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

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Date:	Thu, 14 Jan 2016 23:39:37 +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


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