linux-kernel - Re: [PATCH 1/2] linux/kconfig.h: generalize IS

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Date:	Wed, 7 Oct 2015 10:15:27 +0200
From:	Michal Marek <mmarek@...e.cz>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Jiri Slaby <jslaby@...e.cz>, Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] linux/kconfig.h: generalize IS_ENABLED logic

On 2015-10-06 23:05, Rasmus Villemoes wrote:
> It's not hard to generalize the macro magic used to build the
> IS_ENABLED macro and friends to produce a few other potentially useful
> macros:
> 
> CHOOSE_EXPR(CONFIG_FOO, expr): if CONFIG_FOO is set expands to
> expr, otherwise expands to nothing.
> 
> CHOOSE_EXPR(CONFIG_FOO, expr1, expr2): if CONFIG_FOO is set,
> expands to expr1, otherwise expands to expr2.

FWIW, I agree with Ingo that the CHOOSE_EXPR name is not really obvious.
IF_CONFIG is a better alternative IMO, since the average programmer
probably does not know __builtin_choose_expr() to see the analogy.


> Similarly, we can define helpers for conditional struct members and
> their associated initializers. It would probably take some time to get
> used to reading, to pick another random example,
> 
> struct task_struct {
>    ...
>    COND_DECLARATION(CONFIG_KASAN, unsigned int kasan_depth)
>    ...
> }

While the C standard syntax requires struct-declaration to actually
declare a member, the compiler will happily ignore the extra semicolon
if you write

truct task_struct {
   ...
   CHOOSE_EXPR(CONFIG_KASAN, unsigned int kasan_depth);
   ...
}

So I think that the COND_DECLARATION macro is not necessary.


> #define INIT_KASAN(tsk) COND_INITIALIZER(CONFIG_KASAN, .kasan_depth = 1)

COND_INITIALIZER on the other hand is useful (CHOOSE_EXPR(CONFIG_KASAN,
.kasan_depth = 1 _COMMA) does does not work, unfortunately).


> [and I'm certainly not proposing any mass conversion], but I think it
> might be nice to avoid lots of short #ifdef/#else/#endif sections.

It should be accompanied by a patch to scripts/tags.sh teaching
ctags/etags about the new macros.

Michal

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