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Message-ID: <AANLkTinbLbdPDS-MaSQ3b2BP_ykERx6EfBoJuD0ZxWdr@mail.gmail.com> Date: Wed, 9 Feb 2011 20:32:39 -0500 From: Jidong Xiao <jidong.xiao@...il.com> To: David Miller <davem@...emloft.net> Cc: hpa@...or.com, linux-kernel@...r.kernel.org Subject: Re: Can anyone explain "movl %eax %eax"? Thank you David and Peter. And I found more information in the following website, other people who are not very clear can have a look at this document: http://www.x86-64.org/documentation/assembly.html ====================== Implicit zero extend Results of 32-bit operations are implicitly zero extended to 64-bit values. This differs from 16 and 8 bit operations, that don't affect the upper part of registers. This can be used for code size optimisations in some cases, such as: movl $1, %eax # one byte shorter movq $1, %rax xorq %rax, %rax # three byte equivalent of mov $0,%rax andl $5, %eax # equivalent for andq $5, %eax ====================== Regards Jidong On Wed, Feb 9, 2011 at 6:05 PM, David Miller <davem@...emloft.net> wrote: > From: Jidong Xiao <jidong.xiao@...il.com> > Date: Wed, 9 Feb 2011 18:02:09 -0500 > >> 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. > > The xorl clears the entire register. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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