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From: Joe Perches <joe@perches.com>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Julia Lawall <julia.lawall@lip6.fr>,
	Denis Efremov <efremov@linux.com>
Cc: Michal Marek <michal.lkml@markovi.net>,
	Nicolas Palix <nicolas.palix@imag.fr>,
	cocci@systeme.lip6.fr, linux-kernel@vger.kernel.org
Subject: Re: [Cocci] [PATCH] scripts: coccinelle: check for !(un)?likely usage
Date: Wed, 28 Aug 2019 04:59:29 -0700	[thread overview]
Message-ID: <84c7410d1d3ef56370c698c4e603e5422e337abc.camel@perches.com> (raw)
In-Reply-To: <95c32d19-eb4d-a214-6332-038610ec3dbd@rasmusvillemoes.dk>

On Wed, 2019-08-28 at 13:33 +0200, 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).

likely and unlikely use __builtin_expect

https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fexpect
https://stackoverflow.com/questions/7346929/what-is-the-advantage-of-gccs-builtin-expect-in-if-else-statements

It's probable that of the more than 20K uses of
likely and unlikely in the kernel, most have no
real performance effect.


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  reply	other threads:[~2019-08-28 12:06 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 [this message]
2019-08-28 12:33         ` Denis Efremov
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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