On Mon, Feb 28, 2022 at 8:25 PM Mark Rutland [off-list ref] wrote:

Hi Arnd,

This is great!

On Mon, Feb 28, 2022 at 11:27:43AM +0100, Arnd Bergmann wrote:

quoted

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
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.

FWIW, I had a go at moving to c99 a few weeks ago (to be able to use
for-loop-declarations in some concurrency primitives), and when I tried, I also
saw declaration-after-statement warnings when building modpost.c, which is easy
enough to fix:

  https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=treewide/gnu99&id=505775bd6fd0bc1883f3271f826963066bbdc194

I do not understand this statement:

"Usually such warnings are implciitly enabled as part of `-std=gnu89`,
 and in preparation for changing the standard used, this patch explciitly
enales the warnings with `-Wdeclaration-after-statement`, which takes
effect regardless of which version of the C standard is in use."



modpost is already built with -std=gnu89.

If  Wdeclaration-after-statement is implied by gnu89,
why did nobody notice this before?


-- 
Best Regards
Masahiro Yamada

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