On Mon, 2019-08-12 at 16:11 -0700, Nick Desaulniers wrote: > On Mon, Aug 12, 2019 at 3:40 PM Joe Perches <joe@xxxxxxxxxxx> wrote: > > On Mon, 2019-08-12 at 15:14 -0700, Nathan Huckleberry wrote: > > > Clang does not support the use of comments to label > > > intentional fallthrough. This patch replaces some uses > > > of comments to attributesto cut down a significant number > > > of warnings on clang (from ~50000 to ~200). Only comments > > > in commonly used header files have been replaced. > > > > > > Since there is still quite a bit of noise, this > > > patch moves -Wimplicit-fallthrough to > > > Makefile.extrawarn if you are compiling with > > > clang. > > > > Unmodified clang does not emit this warning without a patch. > > Correct, Nathan is currently implementing support for attribute > fallthrough in Clang in: > https://reviews.llvm.org/D64838 > > I asked him in person to evaluate how many warnings we'd see in an > arm64 defconfig with his patch applied. There were on the order of > 50k warnings, mostly from these headers. I asked him to send these > patches, then land support in the compiler, that way should our CI > catch fire overnight, we can carry out of tree fixes until they land. > With the changes here to Makefile.extrawarn, we should not need to > carry any out of tree patches. > > > > diff --git a/Makefile b/Makefile > > [] > > > @@ -846,7 +846,11 @@ NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include) > > > KBUILD_CFLAGS += -Wdeclaration-after-statement > > > > > > # Warn about unmarked fall-throughs in switch statement. > > > +# If the compiler is clang, this warning is only enabled if W=1 in > > > +# Makefile.extrawarn > > > +ifndef CONFIG_CC_IS_CLANG > > > KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,) > > > +endif > > > > It'd be better to remove CONFIG_CC_IS_CLANG everywhere > > eventually as it adds complexity and makes .config files > > not portable to multiple systems. > > > > > diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h > > [] > > > @@ -253,4 +253,8 @@ > > > */ > > > #define __weak __attribute__((__weak__)) > > > > > > +#if __has_attribute(fallthrough) > > > +#define __fallthrough __attribute__((fallthrough)) > > > > This should be __attribute__((__fallthrough__)) > > Agreed. I think the GCC documentation on attributes had a point about > why the __ prefix/suffix was important, which is why we went with that > in Miguel's original patchset. > > > And there is still no agreement about whether this should > > be #define fallthrough or #define __fallthrough > > > > https://lore.kernel.org/patchwork/patch/1108577/ > > > > > diff --git a/include/linux/jhash.h b/include/linux/jhash.h > > [] > > > @@ -86,19 +86,43 @@ static inline u32 jhash(const void *key, u32 length, u32 initval) > > [] > > > + case 12: > > > + c += (u32)k[11]<<24; > > > + __fallthrough; > > > > You might consider trying out the scripted conversion tool > > attached to this email: > > > > https://lore.kernel.org/lkml/61ddbb86d5e68a15e24ccb06d9b399bbf5ce2da7.camel@xxxxxxxxxxx/ > > I guess the thing I'm curious about is why /* fall through */ is being > used vs __attribute__((__fallthrough__))? Surely there's some > discussion someone can point me to? AFAIK: It's historic. https://lkml.org/lkml/2019/8/4/83 coverity and lint do not support __attribute__((__fallthrough__)) but do support /* fallthrough */ comments in their analysis output. I prefer converting all the comments to a macro / pseudo keyword. The cvt_style.pl script does a reasonable job of conversion.
