ARM: Count ones (bit count)
From ScienceZero
Related terms
- Population-count
- Hamming weight
- Sideways addition
Minimum size
;R0 - value ;R1 = number of ones ;Uses R0-R1 ;81 cycles worst case, 4 cycles best case, exit when r1=0 mov r1,r0,lsr #31 loop movs r0,r0,lsl #2 adc r1,r1,r0,lsr #31 bne loop mov pc,r14
Maximum performance
;R0 - value ;R0 = number of ones ;Uses R0-R5 ;15 cycles constant, 10 cycles when masks can be generated outside the loop mov r2,#0xff ;Masks orr r2,r2,#0xff<<16 ;00000000111111110000000011111111 eor r3,r2,r2,lsl #4 ;00001111000011110000111100001111 eor r4,r3,r3,lsl #2 ;00110011001100110011001100110011 eor r5,r4,r4,lsl #1 ;01010101010101010101010101010101 and r1,r5,r0,lsr #1 sub r0,r0,r1 and r1,r4,r0,lsr #2 and r0,r4,r0 add r0,r0,r1 add r0,r0,r0,lsr #4 and r0,r0,r3 add r0,r0,r0,lsr #8 add r0,r0,r0,lsr #16 and r0,r0,#63 mov pc,r14
Thumb-2 version
;R0 - Value ;R0 = Number of ones ;Uses R0-R1 ;Thumb-2 version and r1,r0,#0xaaaaaaaa sub r0,r0,r1,lsr #1 and r1,r0,#0xcccccccc and r0,r0,#0x33333333 add r0,r0,r1,lsr #2 add r0,r0,r0,lsr #4 and r0,r0,#0x0f0f0f0f add r0,r0,r0,lsr #8 add r0,r0,r0,lsr #16 and r0,r0,#63 bx lr