Re: [PATCH] scripts: coccinelle: check for !(un)?likely usage
From: Joe Perches
Date: Wed Aug 28 2019 - 07:59:34 EST
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@xxxxxxxxx> 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
likely and unlikely use __builtin_expect
It's probable that of the more than 20K uses of
likely and unlikely in the kernel, most have no
real performance effect.