution.h (+10-4)

diff --git a/libcxx/include/__random/binomial_distribution.h b/libcxx/include/__random/binomial_distribution.h
index b4b4340827761..80a910b611adf 100644
--- a/libcxx/include/__random/binomial_distribution.h
+++ b/libcxx/include/__random/binomial_distribution.h
@@ -97,13 +97,19 @@ class binomial_distribution {
   }
 };
 
-// The LLVM C library provides this with conflicting `noexcept` attributes.
-#if !defined(_LIBCPP_MSVCRT_LIKE) && !defined(__LLVM_LIBC__)
-extern "C" double lgamma_r(double, int*);
+// Some libc declares the math functions to be `noexcept`.
+#if _LIBCPP_GLIBC_PREREQ(2, 8) || 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) || defined(__LLVM_LIBC__)
+#if defined(_LIBCPP_MSVCRT_LIKE)
   return lgamma(__d);
 #else
   int __sign;

[yuxuanchen1997]

I think #153631 (comment) is rather what we should go for.

Hi. Would appreciate progress on that work. If no one is working on it, I can also contribute some cycles. In the meantime, this patch can unblock some libc++ CUDA usages.