[libc] Allow each function can have extra attributes by defining LLVM_LIBC_FUNCTION_ATTR_func macro. (#116160)

Author: lntue

libc/src/__support/common.h

@@ -21,9 +21,25 @@
 #define LLVM_LIBC_FUNCTION_ATTR
 #endif
 
+// Allow each function `func` to have extra attributes specified by defining:
+// `LLVM_LIBC_FUNCTION_ATTR_func` macro, which should always start with
+// "LLVM_LIBC_EMPTY, "
+//
+// For examples:
+// #define LLVM_LIBC_FUNCTION_ATTR_memcpy LLVM_LIBC_EMPTY, [[gnu::weak]]
+// #define LLVM_LIBC_FUNCTION_ATTR_memchr LLVM_LIBC_EMPTY, [[gnu::weak]]       \
+//                                        [[gnu::visibility("default")]]
+#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)
+
 // 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)                           \
+  LLVM_LIBC_ATTR(name)                                                         \
   LLVM_LIBC_FUNCTION_ATTR decltype(LIBC_NAMESPACE::name)                       \
       __##name##_impl__ __asm__(#name);                                        \
   decltype(LIBC_NAMESPACE::name) name [[gnu::alias(#name)]];                   \

utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl

@@ -59,7 +59,7 @@ def libc_function(
         srcs,
         weak = False,
         copts = None,
-        local_defines = None,
+        local_defines = [],
         **kwargs):
     """Add target for a libc function.
 
@@ -108,16 +108,20 @@ def libc_function(
         name = libc_internal_target(name),
         srcs = srcs,
         copts = copts,
+        local_defines = local_defines,
         **kwargs
     )
 
     # This second target is the llvm libc C function with either a default or hidden visibility.
     # All other functions are hidden.
-    func_attrs = ["__attribute__((visibility(\"default\")))"]
-    if weak:
-        func_attrs = func_attrs + ["__attribute__((weak))"]
-    local_defines = local_defines or ["LIBC_COPT_PUBLIC_PACKAGING"]
-    local_defines = local_defines + ["LLVM_LIBC_FUNCTION_ATTR='%s'" % " ".join(func_attrs)]
+    func_attrs = [
+        "LLVM_LIBC_FUNCTION_ATTR_" + name + "='LLVM_LIBC_EMPTY, [[gnu::weak]]'",
+    ] if weak else []
+        
+    local_defines = (local_defines
+                    + ["LIBC_COPT_PUBLIC_PACKAGING"]
+                    + ["LLVM_LIBC_FUNCTION_ATTR='[[gnu::visibility(\"default\")]]'"]
+                    + func_attrs)
     _libc_library(
         name = name,
         hidden = True,