[libc-commits] [libc] [libc][CPP] Define max_align_t (PR #94065)

[via libc-commits]

https://github.com/PiJoules created https://github.com/llvm/llvm-project/pull/94065

None

>From 000dccfffc19b0df0114ac3603c5fffd5ecf76fa Mon Sep 17 00:00:00 2001
From: Leonard Chan <leonardchan at google.com>
Date: Fri, 31 May 2024 15:27:41 -0700
Subject: [PATCH] [libc][CPP] Define max_align_t

---
 libc/src/__support/CPP/cstddef.h       | 4 ++++
 libc/src/__support/macros/attributes.h | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/libc/src/__support/CPP/cstddef.h b/libc/src/__support/CPP/cstddef.h
index 1da51fd253fb5..111265d38cde7 100644
--- a/libc/src/__support/CPP/cstddef.h
+++ b/libc/src/__support/CPP/cstddef.h
@@ -16,6 +16,10 @@ namespace LIBC_NAMESPACE::cpp {
 
 enum class byte : unsigned char {};
 
+// When built with clang, this will use the definition from the clang header
+// __stddef_max_align_t.h
+using ::max_align_t LIBC_USING_IF_EXISTS;
+
 template <class IntegerType>
 LIBC_INLINE constexpr enable_if_t<is_integral_v<IntegerType>, byte>
 operator>>(byte b, IntegerType shift) noexcept {
diff --git a/libc/src/__support/macros/attributes.h b/libc/src/__support/macros/attributes.h
index 8637e165fe3bc..40b67281767f8 100644
--- a/libc/src/__support/macros/attributes.h
+++ b/libc/src/__support/macros/attributes.h
@@ -30,4 +30,10 @@
 #define LIBC_THREAD_LOCAL thread_local
 #endif
 
+#if __has_attribute(__using_if_exists__)
+#define LIBC_USING_IF_EXISTS __attribute__((__using_if_exists__))
+#else
+#define LIBC_USING_IF_EXISTS
+#endif
+
 #endif // LLVM_LIBC_SRC___SUPPORT_MACROS_ATTRIBUTES_H