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

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Message-ID: <1329924752.24994.25.camel@twins>
Date:	Wed, 22 Feb 2012 16:32:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Jason Baron <jbaron@...hat.com>,
	mathieu.desnoyers@...icios.com, davem@...emloft.net,
	ddaney.cavm@...il.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	rth <rth@...hat.com>
Subject: Re: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups
 + docs

On Wed, 2012-02-22 at 16:13 +0100, Peter Zijlstra wrote:
> On Wed, 2012-02-22 at 08:54 -0500, Steven Rostedt wrote:
> > But we could push a "unlikely(static_branch())" for
> > such a case (or keep it as very_unlikely()). 
> 
> If only that would work.. GCC assumes a bunch of things when you use
> 'asm goto'. An unbiased 'asm goto' that would take likely() and
> unlikely() hints would be ideal, but alas that's not how the thing got
> implemented.
> 
> Now arguable we could maybe just stick with the static_branch() thing
> and decide that GCC is broken for adding bias and not respecting
> likely() and unlikely(), but the GCC folks might have an opinion there.
> 
> Anyway, if we have to stick with the current implementation then an
> unbiased version is impossible to implement.
> 
> If we get GCC folks on board to change stuff, who knows.

So I clicked the link Jason provided in his 10/10 Documentation patch
and stumbled upon:

  http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01558.html

Where rth suggests that __attribute__((hot,cold)) might work on the
destination labels. Trying this my compiler (4.6.1+crap) pukes all over
me suggesting this isn't (yet) implemented.

Richard, is something like that still on the table?
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