linux-kernel - Re: free_pages

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Message-Id: <200801081844.22448.nickpiggin@yahoo.com.au>
Date:	Tue, 8 Jan 2008 18:44:22 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Yinghai Lu <yhlu.kernel@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Adrian Bunk <bunk@...sta.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: free_pages_check

On Tuesday 08 January 2008 16:44, H. Peter Anvin wrote:
> Nick Piggin wrote:
> > On Tuesday 08 January 2008 13:43, Yinghai Lu wrote:
> >> wonder why free_pages_check mm/page_alloc.c is using bit OR than logical
> >> OR
> >>
> >> @@ -450,9 +450,9 @@ static inline void __free_one_page(struc
> >>
> >>  static inline int free_pages_check(struct page *page)
> >>  {
> >> -       if (unlikely(page_mapcount(page) |
> >> -               (page->mapping != NULL)  |
> >> -               (page_count(page) != 0)  |
> >> +       if (unlikely(page_mapcount(page) ||
> >> +               (page->mapping != NULL)  ||
> >> +               (page_count(page) != 0)  ||
> >>                 (page->flags & (
> >>                         1 << PG_lru     |
> >>                         1 << PG_private |
> >
> > Because the positive case is extremely rare, so there is no benefit (nor
> > any correctness requirement) for short-circuit evaluation, and we don't
> > want to have all the branches that it involves. I think it is 3 more
> > conditional jumps.
>
> Depends on how smart the compiler is.  If the page_() functions are
> inlines or macros, there is only one pointer reference involved and it
> should be able to do that transformation.  Whether or not gcc is that
> smart is another matter.

Mine is not.
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