In file C:\Program Files\Microchip\xc8\v2.31\avr\avr\include\avr\sfr_defs.h you can fing used definition:

#define _MMIO_BYTE(mem_addr) (*(volatile uint8_t *)(mem_addr))
#define _SFR_IO8(io_addr) _MMIO_BYTE((io_addr) + __SFR_OFFSET)