linux-kernel - Re: [PATCH 1/4] compiler.h: introduce unused

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Date:	Sat, 28 Apr 2012 07:50:10 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/4] compiler.h: introduce unused_expression() macro

H. Peter Anvin wrote:
> On 04/25/2012 04:26 AM, Konstantin Khlebnikov wrote:
>> Sometimes we want to check some expressions correctness in compile-time without
>> generating extra code. "(void)(e)" does not work if expression has side-effects.
>> This patch introduces macro unused_expression() which helps in this situation.
>>
>> Cast to "long" required because sizeof does not work for bit-fields.
>>
>> Signed-off-by: Konstantin Khlebnikov<khlebnikov@...nvz.org>
>> ---
>>   include/linux/compiler.h |    2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
>> index 923d093..46fbda3 100644
>> --- a/include/linux/compiler.h
>> +++ b/include/linux/compiler.h
>> @@ -310,4 +310,6 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
>>    */
>>   #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
>>
>> +#define unused_expression(e) ((void)(sizeof((__force long)(e))))
>> +
>
> OK, bikeshedding a bit:
>
> "unused_expression()" doesn't sound like something that inhibits side
> effects, especially since "unused" applied to an argument or variable
> means "don't make a fuss over this not being used."
>
> 	-hpa
>

Ok, in v2 I'll rename it into BUILD_BUG_ON_INVALID()
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