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

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Message-ID: <84c7410d1d3ef56370c698c4e603e5422e337abc.camel@perches.com>
Date:   Wed, 28 Aug 2019 04:59:29 -0700
From:   Joe Perches <joe@...ches.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Julia Lawall <julia.lawall@...6.fr>,
        Denis Efremov <efremov@...ux.com>
Cc:     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 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@...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).

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.