From: Nick Piggin <nickpiggin@yahoo.com.au>
To: "Yinghai Lu" <yhlu.kernel@gmail.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
"Christoph Lameter" <clameter@sgi.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
"Adrian Bunk" <bunk@stusta.de>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: free_pages_check
Date: Tue, 8 Jan 2008 14:34:14 +1100 [thread overview]
Message-ID: <200801081434.14723.nickpiggin@yahoo.com.au> (raw)
In-Reply-To: <86802c440801071843p583f390as53f8b600622b93b2@mail.gmail.com>
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.
next prev parent reply other threads:[~2008-01-08 3:34 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-01-08 2:43 free_pages_check Yinghai Lu
2008-01-08 3:08 ` free_pages_check Andrew Morton
2008-01-08 3:34 ` Nick Piggin [this message]
2008-01-08 5:44 ` free_pages_check H. Peter Anvin
2008-01-08 7:44 ` free_pages_check Nick Piggin
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