From: Oliver Hartkopp <socketcan@hartkopp.net>
To: Marc Kleine-Budde <mkl@pengutronix.de>, linux-can@vger.kernel.org
Subject: Re: [PATCH 2/3] can: add hash based access to single EFF frame filters
Date: Wed, 02 Apr 2014 19:26:16 +0200 [thread overview]
Message-ID: <533C4838.2000908@hartkopp.net> (raw)
In-Reply-To: <533BE133.8020600@pengutronix.de>
On 02.04.2014 12:06, Marc Kleine-Budde wrote:
>
> [1] After a long long long search I found a compiler (Debian clang
> version 3.4-2 (tags/RELEASE_34/final) (based on LLVM 3.4) in 32 Bit
> mode) that produces a function with more more instruction for your
> "backwards" hash compared to a "forward" hash:
>
> hash = can_id;
> hash ^= can_id >> CAN_EFF_RCV_HASH_BITS;
> hash ^= can_id >> (2 * CAN_EFF_RCV_HASH_BITS);
>
> return hash & ((1 << CAN_EFF_RCV_HASH_BITS) - 1);
>
> In 64 bit mode it issues the same number of instructions, though your
> backwards hash is a byte shorter :) gcc 4.7 in contrast creates same
> number of instructions and bytes for 32 and 64 bit.
>
Ok, I see you really had fun with this review :-)
Will change the hash function like this - and the other stuff too.
Thanks,
Oliver
next prev parent reply other threads:[~2014-04-02 17:26 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-01 18:57 [PATCH 0/3] can: add support for hash based access to EFF frame filters Oliver Hartkopp
2014-04-01 18:57 ` [PATCH 1/3] can: proc: make array printing function indenpendent from sff frames Oliver Hartkopp
2014-04-02 7:16 ` Marc Kleine-Budde
2014-04-01 18:57 ` [PATCH 2/3] can: add hash based access to single EFF frame filters Oliver Hartkopp
2014-04-02 10:06 ` Marc Kleine-Budde
2014-04-02 17:26 ` Oliver Hartkopp [this message]
2014-04-01 18:57 ` [PATCH 3/3] can: add documentation for CAN filter usage optimisation Oliver Hartkopp
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