[avr-gcc-list] a newbie's question

[TeeN]

Hi Guru,

I have used the following CFLAGS to make the gcctest8.c:

CFLAGS =  -O1 -Wall -Wstrict-prototypes -Wa,-ahlms=$(<:.c=.lst)

the original C code:

for (j=0; j<255; j++)
    k++;

generates:

                 ldi     r24, 0          ; for (j=0;
 loc_5F:                                 ; 
                 subi    r24, -1         ; ;j++)
                 cpi     r24, -1
                 brcs    loc_5F          ; r24 == 0x00-0xFE : loop 
                                           r24 == 0xFF : fall thru

1. k++ line is optimised. it is ok.
2. since the "subi r24,0xFF" has increased r24 before the 
comparision, the loop actually executed 253 times instead of 254 
times.
   
if i use -O3 to optimise it, the disassembled code is even funny. Is 
it something wrong with my system ?
thanks!


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