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

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Date:	Mon, 05 Aug 2013 15:04:29 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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.apana.org.au>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections

On Mon, 2013-08-05 at 11:34 -0700, Linus Torvalds wrote:
> On Mon, Aug 5, 2013 at 11:24 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Ugh. I can see the attraction of your section thing for that case, I
> > just get the feeling that we should be able to do better somehow.
> 
> Hmm.. Quite frankly, Steven, for your use case I think you actually
> want the C goto *labels* associated with a section. Which sounds like
> it might be a cleaner syntax than making it about the basic block
> anyway.

I would love to. But IIRC, the asm_goto() has some strict constraints.
We may be able to jump to a different section, but we have no way of
coming back. Not to mention, you must tell the asm goto() what label you
may be jumping to.

I don't know how safe something like this may be:


static inline trace_sched_switch(prev, next)
{
	asm goto("jmp foo1\n" : : foo2);
 foo1:
	return;

	asm goto(".pushsection\n"
		"section \".foo\"\n");
 foo2:
	__trace_sched_switch(prev, next);
	asm goto("jmp foo1"
		".popsection\n" : : foo1);
}


The above looks too fragile for my taste. I'm afraid gcc will move stuff
out of those "asm goto" locations, and make things just fail. But I can
play with this, but I don't like it.

-- Steve


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