SAD, sum of absolute differences in C++ (NEON instrinsics)

#include <arm_neon.h>
#include "rect_sad.h"
#include <assert.h>

typedef unsigned char uint8_t;
typedef unsigned int uint32_t;

#ifndef ABS
#define ABS(a) (((a)>=0)?(a):(-(a)))
#endif

uint32_t rect_sad(
        const uint8_t * src_line_r0,
        const uint8_t * targ_line_r1,
        unsigned rect_w_r2, unsigned rect_h_r3,
        unsigned stride_r4)
{
    uint32x4_t neon_sad_r5 = vdupq_n_u32(0);
    unsigned gap_r6 = stride_r4 - rect_w_r2;
    const uint8_t * end_r7 = src_line_r0 + rect_h_r3 * stride_r4;

    for (; src_line_r0 < end_r7;
            src_line_r0 += gap_r6, targ_line_r1 += gap_r6)
    {
        uint16x8_t line_sad16 = vdupq_n_u16(0);

        const uint8_t * line_end_r8 = &src_line_r0[rect_w_r2];
        for (; src_line_r0 < line_end_r8; src_line_r0+=8, targ_line_r1+=8)
        {
            uint8x8_t s = vld1_u8(src_line_r0);
            uint8x8_t t = vld1_u8(targ_line_r1);
            line_sad16 = vabal_u8(line_sad16, s, t);
        }
        neon_sad_r5 = vpadalq_u16(neon_sad_r5, line_sad16);
    }
    uint32_t neon_sad_store[4];
    uint32_t ret = 0;
    vst1q_u32(neon_sad_store, neon_sad_r5);
    for(int i = 0; i < 4; ++i) { ret += neon_sad_store[i];}

    return ret;
}