On Mon, Feb 28, 2022 at 2:32 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote: > > From: Arnd Bergmann <arnd@xxxxxxxx> > > 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. 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 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@xxxxxxxxxx> > > Link: https://lore.kernel.org/lkml/CAHk-=wiyCH7xeHcmiFJ-YgXUy2Jaj7pnkdKpcovt8fYbVFW3TA@xxxxxxxxxxxxxx/ > Link: https://github.com/ClangBuiltLinux/linux/issues/1603 > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx> > Cc: linux-kbuild@xxxxxxxxxxxxxxx > Cc: llvm@xxxxxxxxxxxxxxx > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> -- Thanks, ~Nick Desaulniers
