linux-kernel - Re: [PATCH 00/10] jump label: introduce very

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Message-ID: <20120221211617.GB15210@Krystal>
Date:	Tue, 21 Feb 2012 16:16:17 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jbaron@...hat.com>, a.p.zijlstra@...llo.nl,
	mingo@...e.hu, davem@...emloft.net, ddaney.cavm@...il.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups
	+ docs

* H. Peter Anvin (hpa@...or.com) wrote:
> On 02/21/2012 12:39 PM, Steven Rostedt wrote:
> > On Tue, 2012-02-21 at 15:20 -0500, Jason Baron wrote:
> > 
> >> I'm not really too hung up on the naming, but I did think that
> >> very_[un]likely were an interesting possibility.
> > 
> > The problem comes from what Peter said. They are too similar to
> > "likely()" and "unlikely()", and can become confusing.
> > 
> > Maybe "static_likely()" and "static_unlikely()" as the word "static" can
> > imply something strange about these. Or perhaps a "const_likely()"?
> > 
> > Maybe "dynamic_branch_true()" and "dynamic_branch_false()". This may be
> > the most descriptive.
> > 
> 
> I thought about this some more, and the very_[un]likely() naming is even
> worse than I originally thought: the jump label stuff isn't about the
> bias level, but rather if a static decision (on the order or once per
> boot) can be made to go one way or the other.
> 
> 	-hpa

I agree that this decision is taken typically once at boot time, so
claiming it is only a strong compiler bias hint for block placement
would be a lie. However, I think the fact that the fall-through is for
either true or false branch seems to be an implicit bias. Therefore, I
start to like the static_likely()/static_unlikely(), which conveys both
the static nature of the branch, as well as the bias.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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