Re: [GIT PULL] Enable -Wimplicit-fallthrough for Clang for 5.16-rc1

[Linus Torvalds]

On Sat, Nov 13, 2021 at 1:36 PM Gustavo A. R. Silva
<gustavoars@xxxxxxxxxx> wrote:
>
> This patch has been baking in linux-next for a couple of developement
> cycles, now. So, I think we are pretty much ready to merge it into
> mainline.

Ugh. It's also very ugly.

Wouldn't something like this (TOTALLY UNTESTED!) work and do the right thing?

This seems like a natural for a Kconfig decision.

Also, does -Wimplicit-fallthrough=5 work with clang too? That would
simplify things a bit, and then we could just use a regular boolean
and do

  KBUILD_CFLAGS-$(CC_IMPLICIT_FALLTHROUGH) += -Wimplicit-fallthrough=5

in the Makefile, which is more in like with what we do for other
config-time cflags..

           Linus
 Makefile     | 6 +-----
 init/Kconfig | 5 +++++
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/Makefile b/Makefile
index d83d72c26aaa..9cf9d5c1ed8f 100644
--- a/Makefile
+++ b/Makefile
@@ -789,7 +789,7 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
 KBUILD_CFLAGS += $(stackp-flags-y)
 
 KBUILD_CFLAGS-$(CONFIG_WERROR) += -Werror
-KBUILD_CFLAGS += $(KBUILD_CFLAGS-y)
+KBUILD_CFLAGS += $(KBUILD_CFLAGS-y) $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
 
 ifdef CONFIG_CC_IS_CLANG
 KBUILD_CPPFLAGS += -Qunused-arguments
@@ -801,10 +801,6 @@ KBUILD_CFLAGS += -Wno-gnu
 KBUILD_CFLAGS += -mno-global-merge
 else
 
-# Warn about unmarked fall-throughs in switch statement.
-# Disabled for clang while comment to attribute conversion happens and
-# https://github.com/ClangBuiltLinux/linux/issues/636 is discussed.
-KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough=5,)
 # gcc inanely warns about local variables called 'main'
 KBUILD_CFLAGS += -Wno-main
 endif
diff --git a/init/Kconfig b/init/Kconfig
index 45bcaa8e7481..036b750e8d8a 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -885,6 +885,11 @@ config ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
 config CC_HAS_INT128
 	def_bool !$(cc-option,$(m64-flag) -D__SIZEOF_INT128__=0) && 64BIT
 
+config CC_IMPLICIT_FALLTHROUGH
+	string
+	default "-Wimplicit-fallthrough=5" if CC_IS_GCC
+	default "-Wimplicit-fallthrough" if CC_IS_CLANG && $(cc-option,-Wunreachable-code-fallthrough)
+
 #
 # For architectures that know their GCC __int128 support is sound
 #