leaq (_bss-8)(%rip), %rsi
leaq (_bss-8)(%rbx), %rdi
movq $_bss /* - $startup_32 */, %rcx
shrq $3, %rcx
std
rep movsq
cld
The movsq instruction copys 8 bytes each time and for the first time it will copy [_bss-8, _bss) to the new location. If we don't minus 8, the data in [_bss, _bss+8) will also be copied on the first execution of movsq. Obviously, we don't want to copy the bss section.
Regards,
Hao Lee
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