netdev - Re: Poll about irqsafe_cpu

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Message-ID: <alpine.DEB.2.00.1103171338350.18529@router.home>
Date:	Thu, 17 Mar 2011 13:42:10 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: Poll about irqsafe_cpu_add and others

On Thu, 17 Mar 2011, Eric Dumazet wrote:

> By the way, I noticed :
>
> DECLARE_PER_CPU(u64, xt_u64);
> __this_cpu_add(xt_u64, 2) translates to following x86_32 code :
>
> 	mov	$xt_u64,%eax
> 	add	%fs:0x0,%eax
> 	addl	$0x2,(%eax)
> 	adcl	$0x0,0x4(%eax)
>
>
> I wonder why we dont use :
>
> 	addl	$0x2,%fs:xt_u64
> 	addcl	$0x0,%fs:xt_u64+4

The compiler is fed the following

	*__this_cpu_ptr(xt_u64) += 2

__this_cpu_ptr makes it:

	*(xt_u64 + __my_cpu_offset) += 2

So the compiler calculates the address first and then increments it.

The compiler could optimize this I think. Wonder why that does not happen.




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