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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>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "x86@kernel.org" <x86@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Bill Metzenthen <billm@melbpc.org.au>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Joerg Roedel <jroedel@suse.de>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Michael Roth <michael.roth@amd.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sean Christopherson <seanjc@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ammar Faizi <ammarfaizi2@gnuweeb.org>,
        GNU/Weeb Mailing List <gwml@vger.gnuweeb.org>,
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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>
In-Reply-To: <126271e264204581a42b079b51481740@AcuMS.aculab.com>
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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