From: Andi Kleen <ak@suse.de>
To: David Miller <davem@davemloft.net>
Cc: dada1@cosmosbay.com, mchan@broadcom.com, netdev@vger.kernel.org
Subject: Re: [PATCH] tg3 : avoid an expensive divide
Date: 07 Feb 2007 10:54:49 +0100 [thread overview]
Message-ID: <p73ireemgwm.fsf@bingen.suse.de> (raw)
In-Reply-To: <20070206.131908.13769204.davem@davemloft.net>
David Miller <davem@davemloft.net> writes:
>
> Because I've seen gcc optimize this properly before (at least on
> sparc64), it means that either:
>
> 1) There is a GCC bug where the properties of the constants
> do not propagate.
>
> 2) GCC really thinks the divide is cheaper (code density vs.
> cycle count tradeoffs etc.)
Probably Eric compiled with the now default CONFIG_CC_OPTIMIZE_FOR_SIZE/-Os.
With that gcc decides to use the shorter hardware divide instruction, even
though it is significantly slower than an expanded optimized sequence
for constant dividend.
We've seen this in a few other cases while during performance regression
testing between kernels that still used -O2 vs the newer -Os.
No good solution found unfortunately.
-Andi
next prev parent reply other threads:[~2007-02-07 8:54 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-01-03 16:33 [PATCH/RFC 00/10] Transparent proxying patches version 4 KOVACS Krisztian
2007-01-03 16:34 ` [PATCH/RFC 01/10] Implement local diversion of IPv4 skbs KOVACS Krisztian
2007-01-10 6:46 ` Patrick McHardy
2007-01-10 9:31 ` Balazs Scheidler
2007-01-10 12:32 ` Patrick McHardy
2007-01-10 13:27 ` Ingo Oeser
2007-01-10 13:42 ` Patrick McHardy
2007-01-11 14:05 ` KOVACS Krisztian
2007-01-10 10:17 ` KOVACS Krisztian
2007-01-10 12:19 ` Patrick McHardy
2007-01-16 12:49 ` KOVACS Krisztian
2007-01-16 13:19 ` Patrick McHardy
2007-01-03 16:34 ` [PATCH/RFC 02/10] Port redirection support for TCP KOVACS Krisztian
2007-01-03 16:35 ` [PATCH/RFC 03/10] Don't do the TCP socket lookup if we already have one attached KOVACS Krisztian
2007-01-03 16:35 ` [PATCH/RFC 04/10] Don't do the UDP " KOVACS Krisztian
2007-01-03 16:36 ` [PATCH/RFC 05/10] Remove local address check on IP output KOVACS Krisztian
2007-01-10 6:47 ` Patrick McHardy
2007-01-10 10:01 ` KOVACS Krisztian
2007-02-06 14:36 ` IP_FREEBIND and CAP_NET_ADMIN (was: Re: [PATCH/RFC 05/10] Remove local address check on IP output) KOVACS Krisztian
2007-02-06 19:46 ` IP_FREEBIND and CAP_NET_ADMIN David Miller
2007-02-06 20:53 ` [PATCH] tg3 : avoid an expensive divide Eric Dumazet
2007-02-06 21:19 ` David Miller
2007-02-06 22:09 ` Michael Chan
2007-02-06 21:27 ` David Miller
2007-02-07 9:54 ` Andi Kleen [this message]
2007-02-07 9:45 ` David Miller
2007-02-07 9:56 ` Eric Dumazet
2007-02-07 10:27 ` Andi Kleen
2007-02-06 22:05 ` Michael Chan
2007-02-06 21:25 ` David Miller
2007-02-06 21:35 ` Eric Dumazet
2007-02-06 22:17 ` David Miller
2007-01-03 16:36 ` [PATCH/RFC 06/10] Create a tproxy flag in struct sk_buff KOVACS Krisztian
2007-01-03 16:37 ` [PATCH/RFC 07/10] Export UDP socket lookup function KOVACS Krisztian
2007-01-03 16:37 ` [PATCH/RFC 08/10] iptables tproxy table KOVACS Krisztian
2007-01-10 12:40 ` Patrick McHardy
2007-01-03 16:38 ` [PATCH/RFC 09/10] iptables TPROXY target KOVACS Krisztian
2007-01-10 12:45 ` Patrick McHardy
2007-01-03 16:38 ` [PATCH/RFC 10/10] iptables tproxy match KOVACS Krisztian
2007-01-03 17:23 ` [PATCH/RFC 00/10] Transparent proxying patches version 4 Evgeniy Polyakov
2007-01-08 20:30 ` KOVACS Krisztian
2007-01-03 19:33 ` Lennert Buytenhek
2007-01-04 12:13 ` KOVACS Krisztian
2007-01-04 12:16 ` Lennert Buytenhek
2007-01-07 14:11 ` Harald Welte
2007-01-07 16:11 ` Lennert Buytenhek
2007-01-07 23:58 ` Harald Welte
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