Re: [PATCH 3/4] fib_trie: print statistics for multiple tables

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 12 Feb 2008 16:50:45 -0800 Stephen Hemminger <shemminger@vyatta.com> wrote:

> +/**
> + * hlist_for_each_entry_continue_rcu - iterate over rcu hlist after current point
> + * @tpos:	the type * to use as a loop cursor.
> + * @pos:	the &struct hlist_node to use as a loop cursor.
> + * @member:	the name of the hlist_node within the struct.
> + */
> +#define hlist_for_each_entry_continue_rcu(tpos, pos, member)		\
> +	for (pos = (pos)->next;						\
> +	     rcu_dereference(pos) && ({ prefetch(pos->next); 1;}) &&	\
> +		     ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
> +	     pos = pos->next)

Is the compiler allowed to look at a term such as

	({ prefetch(pos->next); 1;})

and, when it is used as a truth value, say "hey, that's always true" and
then elide the call to prefetch()?  We've no way of telling because this
remains gcc-specific territory, afaik.

(cc Paul for rcu stuff)