linux-kernel - Re: [PATCH] compiler.h: fix error in BUILD_BUG

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Date:   Tue, 31 Mar 2020 21:08:42 +0200
From:   Vegard Nossum <vegard.nossum@...cle.com>
To:     Joe Perches <joe@...ches.com>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Daniel Santos <daniel.santos@...ox.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Ian Abbott <abbotti@....co.uk>
Subject: Re: [PATCH] compiler.h: fix error in BUILD_BUG_ON() reporting


On 3/31/20 9:00 PM, Joe Perches wrote:
> On Tue, 2020-03-31 at 20:56 +0200, Rasmus Villemoes wrote:
>> On 31/03/2020 20.20, Joe Perches wrote:
>>> On Tue, 2020-03-31 at 13:26 +0200, Vegard Nossum wrote:
>>>>   #define compiletime_assert(condition, msg) \
>>>> -	_compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
>>>> +	_compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>>>
>>> This might be better using something like __LINE__ ## _ ## __COUNTER__
>>>
>>> as line # is somewhat useful to identify the specific assert in a file.
>>>
>>
>> Eh, if the assert fires, doesn't the compiler's diagnostics already
>> contain all kinds of location information?
> 
> I presume if that were enough,
> neither __LINE__ nor __COUNTER__
> would be useful.
> 

__LINE__ is only used currently for creating a unique identifier, as far
as I can tell.

The way it works is that it creates a function declaration with the
attribute __attribute__((error(message))), which makes gcc throw an
error if the function is ever used (i.e. calls are not compiled out).

The number does appear in the output, but it's not even really obvious
that it's a line number. And the compiler's diagnostics are pretty good
at showing the whole "stack trace" of where the call came from
(including the proper line numbers).


Vegard