The Timer/Counter Registers are registers incremented by each timer clock.

The Timer/Counter registers related codes are defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.

#define TCNT0 _SFR_IO8(0x26)
#define TCNT0_0 0
#define TCNT0_1 1
#define TCNT0_2 2
#define TCNT0_3 3
#define TCNT0_4 4
#define TCNT0_5 5
#define TCNT0_6 6
#define TCNT0_7 7

#define TCNT1 _SFR_MEM16(0x84)

#define TCNT1L _SFR_MEM8(0x84)
#define TCNT1L0 0
#define TCNT1L1 1
#define TCNT1L2 2
#define TCNT1L3 3
#define TCNT1L4 4
#define TCNT1L5 5
#define TCNT1L6 6
#define TCNT1L7 7

#define TCNT1H _SFR_MEM8(0x85)
#define TCNT1H0 0
#define TCNT1H1 1
#define TCNT1H2 2
#define TCNT1H3 3
#define TCNT1H4 4
#define TCNT1H5 5
#define TCNT1H6 6
#define TCNT1H7 7

#define TCNT2 _SFR_MEM8(0xB2)
#define TCNT2_0 0
#define TCNT2_1 1
#define TCNT2_2 2
#define TCNT2_3 3
#define TCNT2_4 4
#define TCNT2_5 5
#define TCNT2_6 6
#define TCNT2_7 7

They call _SFR_IO8(), _SFR_MEM8() and _SFR_MEM16().