netdev - Re: [PATCH 1/2] ipmr: delete redundant variable

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Message-Id: <200807231003.05848.netdev@axxeo.de>
Date:	Wed, 23 Jul 2008 10:03:05 +0200
From:	Ingo Oeser <netdev@...eo.de>
To:	Wang Chen <wangchen@...fujitsu.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	NETDEV <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/2] ipmr: delete redundant variable

Hi Wang Chen,

Wang Chen schrieb:
> *v can be removed as this patch showing.

You are right, but did you check the resulting asm?
 
> Signed-off-by: Wang Chen <wangchen@...fujitsu.com>
> ---
> diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
> index c519b8d..6e715c7 100644
> --- a/net/ipv4/ipmr.c
> +++ b/net/ipv4/ipmr.c
> @@ -1129,9 +1128,9 @@ static int ipmr_device_event(struct notifier_block *this, unsigned long event, v
>  
>  	if (event != NETDEV_UNREGISTER)
>  		return NOTIFY_DONE;
> -	v=&vif_table[0];
> -	for (ct=0;ct<maxvif;ct++,v++) {
> -		if (v->dev==dev)

This is ptr += sizeof(vif_table[0])

> +
> +	for (ct = 0; ct < maxvif; ct++) {
> +		if (vif_table[ct].dev == dev)

This is ptr + ct * sizeof(vif_table[0])

On architectures, where the second address variant is
not supported, it spills a register with the multiply/shift.

But the second variant could be easily auto vectorized, 
if we had no if.

So just check the asm on a CISC and a RISC architecture 
with a cross compile, before you transform these patterns.

Maybe GCC even transform one into the other these days :-)


Best Regards

Ingo Oeser
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