From: Denis Efremov <efremov@linux.com>
To: Julia Lawall <julia.lawall@lip6.fr>
Cc: Michal Marek <michal.lkml@markovi.net>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Nicolas Palix <nicolas.palix@imag.fr>,
linux-kernel@vger.kernel.org,
Markus Elfring <Markus.Elfring@web.de>,
Joe Perches <joe@perches.com>,
cocci@systeme.lip6.fr
Subject: Re: [Cocci] [PATCH v2] scripts: coccinelle: check for !(un)?likely usage
Date: Fri, 30 Aug 2019 09:56:26 +0300 [thread overview]
Message-ID: <7933d51f-5c2e-26a4-2dee-e13e61d0ac8c@linux.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1908300842060.2184@hadrien>
On 30.08.2019 03:42, Julia Lawall wrote:
>
>
> On Thu, 29 Aug 2019, Denis Efremov wrote:
>
>> On 8/29/19 8:10 PM, Denis Efremov wrote:
>>> This patch adds coccinelle script for detecting !likely and
>>> !unlikely usage. These notations are confusing. It's better
>>> to replace !likely(x) with unlikely(!x) and !unlikely(x) with
>>> likely(!x) for readability.
>>
>> I'm not sure that this rule deserves the acceptance.
>> Just to want to be sure that "!unlikely(x)" and "!likely(x)"
>> are hard-readable is not only my perception and that they
>> become more clear in form "likely(!x)" and "unlikely(!x)" too.
>
> Is likely/unlikely even useful for anything once it is a subexpression?
>> julia
>
Well, as far as I understand it,
It's correct since it sets the probability of likely/unlikely subexpression
is true to 90% (see https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc/Other-Builtins.html).
The probability of a whole expression is then computed by GCC
in this case. It's kind of assigning individual weights to conjuncts/disjuncts.
I think that it can be useful when you are not sure about the probability
of the whole expression but you know something about subexpressions it consists, e.g.,
likely(E1) && E2. However, I think that "!unlikely(x)" is fully equivalent in this sense
to "likely(!x)". I tested it once again for allyesconfig with branch profiling
disabled and bloat-o-meter shows no diff in binary size.
Denis
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next prev parent reply other threads:[~2019-08-30 6:56 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
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 [this message]
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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