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Message-ID: <97c052a1-44e4-3c59-d4c8-34176d1d601a@gnuweeb.org>
Date: Mon, 8 Aug 2022 23:38:34 +0700
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Subject: Re: [PATCH 1/1] x86: Change mov $0, %reg with xor %reg, %reg
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To: David Laight <David.Laight@ACULAB.COM>
Cc: Borislav Petkov <bp@alien8.de>, Thomas Gleixner <tglx@linutronix.de>,
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References: <20220804152656.8840-1-knscarlet@gnuweeb.org>
 <20220804152656.8840-2-knscarlet@gnuweeb.org> <Yuvrd2yWLnyxOVLU@zn.tnic>
 <20220804180805.9077-1-knscarlet@gnuweeb.org>
 <126271e264204581a42b079b51481740@AcuMS.aculab.com>
From: Kanna Scarlet <knscarlet@gnuweeb.org>
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On 8/5/22 4:26 PM, David Laight wrote:
> Use "xor %rax,%rax" instead of "xor %eax,%eax" to save
> the 'reg' prefix.

hello David Laight

"xor %rax,%rax" is bigger because of rex prefix, "xor %eax,%eax" is smaller
because it doesn't need rex prefix.

asm:
    0:   48 31 c0  xor    %rax,%rax
    3:   31 c0     xor    %eax,%eax

so i think to save from rex prefix, use xor %eax,%eax instead of xor %rax,%rax.

Best regards,
-- 
Kanna Scarlet