[libc] Add _Returns_twice to C++ code

[saturn691]

Fixes issue with <csetjmp> which requires _Returns_twice but in C++ mode

[llvmbot]

@llvm/pr-subscribers-libc

Author: William Huynh (saturn691)

Changes

Fixes issue with <csetjmp> which requires _Returns_twice but in C++ mode

---

Full diff: https://github.com/llvm/llvm-project/pull/153602.diff

1 Files Affected:

• (modified) libc/include/__llvm-libc-common.h (+3)

diff --git a/libc/include/__llvm-libc-common.h b/libc/include/__llvm-libc-common.h
index c6fd33a55532c..b376b05df1678 100644
--- a/libc/include/__llvm-libc-common.h
+++ b/libc/include/__llvm-libc-common.h
@@ -47,6 +47,9 @@
 #define __NOEXCEPT throw()
 #endif
 
+#undef _Returns_twice
+#define _Returns_twice [[gnu::returns_twice]]
+
 // This macro serves as a generic cast implementation for use in both C and C++,
 // similar to `__BIONIC_CAST` in Android.
 #undef __LLVM_LIBC_CAST

[michaelrj-google]

given that this is on both sides of the #ifdef __cplusplus branch is there a way we can unify it instead of having two copies? Also is there a reason you used the gnu attribute instead of the generic one?

[saturn691]

1. returns_twice needs to be namespaced (https://en.cppreference.com/w/cpp/language/attributes.html)

❯ clang test.c                                                               
test.c:3:3: warning: unknown attribute 'returns_twice' ignored [-Wunknown-attributes]
    3 | [[returns_twice]] int f() {
      |   ^~~~~~~~~~~~~
1 warning generated.

2. [[attribute]] syntax is only supported in C++11. It seems to compile for older versions and this has even been back-ported to C code in clang. I'll push an update to unify this.

[petrhosek]

The [[attribute]] syntax was only introduced in C20 so we shouldn't be using it in C code since we want our headers are compatible with older C standards.

[saturn691]

Good point, I've settled with preferring to generate the new attribute syntax if the compiler supports it (C++11) onwards.