linux-kernel - Re: [PATCH v3 7/7] dynamic_debug: add jump label support

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Date:	Mon, 11 Jul 2016 09:18:21 -0400
From:	Jason Baron <jbaron@...mai.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	<joe@...ches.com>, <peterz@...radead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 7/7] dynamic_debug: add jump label support

On 07/08/2016 05:41 PM, Andrew Morton wrote:
> On Wed,  6 Jul 2016 17:42:36 -0400 Jason Baron <jbaron@...mai.com> wrote:
>
>> Although dynamic debug is often only used for debug builds, sometimes its
>> enabled for production builds as well. Minimize its impact by using jump
>> labels. This reduces the text section by 7000+ bytes in the kernel image
>> below. It does increase data, but this should only be referenced when
>> changing the direction of the branches, and hence usually not in cache.
>>
>> ...
>>
>> +#ifdef HAVE_JUMP_LABEL
>> +
>> +#define dd_key_init(key, init) key = (init)
>>
>> ...
>>
>> +#else
>> +
>> +#define dd_key_init(key, init)
>> +
> umm, lazy.  One is an lval and returns a value and the other does
> neither.  Lack of parenthesization in the first version doubtless
> exposes various horrors.
>
> Care to do something more robust and conventional here?  Presumably use
> symmetrical do{}while(0) things, neither of which is an lval, both of
> which don't return anything.
>

Hi,

The 'dd_key_init()' macro is being used here to help initialize
the 'key' field in the 'struct _ddebug', and its not being used as a
statement.

In the 'HAVE_JUMP_LABEL' case, we are initializing the 'key' field, while in
the not-'HAVE_JUMP_LABEL' case, the 'key' field is simply not present
in the structure (to conserve space).

Thanks,

-Jason