Re: Can anyone explain "movl %eax %eax"?

[Jidong Xiao]

Oh, I see. Thank you. So similarly, the operation "xorl %eax,%eax" is
used for the same reason, right? I see that appears in more files.

Regards
Jidong

On Wed, Feb 9, 2011 at 5:32 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> 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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