[PATCH 2/2] kbuild: Use '-fms-anonymous-structs' if it is available

[Nathan Chancellor]

Clang recently added '-fms-anonymous-structs' [1] to specifically enable
the Microsoft tagged anonymous structure / union extension, for which
the kernel added '-fms-extensions' in commit c4781dc3d1cf ("Kbuild:
enable -fms-extensions"). Switch to this more narrow option if it is
available, which would have helped avoid the issue addressed by
commit a6773e6932cb ("jfs: Rename _inline to avoid conflict with clang's
'-fms-extensions'"). GCC has talked about adding a similar flag [2] as
well but potentially naming it differently.

Move the selection of the flag to Kconfig to make it easier to use
cc-option (as CC_FLAGS_DIALECT may be used in arch Makefiles, which may
be too early for cc-option in Kbuild) and customize based on compiler
flag names.

Link: https://github.com/llvm/llvm-project/commit/c391efe6fb67329d8e2fd231692cc6b0ea902956 [1]
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123623 [2]
Signed-off-by: Nathan Chancellor <nathan at kernel.org>
---
 Makefile     | 2 +-
 init/Kconfig | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 06ff3032a6bc..1d5c0ab9ed5c 100644
--- a/Makefile
+++ b/Makefile
@@ -791,7 +791,7 @@ endif
 
 CC_FLAGS_DIALECT := -std=gnu11
 # Allow including a tagged struct or union anonymously in another struct/union.
-CC_FLAGS_DIALECT += -fms-extensions
+CC_FLAGS_DIALECT += $(CONFIG_CC_MS_EXTENSIONS)
 # Clang enables warnings about GNU and Microsoft extensions by default, disable
 # them because this is expected with the above options.
 ifdef CONFIG_CC_IS_CLANG
diff --git a/init/Kconfig b/init/Kconfig
index c25869cf59c1..c4282d049463 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -960,6 +960,11 @@ config CC_IMPLICIT_FALLTHROUGH
 	default "-Wimplicit-fallthrough=5" if CC_IS_GCC && $(cc-option,-Wimplicit-fallthrough=5)
 	default "-Wimplicit-fallthrough" if CC_IS_CLANG && $(cc-option,-Wunreachable-code-fallthrough)
 
+config CC_MS_EXTENSIONS
+	string
+	default "-fms-anonymous-structs" if $(cc-option,-fms-anonymous-structs)
+	default "-fms-extensions"
+
 # Currently, disable gcc-10+ array-bounds globally.
 # It's still broken in gcc-13, so no upper bound yet.
 config GCC10_NO_ARRAY_BOUNDS

-- 
2.53.0