avr-gcc adding useless (?) subi instructions, why?

gistfile1.c

// compiler commandline as below:
// avr-gcc -mmcu=attiny85 -O2 -DF_CPU=16000000L -I. -Iusbdrv -std=c99 -Wall -g   -c -o main.o main.c

ISR(ADC_vect) {
        adclast = ADMUX & ~_BV(ADLAR);
        adcreading[adclast] = ADCH;
        ADMUX = adcmux[adclast];
}

gistfile2.txt

section from avr-objdump -D
00000124 <__vector_8>:
 124:   1f 92           push    r1
 126:   0f 92           push    r0
 128:   0f b6           in      r0, 0x3f        ; 63
 12a:   0f 92           push    r0
 12c:   11 24           eor     r1, r1
 12e:   2f 93           push    r18
 130:   8f 93           push    r24
 132:   9f 93           push    r25
 134:   ef 93           push    r30
 136:   ff 93           push    r31
 138:   87 b1           in      r24, 0x07       ; 7
 13a:   8f 7d           andi    r24, 0xDF       ; 223
 13c:   80 93 00 00     sts     0x0000, r24
 140:   90 e0           ldi     r25, 0x00       ; 0
 142:   25 b1           in      r18, 0x05       ; 5
 144:   fc 01           movw    r30, r24
 146:   e0 50           subi    r30, 0x00       ; 0
 148:   f0 40           sbci    r31, 0x00       ; 0
 14a:   20 83           st      Z, r18
 14c:   fc 01           movw    r30, r24
 14e:   e0 50           subi    r30, 0x00       ; 0
 150:   f0 40           sbci    r31, 0x00       ; 0
 152:   80 81           ld      r24, Z
 154:   87 b9           out     0x07, r24       ; 7
 156:   ff 91           pop     r31
 158:   ef 91           pop     r30
 15a:   9f 91           pop     r25
 15c:   8f 91           pop     r24
 15e:   2f 91           pop     r18
 160:   0f 90           pop     r0
 162:   0f be           out     0x3f, r0        ; 63
 164:   0f 90           pop     r0
 166:   1f 90           pop     r1
 168:   18 95           reti