Fastest way to count number of 1s in a register, ARM assembly

Question

So I had an interview question before regarding bit manipulation. The company is a well known GPU company. I had very little background in assembly language (weird despite b

Answer 1

    LDR r0, = 0x000000FF;
    MOV r1, #0;
    MOV r3, #0; this will always be zero
    MOV r2,r0;
rep MOVS r2, r2, LSR #1;
    ADC r1,r1, r3;  this adds r1 with zero plus the carry bit
    CMP r2, #0;
    BNE rep

This will do it, r3 is just a dummy register with 0 to make ADC work properly.