Re: [PATCH 1/4] compiler.h: introduce unused_expression() macro

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Andrew Morton wrote:
> On Wed, 25 Apr 2012 15:26:23 +0400
> Konstantin Khlebnikov<khlebnikov@openvz.org>  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@openvz.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))))
>> +
>
> btw, please let's not add undocumented stuff, particularly obscure
> undocumented stuff.
>
> I typed this in:

Thanks!

>
> --- a/include/linux/compiler.h~compilerh-introduce-unused_expression-macro-fix
> +++ a/include/linux/compiler.h
> @@ -310,6 +310,11 @@ void ftrace_likely_update(struct ftrace_
>    */
>   #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
>
> +/*
> + * unused_expression() permits the compiler to check the validity of the
> + * expression but avoids the generation of any code, even if that expression
> + * has side-effects.
> + */
>   #define unused_expression(e) ((void)(sizeof((__force long)(e))))
>
>   #endif /* __LINUX_COMPILER_H */
>
> but then decided I didn't want to apply the patches (yet?).

I'm OK with that. But probably Hugh Dickins (now in CC) aggressively wants to
remove side-effects of VM_BUG_ON's in case CONFIG_DEBUG_VM=n

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