Question

saturating instructions saturate unsigned to unsigned or signed to signed int.

What’s the best way to saturate signed 16-bit ints to unsigned byte?

In short, here’s the logic

uint8_t usat8(uint8_t u8, int16_t s16)
{
    s16 += u8;
    if(s16 <= 0) {
        return 0;
    } else if(s16 >=255){
        return 255;
    }else{
        return (uint8_t)s16;
    }
}

void add_row(uint8_t * dst, uint8_t * u8, int16_t * s16)
{
    for(int i=0; i<XXX; ++i)
    {
        dst[i] = usat8(u8[i] + s16[i]);
    }
}

values of s16 are usually not much off from the [0, 255] range, e.g. it’s safe to assume that abs(s16[x]) < 1000.

EDIT: I just realized that USAT16 actually saturates signed 16-bit int to unsigned integer. Simple USAT16 is the solution to the problem.

Answer 1

After 5 mins of thinking I have this idea (pseudo arm-asm):

sadd16  sum, s16, u8                  @ do two additions in parallel

orr     signs, 0x1001, sum, lsr #15   @ extract signs of the two 16 bit results
usat16  sum, sum, #8                  @ saturate both of the 16-bit sums to unsigned 8-byte range
uadd16  sum, sum, signs

this way, if sign bit was set for any of the sums the resulting sum will become 256, or 0x100. When writing back the data the shifted out 0x1 will be discarded.

Any comments, does that seem like the optimal approach, is there any better alternative?

PS. I do it for an armv6 device, no NEON or armv6t2