linux-kernel - Re: [PATCH] scripts: coccinelle: check for !(un)?likely usage

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Date:   Sun, 1 Sep 2019 20:39:30 +0300
From:   Denis Efremov <efremov@...ux.com>
To:     Pavel Machek <pavel@....cz>, Joe Perches <joe@...ches.com>
Cc:     cocci@...teme.lip6.fr, linux-kernel@...r.kernel.org,
        Julia Lawall <Julia.Lawall@...6.fr>,
        Gilles Muller <Gilles.Muller@...6.fr>,
        Nicolas Palix <nicolas.palix@...g.fr>,
        Michal Marek <michal.lkml@...kovi.net>
Subject: Re: [PATCH] scripts: coccinelle: check for !(un)?likely usage



On 01.09.2019 20:24, Pavel Machek wrote:
> Hi!
> 
>>> This patch adds coccinelle script for detecting !likely and !unlikely
>>> usage. It's better to use unlikely instead of !likely and vice versa.
>>
>> Please explain _why_ is it better in the changelog.
>>
>> btw: there are relatively few uses like this in the kernel.
>>
>> $ git grep -P '!\s*(?:un)?likely\s*\(' | wc -l
>> 40
>>
>> afaict: It may save 2 bytes of x86/64 object code.
>>
>> For instance:
>>
>> $ diff -urN kernel/tsacct.lst.old kernel/tsacct.lst.new|less
>> --- kernel/tsacct.lst.old       2019-08-25 09:21:39.936570183 -0700
>> +++ kernel/tsacct.lst.new       2019-08-25 09:22:20.774324886 -0700
>> @@ -24,158 +24,153 @@
>>    15:  48 89 fb                mov    %rdi,%rbx
>>         u64 time, delta;
>>  
>> -       if (!likely(tsk->mm))
>> +       if (unlikely(tsk->mm))
> 
> Are you sure this is equivalent?
> 
> 									Pavel

Hi,

No, it's not correct. Thanks Rasmus for clarifying the things here.
This was my mistake. As a human, I failed to parse "likely" and "!"
and made too hasty conclusions :)

The correct transformation is in v2. This will be
!likely(tsk->mm) -> unlikely(!tsk->mm)

Thanks,
Denis