[libc] Allow each function can have extra attributes by defining LLVM_LIBC_FUNCTION_ATTR_func macro.

[lntue]

No description provided.

[llvmbot]

@llvm/pr-subscribers-libc

Author: None (lntue)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/116160.diff

1 Files Affected:

• (modified) libc/src/__support/common.h (+15)

diff --git a/libc/src/__support/common.h b/libc/src/__support/common.h
index 48c773fa02c176..489ca41770e998 100644
--- a/libc/src/__support/common.h
+++ b/libc/src/__support/common.h
@@ -21,9 +21,24 @@
 #define LLVM_LIBC_FUNCTION_ATTR
 #endif
 
+// Allow each function `func` can have extra attributes specified by defining:
+// `LLVM_LIBC_FUNCTION_ATTR_func` macro, which should always start with
+// "LLVM_LIBC_EMPTY,"
+//
+// For example:
+// #define LLVM_LIBC_FUNCTION_ATTR_memcpy LLVM_LIBC_EMPTY, __attribute__((weak))
+#define LLVM_LIBC_EMPTY
+
+#define GET_SECOND(first, second, ...) second
+#define EXPAND_THEN_SECOND(name) GET_SECOND(name, LLVM_LIBC_EMPTY, )
+
+#define LLVM_LIBC_ATTR(name) EXPAND_THEN_SECOND(LLVM_LIBC_FUNCTION_ATTR_##name)
+#define EXPAND_ATTR(name) LLVM_LIBC_ATTR(name)
+
 // MacOS needs to be excluded because it does not support aliasing.
 #if defined(LIBC_COPT_PUBLIC_PACKAGING) && (!defined(__APPLE__))
 #define LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)                           \
+  EXPAND_ATTR(name)                                                            \
   LLVM_LIBC_FUNCTION_ATTR decltype(LIBC_NAMESPACE::name)                       \
       __##name##_impl__ __asm__(#name);                                        \
   decltype(LIBC_NAMESPACE::name) name [[gnu::alias(#name)]];                   \

[nickdesaulniers]

Suggested change

	// #define LLVM_LIBC_FUNCTION_ATTR_memcpy LLVM_LIBC_EMPTY, __attribute__((weak))
	// #define LLVM_LIBC_FUNCTION_ATTR_memcpy LLVM_LIBC_EMPTY, [[gnu::weak]]

Let's encourage the usage of the C++11/C23 style attribute syntax, rather than the GNU C extension syntax.

Though, don't we expect this to be used by the command line cmake invocation? Perhaps an example of that in this comment, or moving our bazel files over to use that would be welcome additions to this PR?

[nickdesaulniers]

Perhaps worth documenting any new command line options in docs/dev/implementation_standard.rst.

[vonosmas]

I don't think you need EXPAND_ATTR and can just use LLVM_LIBC_ATTR below?

[lntue]

I think you're right, EXPAND_ATTR is not needed.

[vonosmas]

LGTM, but please get confirmation from @nickdesaulniers as well.