Re: [PATCH v2 1/3] kbuild: add macro for controlling warnings to linux/compiler.h

[Masahiro Yamada <yamada.masahiro@socionext.com>]

Hi.


2018-06-20 5:14 GMT+09:00 Paul Burton <paul.burton@mips.com>:
> From: Arnd Bergmann <arnd@arndb.de>
>
> I have occasionally run into a situation where it would make sense to
> control a compiler warning from a source file rather than doing so from
> a Makefile using the $(cc-disable-warning, ...) or $(cc-option, ...)
> helpers.
>
> The approach here is similar to what glibc uses, using __diag() and
> related macros to encapsulate a _Pragma("GCC diagnostic ...") statement
> that gets turned into the respective "#pragma GCC diagnostic ..." by
> the preprocessor when the macro gets expanded.
>
> Like glibc, I also have an argument to pass the affected compiler
> version, but decided to actually evaluate that one. For now, this
> supports GCC_4_6, GCC_4_7, GCC_4_8, GCC_4_9, GCC_5, GCC_6, GCC_7,
> GCC_8 and GCC_9. Adding support for CLANG_5 and other interesting
> versions is straightforward here. GNU compilers starting with gcc-4.2
> could support it in principle, but "#pragma GCC diagnostic push"
> was only added in gcc-4.6, so it seems simpler to not deal with those
> at all. The same versions show a large number of warnings already,
> so it seems easier to just leave it at that and not do a more
> fine-grained control for them.
>
> The use cases I found so far include:
>
> - turning off the gcc-8 -Wattribute-alias warning inside of the
>   SYSCALL_DEFINEx() macro without having to do it globally.
>
> - Reducing the build time for a simple re-make after a change,
>   once we move the warnings from ./Makefile and
>   ./scripts/Makefile.extrawarn into linux/compiler.h
>
> - More control over the warnings based on other configurations,
>   using preprocessor syntax instead of Makefile syntax. This should make
>   it easier for the average developer to understand and change things.
>
> - Adding an easy way to turn the W=1 option on unconditionally
>   for a subdirectory or a specific file. This has been requested
>   by several developers in the past that want to have their subsystems
>   W=1 clean.
>
> - Integrating clang better into the build systems. Clang supports
>   more warnings than GCC, and we probably want to classify them
>   as default, W=1, W=2 etc, but there are cases in which the
>   warnings should be classified differently due to excessive false
>   positives from one or the other compiler.
>
> - Adding a way to turn the default warnings into errors (e.g. using
>   a new "make E=0" tag) while not also turning the W=1 warnings into
>   errors.
>
> This patch for now just adds the minimal infrastructure in order to
> do the first of the list above. As the #pragma GCC diagnostic
> takes precedence over command line options, the next step would be
> to convert a lot of the individual Makefiles that set nonstandard
> options to use __diag() instead.
>
> [paul.burton@mips.com:
>   - Rebase atop current master.
>   - Add __diag_GCC, or more generally __diag_<compiler>, abstraction to
>     avoid code outside of linux/compiler-gcc.h needing to duplicate
>     knowledge about different GCC versions.
>   - Add a comment argument to __diag_{ignore,warn,error} which isn't
>     used in the expansion of the macros but serves to push people to
>     document the reason for using them - per feedback from Kees Cook.
>   - Translate severity to GCC-specific pragmas in linux/compiler-gcc.h
>     rather than using GCC-specific in linux/compiler_types.h.
>   - Drop all but GCC 8 macros, since we only need to define macros for
>     versions that we need to introduce pragmas for, and as of this
>     series that's just GCC 8.
>   - Capitalize comments in linux/compiler-gcc.h to match the style of
>     the rest of the file.
>   - Line up macro definitions with tabs in linux/compiler-gcc.h.]
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Paul Burton <paul.burton@mips.com>
> Tested-by: Christophe Leroy <christophe.leroy@c-s.fr>
> Tested-by: Stafford Horne <shorne@gmail.com>
> Cc: Michal Marek <michal.lkml@markovi.net>
> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> Cc: Douglas Anderson <dianders@chromium.org>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> Cc: Matthew Wilcox <matthew@wil.cx>
> Cc: Matthias Kaehlcke <mka@chromium.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Gideon Israel Dsouza <gidisrael@gmail.com>
> Cc: Christophe Leroy <christophe.leroy@c-s.fr>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Stafford Horne <shorne@gmail.com>
> Cc: Khem Raj <raj.khem@gmail.com>
> Cc: He Zhe <zhe.he@windriver.com>
> Cc: linux-kbuild@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mips@linux-mips.org
> Cc: linuxppc-dev@lists.ozlabs.org
>
> ---
>
> Changes in v2:
> - Add version argument to fallback __diag_GCC definition.
> - Translate severity from generic ignore,warn,error to GCC-specific
>   pragma content ignored,warning,error in linux/compiler-gcc.h in order
>   to keep linux/compiler_types.h generic per feedback from Masahiro
>   Yamada.
> - Drop all but GCC 8 macros, since we only need to define macros for
>   versions that we need to introduce pragmas for, and as of this series
>   that's just GCC 8.
> - Capitalize comments in linux/compiler-gcc.h to match the style of the
>   rest of the file.
> - Line up macro definitions with tabs in linux/compiler-gcc.h.
>
>  include/linux/compiler-gcc.h   | 27 +++++++++++++++++++++++++++
>  include/linux/compiler_types.h | 18 ++++++++++++++++++
>  2 files changed, 45 insertions(+)
>
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index f1a7492a5cc8..5067a90af9c3 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -347,3 +347,30 @@
>  #if GCC_VERSION >= 50100
>  #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
>  #endif
> +
> +/*
> + * Turn individual warnings and errors on and off locally, depending
> + * on version.
> + */
> +#define __diag_GCC(version, severity, s) \
> +       __diag_GCC_ ## version(__diag_GCC_ ## severity s)
> +
> +/* Severity used in pragma directives */
> +#define __diag_GCC_ignore      ignored
> +#define __diag_GCC_warn                warning
> +#define __diag_GCC_error       error
> +
> +/* Compilers before gcc-4.6 do not understand "#pragma GCC diagnostic push" */
> +#if GCC_VERSION >= 40600
> +#define __diag_str1(s)         #s
> +#define __diag_str(s)          __diag_str1(s)
> +#define __diag(s)              _Pragma(__diag_str(GCC diagnostic s))
> +#else
> +#define __diag(s)


You can omit the #else here
because it is covered by compiler_types.h



> +#endif
> +
> +#if GCC_VERSION >= 80000
> +#define __diag_GCC_8(s)                __diag(s)
> +#else
> +#define __diag_GCC_8(s)
> +#endif
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 6b79a9bba9a7..a8ba6b04152c 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -271,4 +271,22 @@ struct ftrace_likely_data {
>  # define __native_word(t) (sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))
>  #endif
>
> +#ifndef __diag
> +#define __diag(string)
> +#endif
> +
> +#ifndef __diag_GCC
> +#define __diag_GCC(version, severity, string)
> +#endif
> +
> +#define __diag_push()  __diag(push)
> +#define __diag_pop()   __diag(pop)
> +
> +#define __diag_ignore(compiler, version, option, comment) \
> +       __diag_ ## compiler(version, ignore, option)
> +#define __diag_warn(compiler, version, option, comment) \
> +       __diag_ ## compiler(version, warn, option)
> +#define __diag_error(compiler, version, option, comment) \
> +       __diag_ ## compiler(version, error, option)
> +
>  #endif /* __LINUX_COMPILER_TYPES_H */
> --
> 2.17.1
>
> --
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-- 
Best Regards
Masahiro Yamada