Messages in this thread | | | Date | Wed, 6 May 2009 16:37:55 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 10/11] ring-buffer: move big if statement down |
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* Steven Rostedt <rostedt@goodmis.org> wrote:
> > 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.
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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