linux-kernel - Re: [PATCH 7/7] Compiler Attributes: use feature checks instead of version checks

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Date:   Sat, 1 Sep 2018 11:56:40 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Eli Friedman <efriedma@...eaurora.org>,
        Christopher Li <sparse@...isli.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Joe Perches <joe@...ches.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH 7/7] Compiler Attributes: use feature checks instead of
 version checks

On Fri, Aug 31, 2018 at 07:05:14PM +0200, Miguel Ojeda wrote:
> Instead of using version checks per-compiler to define (or not)
> each attribute, use __has_attribute to test for them, following
> the cleanup started with commit 815f0ddb346c
> ("include/linux/compiler*.h: make compiler-*.h mutually exclusive").
> 
> All the attributes that are fairly common/standard (i.e. those that
> do not require extra logic to define them) have been moved
> to a new file include/linux/compiler_attributes.h.
> 
> In an effort to make the file as regular as possible, comments
> stating the purpose of attributes have been removed. Instead,
> links to the compiler docs have been added (i.e. to gcc and,
> if available, to clang as well). In addition, they have been sorted.
> 
> Finally, if an attribute is optional (i.e. if it is guarded
> by __has_attribute), the reason has been stated for future reference.
> 
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> new file mode 100644
> index 000000000000..a9dfafc8fd19
> --- /dev/null
> +++ b/include/linux/compiler_attributes.h
> @@ -0,0 +1,226 @@
> +#ifndef __LINUX_COMPILER_ATTRIBUTES_H
> +#define __LINUX_COMPILER_ATTRIBUTES_H
> +
> +/*
> + * This file is meant to be sorted (by actual attribute name,
> + * not by #define identifier).
> + *
> + * Do not add here attributes which depend on others or require extra logic.
> + *
> + * If an attribute is optional, state the reason in the comment.
> + */
> +
> +/*
> + * To check for optional attributes, we use __has_attribute, which is supported
> + * on gcc >= 5, clang >= 2.9 and icc >= 17. In the meantime, to support
> + * 4.6 <= gcc < 5, we implement __has_attribute by hand.
> + */
> +#ifndef __has_attribute
> +#define __has_attribute(x) __GCC4_has_attribute_##x
> +#define __GCC4_has_attribute_externally_visible 1
> +#define __GCC4_has_attribute_noclone 1
> +#define __GCC4_has_attribute_optimize 1
> +#if __GNUC_MINOR__ >= 8
> +#define __GCC4_has_attribute_no_sanitize_address 1
> +#endif
> +#if __GNUC_MINOR__ >= 9
> +#define __GCC4_has_attribute_assume_aligned 1
> +#endif
> +#endif

For sparse (which doesn't support __has_attribute() yet and defines
__GNUC_MINOR__ depending on the compiler used to build it) it won't
be totally correct since the concerned attributes here will be
incorrectly considered as not supported. But, since these attributes
have no semantic effects for sparse, it won't matter.

So, for sparse:
Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@...il.com>

-- Luc