Re: [PATCH 1/1] x86: Change mov $0, %reg with xor %reg, %reg

[Kanna Scarlet <knscarlet@gnuweeb.org>]

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