linux-kernel - Re: [PATCH] fs: dcache: Use bool return value instead of int

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Date:	Mon, 25 Jan 2016 05:19:43 +0800
From:	Chen Gang <chengang@...ndsoft.com.cn>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	dhowells@...hat.com, akpm@...ux-foundation.org,
	nicolas.iooss_linux@....org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: dcache: Use bool return value instead of int

Hello all:

Is this patch OK? shall I send the other patch based on this one? (the
other patch is v3 trivial patch for include/linux/dcache.h).

And sorry for replying late: the last week, I was not in Beijing, had to
be busy for analyzing a Linux kernel usb related issue for my company's
customer in Guangzhou (but at last, I guess, it is not kernel issue).


Thanks.

On 1/14/16 23:39, Chen Gang wrote:
> 
> On 1/14/16 06:54, Al Viro wrote:
>> On Thu, Jan 14, 2016 at 06:39:53AM +0800, Chen Gang wrote:
>>
>>>> As for the inlines... frankly, if gcc generates a different code from having
>>>> replaced int with bool in those, it's time to do something very nasty to
>>>> gcc developers.
>>>>
>>>
>>> Could you provide the related proof?
>>
>> static inline _Bool f(.....)
>> {
>> 	return <int expression>;
>> }
>>
>> ...
>> 	if (f(.....))
>>
> 
> For me, your case above isn't suitable for using bool. Please check this
> patch, there is no any cases like you said above.
> 
>  - For d_unhashed() which return hlist_bl_unhashed(), it seems like your
>    case, but in fact hlist_bl_unhashed() also need return bool (which I
>    shall send another patch for, next).
> 
>  - And all the other changes of this patch are all for real, pure bool
>    functions.
> 
> Thanks.
> 
>> should generate the code identical to
>> 	if ((_Bool)<int expression>)
>> which, in turn, should generate the code identical to
>> 	if (<int expression> != 0)
>> and
>> 	if (<int expression>)
>>
>> Neither explicit nor implicit conversion to _Bool (the former by the explicit
>> cast, the latter - by declaring f() to return _Bool) matters at all when the
>> damn thing is inlined in a condition context.  Conversion to _Bool is
>> equivalent to comparison with 0, and so is the use in condition of if() and
>> friends.
>>
>> For something not inlined you might get different code generated due to a
>> difference in calling sequences of _Bool(...) and int(...); for inlined
>> case having one of those variants produce a better code means that compiler
>> has managed to miss some trivial optimization in all other variants.
>>
>> And I'm yet to see any proof that gcc *does* fuck up in that fashion.  It
>> might - dumb bugs happen to everyone, but I would not assume that they'd
>> managed to do something that bogys without experimental evidence.
>>
> 
> For your cases, what you said sounds OK to me (although I am not quite
> sure what you said above whether precise or not).
> 
> Thanks.
> 

-- 
Chen Gang (陈刚)

Open, share, and attitude like air, water, and life which God blessed