Optimization of embedded C language for indefinite cycles

1、 Indefinite number of cycles , have access to for、while、do{}while

2、 Examples of indefinite cycles

int checksum_v6(int *data, unsigned int N)
{
    int sum = 0;
    for(; N != 0; N--)   
    {
            sum += *(data++);
    }
    return sum;
}

The assembly code of the above program is ：

checksum_v7_s

        MOV r2,#0                ;sum = 0
        
        CMP r0,#0                ; compare N,0

        BEQ checksum_v7_end      ; if(N == 0) goto end

checksum_v7_loop

       LDR r3,[r2] #4               ;r3 = *(data++)

       SUBS r1,r1,#1                ;N-- and set flags

        ADD r2,r3,r2                ;sum +=  r3       

        BNE checksum_v7_loop        ;if(N != 0) goto loop
        
checksum_v7_end

        MOV r0,#r2                ;r0 = sum
        
        MOV pc,r14                 ;return r0

3、 Discuss ： Generally speaking , The length of the array cannot be 0

Use do{}while Rewriting procedures ：

int checksum_v8(int *data, unsigned int N)
{
    int sum = 0;
    do  
    {
            sum += *(data++);
    } while (--N!=0);
    return sum;
}

The assembly code of the above program is ：

checksum_v8_s

        MOV r2,#0                ;sum = 0

checksum_v8_loop

       LDR r3,[r2] #4               ;r3 = *(data++)

       SUBS r1,r1,#1                ;N-- and set flags

        ADD r2,r3,r2                ;sum +=  r3       

        BNE checksum_v8_loop        ;if(N != 0) goto loop      

        MOV r0,#r2                ;r0 = sum
        
        MOV pc,r14                 ;return r0

4、 Conclusion ：

Use do{}while The assembly code of the rewritten program is relatively few lines .

Use do{}while It can show better performance and code density , The premise is to ensure that the loop is executed at least once .