linux-kernel - Re: [PATCH 10/11] ring-buffer: move big if statement down

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Date:	Wed, 6 May 2009 16:37:55 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 10/11] ring-buffer: move big if statement down


* Steven Rostedt <rostedt@...dmis.org> wrote:

> 
> On Wed, 6 May 2009, Ingo Molnar wrote:
> 
> > 
> > * Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > This patch changes it to a goto:
> > > 
> > > 	code;
> > > 
> > > 	if (cross to next page)
> > > 		goto next_page;
> > > 
> > > 	more code;
> > > 
> > > 	return;
> > > 
> > > next_page:
> > > 
> > > 	[ lots of code]
> > 
> > I have pulled it, but could you please change it to a helper 
> > function instead? There's almost never a good reason to combine 
> > 'more code' with 'lots of code' in a single function. It also 
> > documents the unlikeliness, etc.
> 
> As I stated in the change log, I did not want to convert it to a helper 
> function because it uses the variables created before. It would end up 
> going from:
> 
> 	if (write > BUF_PAGE_SIZE)
> 		goto next_page;
> 
> 
> to:
> 
> 	if (write > BUF_PAGE_SIZE)
> 		return rb_move_tail(cpu_buffer, length, tail,
> 				    commit_page, tail_page);
> 
> 
> Although it is the "unlikely" case, it is still a fast path. It 
> happens every time a write into the page buffer crosses a page 
> boundary. Since it is only used once, gcc would hopefully inline 
> it. If it does not, then we are copying a bunch of parameters for 
> nothing.
> 
> I could still do this and see what gcc does with it.

GCC might make a mess of it - but parameters shouldnt be copied 
normally (on 64-bit at least), they'll just be held in parameter 
registers.

	Ingo
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