linux-kernel - Re: Can anyone explain "movl %eax %eax"?

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Message-ID: <4D5315FB.8020004@zytor.com>
Date:	Wed, 09 Feb 2011 14:32:27 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jidong Xiao <jidong.xiao@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Can anyone explain "movl %eax %eax"?

On 02/09/2011 02:24 PM, Jidong Xiao wrote:
> Hi,
> 
> In the kernel source, I see in a couple of places, there is "movl %eax
> %eax". Is this used for alignment purpose?
> 
> For example, in the following piece of code we can see "movl %eax,%eax".
> 

In x86-64, a dword (long) operation clears the upper 32 bits of the
target register, so "movl %eax,%eax" clears the upper 32 bits of %rax.

	-hpa

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