linux-kernel - Re: [PATCH v2] err.h: allow IS_ERR_VALUE to handle properly more types

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Date:	Wed, 03 Feb 2016 11:53:02 +0100
From:	Andrzej Hajda <a.hajda@...sung.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	open list <linux-kernel@...r.kernel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v2] err.h: allow IS_ERR_VALUE to handle properly more types

On 02/03/2016 01:33 AM, Andrew Morton wrote:
> On Thu, 28 Jan 2016 09:27:28 +0100 Andrzej Hajda <a.hajda@...sung.com> wrote:
>
>>  - use '<= 0' instead of '< 0' to silence gcc verbose warnings,
>>  - expand commit message.
>> ---
>>  include/linux/err.h | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/err.h b/include/linux/err.h
>> index 56762ab..43a6adb 100644
>> --- a/include/linux/err.h
>> +++ b/include/linux/err.h
>> @@ -18,7 +18,9 @@
>>  
>>  #ifndef __ASSEMBLY__
>>  
>> -#define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)
>> +#define IS_ERR_VALUE(x) ((typeof(x))(-1) <= 0 \
>> +				? unlikely((x) < 0) \
>> +				: unlikely((x) >= (typeof(x))-MAX_ERRNO))
> I'm still getting a bunch of 
>
> include/linux/err.h: In function 'IS_ERR':
> include/linux/err.h:37: warning: comparison of unsigned expression < 0 is always false
> include/linux/err.h: In function 'IS_ERR_OR_NULL':
> include/linux/err.h:42: warning: comparison of unsigned expression < 0 is always false
>
> with gcc-4.4.4.
>
>

These warnings are false positives and gcc up to 4.7 emits them,
gcc 4.8(which I use) behaves correctly (at least on x86 and arm64). I
have tried
to use __builtin_choose_expr instead of ?: operator but it did not help,
although documentation says "the built-in function does not evaluate
the expression that is not chosen"[1].

The sanest gcc silencer I see for now is to replace:
        ? unlikely((x) < 0) \
with
        ? unlikely((x) <= -1) \

On the other side these warnings are caused by -Wtype-limits switch which
is disabled by default in kernel build and treated as broken by Linus [2].
Maybe it is good enough reason to disregard them? :)

Anyway, I will post another iteration.

[1]:
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-g_t_005f_005fbuiltin_005fchoose_005fexpr-4184
[2]: http://permalink.gmane.org/gmane.linux.kernel/2053963

Regards
Andrzej