Messages in this thread | | | Date | Mon, 28 Feb 2022 13:41:45 -0800 | From | Fangrui Song <> | Subject | Re: [PATCH] [v2] Kbuild: move to -std=gnu11 |
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Thanks for the patch!
(Was always wondering which of binutils and kernel would migrate to C99+ earlier... binutils won)
On 2022-02-28, Nick Desaulniers wrote: >On Mon, Feb 28, 2022 at 2:32 AM Arnd Bergmann <arnd@kernel.org> wrote: >> >> From: Arnd Bergmann <arnd@arndb.de> >> >> During a patch discussion, Linus brought up the option of changing >> the C standard version from gnu89 to gnu99, which allows using variable >> declaration inside of a for() loop. While the C99, C11 and later standards >> introduce many other features, most of these are already available in >> gnu89 as GNU extensions as well. >> >> An earlier attempt to do this when gcc-5 started defaulting to >> -std=gnu11 failed because at the time that caused warnings about >> designated initializers with older compilers. Now that gcc-5.1 is the >> minimum compiler version used for building kernels, that is no longer a >> concern. Similarly, the behavior of 'inline' functions changes between > >More precisely, the semantics of "extern inline" functions changed >between ISO C90 and ISO C99.
Perhaps a clearer explanation to readers is: "extern inline" and "inline" swap semantics with gnu_inline (-fgnu89-inline or __attribute__((__gnu_inline__))).
>That's the only concern I have, which I doubt is an issue. The kernel >is already covered by the function attribute as you note. > >Just to have some measure: >$ git grep -rn "extern inline" | wc -l >116
"^inline" behaves like C99+ "extern inline"
Agree this is handled by
#define inline inline __gnu_inline __inline_maybe_unused notrace
>Most of those are in arch/alpha/ which is curious; I wonder if those >were intentional. > >(I do worry about Makefiles that completely reset KBUILD_CFLAGS >though; the function attributes still take precedence). > >> gnu89 and gnu11, but this was taken care of by defining 'inline' to >> include __attribute__((gnu_inline)) in order to allow building with >> clang a while ago. >> >> One minor issue that remains is an added gcc warning for shifts of >> negative integers when building with -Werror, which happens with the >> 'make W=1' option, as well as for three drivers in the kernel that always >> enable -Werror, but it was only observed with the i915 driver so far. >> To be on the safe side, add -Wno-shift-negative-value to any -Wextra >> in a Makefile. >> >> Nathan Chancellor reported an additional -Wdeclaration-after-statement >> warning that appears in a system header on arm, this still needs a >> workaround. > >Ack; I think we can just fix this in clang. > >> >> The differences between gnu99, gnu11, gnu1x and gnu17 are fairly >> minimal and mainly impact warnings at the -Wpedantic level that the >> kernel never enables. Between these, gnu11 is the newest version >> that is supported by all supported compiler versions, though it is >> only the default on gcc-5, while all other supported versions of >> gcc or clang default to gnu1x/gnu17. > >I agree. With the fixup to s/Werror/Wextra. > >Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> > >> >> Link: https://lore.kernel.org/lkml/CAHk-=wiyCH7xeHcmiFJ-YgXUy2Jaj7pnkdKpcovt8fYbVFW3TA@mail.gmail.com/ >> Link: https://github.com/ClangBuiltLinux/linux/issues/1603 >> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> >> Cc: Masahiro Yamada <masahiroy@kernel.org> >> Cc: linux-kbuild@vger.kernel.org >> Cc: llvm@lists.linux.dev >> Signed-off-by: Arnd Bergmann <arnd@arndb.de> > >-- >Thanks, >~Nick Desaulniers >
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