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Date: Fri, 19 Nov 2010 08:21:25 -0800 (PST) From: David Miller <davem@...emloft.net> To: eric.dumazet@...il.com Cc: hagen@...u.net, netdev@...r.kernel.org, xiaosuo@...il.com Subject: Re: [PATCH net-next-2.6] filter: optimize sk_run_filter From: Eric Dumazet <eric.dumazet@...il.com> Date: Fri, 19 Nov 2010 12:17:52 +0100 > Le vendredi 19 novembre 2010 à 10:54 +0100, Eric Dumazet a écrit : > >> I believe we should revert the u32 f_k = fentry->k; part >> >> fentry->k as is fast as f_k if stored on stack, and avoids one >> instruction if fentry->k is not needed. >> >> > > A revert is not good on arches with decent number of registers (x86_64 > for example). -EFIX_THE_DAMN_COMPILER We never make calls out of this function or touch volatile memory or create a full memory barrier between the assignment of f_k and it's uses. Therefore if common sub-expression elimination is working the compiler will be able to decide properly whether to access things via memory or use a register for the value. Remember this is why we have that ACCESS_ONCE() thing. We can't have it both ways, either ACCESS_ONCE() should be removed or we should never make changes like your's and instead should submit compiler bug reports :-) -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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