linux-kernel - Re: [RFC] gcc feature request: Moving blocks into sections

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Date:	Mon, 05 Aug 2013 23:03:35 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jason Baron <jbaron@...mai.com>
Cc:	Richard Henderson <rth@...ddle.net>,
	Marek Polacek <polacek@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, gcc <gcc@....gnu.org>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Daney <ddaney.cavm@...il.com>,
	Behan Webster <behanw@...verseincode.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Herbert Xu <herbert@...dor.hengli.com.au>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections

On Mon, 2013-08-05 at 22:26 -0400, Jason Baron wrote:

> I think if the 'cold' attribute on the default disabled static_key 
> branch moved the text completely out-of-line, it would satisfy your 
> requirement here?
> 
> If you like this approach, perhaps we can make something like this work 
> within gcc. As its already supported, but doesn't quite go far enough 
> for our purposes.

It may not be too bad to use.

> 
> Also, if we go down this path, it means the 2-byte jump sequence is 
> probably not going to be too useful.

Don't count us out yet :-)


static inline bool arch_static_branch(struct static_key *key)
{
	asm goto("1:"
		[...]
		: : "i" (key) : : l_yes);
	return false;
l_yes:
	goto __l_yes;
__l_yes: __attribute__((cold));
	return false;
}

Or put that logic in the caller of arch_static_branch(). Basically, we
may be able to do a short jump to the place that will do a long jump to
the real work.

I'll have to play with this and see what gcc does with the output.

-- Steve


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