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SubjectRe: [PATCH 10/11] ring-buffer: move big if statement down

On Wed, 6 May 2009, Ingo Molnar wrote:

>
> * Steven Rostedt <rostedt@goodmis.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.

-- Steve



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