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

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Date:   Wed, 28 Aug 2019 20:33:51 +0800 (CST)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
cc:     Denis Efremov <efremov@...ux.com>, 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 Wed, 28 Aug 2019, 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).

Thanks.  Making the change seems like a good idea.

julia