Re: free_pages_check

Nick Piggin
2008-01-08T03:34:44Z

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.