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Date: Thu, 17 Mar 2011 13:42:10 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: Eric Dumazet <eric.dumazet@gmail.com>
cc: David Miller <davem@davemloft.net>, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, netdev@vger.kernel.org,
        netfilter-devel@vger.kernel.org
Subject: Re: Poll about irqsafe_cpu_add and others
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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.