From: Denis Efremov <efremov@linux.com>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Julia Lawall <julia.lawall@lip6.fr>
Cc: Michal Marek <michal.lkml@markovi.net>,
Nicolas Palix <nicolas.palix@imag.fr>,
linux-kernel@vger.kernel.org, Joe Perches <joe@perches.com>,
cocci@systeme.lip6.fr
Subject: Re: [Cocci] [PATCH] scripts: coccinelle: check for !(un)?likely usage
Date: Wed, 28 Aug 2019 15:33:46 +0300 [thread overview]
Message-ID: <16053035-655a-7d53-29d1-ea914e3a21dd@linux.com> (raw)
In-Reply-To: <95c32d19-eb4d-a214-6332-038610ec3dbd@rasmusvillemoes.dk>
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@linux.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
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next prev parent reply other threads:[~2019-08-28 12:34 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-25 13:05 [Cocci] [PATCH] scripts: coccinelle: check for !(un)?likely usage Denis Efremov
2019-08-25 15:30 ` Markus Elfring
2019-08-25 21:06 ` Denis Efremov
2019-08-25 15:30 ` Markus Elfring
2019-08-25 16:37 ` Joe Perches
2019-08-25 18:59 ` Denis Efremov
2019-08-25 19:19 ` Julia Lawall
2019-08-28 11:33 ` Rasmus Villemoes
2019-08-28 11:59 ` Joe Perches
2019-08-28 12:33 ` Denis Efremov [this message]
2019-08-28 12:33 ` Julia Lawall
2019-08-28 12:41 ` Denis Efremov
2019-08-28 13:57 ` Denis Efremov
2019-08-25 21:19 ` Denis Efremov
[not found] ` <20190901172403.GA1047@bug>
2019-09-01 17:39 ` Denis Efremov
2019-08-29 17:10 ` [Cocci] [PATCH v2] " Denis Efremov
2019-08-29 17:13 ` Denis Efremov
2019-08-30 0:42 ` Julia Lawall
2019-08-30 6:56 ` Denis Efremov
2019-08-30 8:06 ` Rasmus Villemoes
2019-08-29 20:07 ` Markus Elfring
2019-08-30 7:55 ` Markus Elfring
2019-09-06 20:19 ` Julia Lawall
2019-09-06 20:55 ` Denis Efremov
2019-09-07 8:05 ` [Cocci] [v2] " Markus Elfring
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