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

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Date:   Wed, 28 Aug 2019 15:33:46 +0300
From:   Denis Efremov <efremov@...ux.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Julia Lawall <julia.lawall@...6.fr>
Cc:     Joe Perches <joe@...ches.com>, cocci@...teme.lip6.fr,
        linux-kernel@...r.kernel.org,
        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 8/28/19 2:33 PM, Rasmus Villemoes wrote:
> On 25/08/2019 21.19, Julia Lawall wrote:
>>
>>
>>> On 26 Aug 2019, at 02:59, Denis Efremov <efremov@...ux.com> wrote:
>>>
>>>
>>>
>>>> On 25.08.2019 19:37, Joe Perches wrote:
>>>>> On Sun, 2019-08-25 at 16:05 +0300, Denis Efremov wrote:
>>>>> 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.
>>>>
>>>
>>> In my naive understanding the negation (!) before the likely/unlikely
>>> could confuse the compiler
>>
>> As a human I am confused. Is !likely(x) equivalent to x or !x?
> 
> #undef likely
> #undef unlikely
> #define likely(x) (x)
> #define unlikely(x) (x)
> 
> should be a semantic no-op. So changing !likely(x) to unlikely(x) is
> completely wrong. If anything, !likely(x) can be transformed to
> unlikely(!x).

As far as I could understand it:

# define likely(x)	__builtin_expect(!!(x), 1)
# define unlikely(x)	__builtin_expect(!!(x), 0)

>From GCC doc:
__builtin_expect compares the values. The semantics of the built-in are that it is expected that exp == c.

if (!likely(cond))
if (!__builtin_expect(!!(cond), 1))
if (!((!!(cond)) == 1))
if ((!!(cond)) != 1) and since !! could result in 0 or 1
if ((!!(cond)) == 0)

if (unlikely(cond))
if (__builtin_expect(!!(cond), 0))
if ((!!(cond)) == 0))

Thanks,
Denis