Effectively calculate max from an array of 8 elements in neon hands

Question

Effectively calculate max from an array of 8 elements in neon hands

How to find the maximum element in an array of 8 bytes, 8 shorts or 8 ints? I may need only the position of the max element, the value of the maximum element, or both of them.

For example :

unsigned FindMax8(const uint32_t src[8]) // returns position of max element
{
    unsigned ret = 0;
    for (unsigned i=0; i<8; ++i)
    {
        if (src[i] > src[ret])
            ret = i;
    }
    return ret;
}

In -O2clang, it loops, but it does not use neon, which should give a decent boost of boost (because it eliminates many data-dependent branches?)

For an approach with 8 bytes and 8 shorts, it should be simpler, since the entire array can be loaded into a single q-register. For arm64, this should be much simpler with vmaxv_u16, but how can I make it effective in 32-bit neon?

Marc , , GTI auto vectorizer neon64:

ldr q0, [x0, 16]
ld1r {v2.4s}, [x0]
ldr q1, [x0]
umax v0.4s, v0.4s, v2.4s
umax v0.4s, v0.4s, v1.4s
umaxv s0, v0.4s
umov w0, v0.s[0]

, , uint32x4_t res, , , - . , , .

( ):

32- .
vst1q_u32 (src, res), C- .
vmov 32- , vget_lane_u64 , - .

, :

unsigned compute(unsigned short *input)
{
    uint32x4_t result = vld1q_u32((uint32_t*)(input));
    // some computations...
    // ... and at the end I end up with res01 and res23
    // and I need to get index of max element from them:
    uint32x2_t res01 = vget_low_u32(result);
    uint32x2_t res23 = vget_high_u32(result);

    // real code below:
    uint64_t xres01 = vget_lane_u64(vreinterpret_u64_u32(res01), 0);
    uint64_t xres23 = vget_lane_u64(vreinterpret_u64_u32(res23), 0);
    unsigned ret = 0;
    uint32_t xmax0 = (uint32_t)(xres01 & 0xffffffff);
    uint32_t xmax1 = (uint32_t)(xres01 >> 32);
    uint32_t xmax2 = (uint32_t)(xres23 & 0xffffffff);
    uint32_t xmax3 = (uint32_t)(xres23 >> 32);
    if (xmax1 > xmax0)
    {
        xmax0 = xmax1;
        ret = 1;
    }
    if (xmax2 > xmax0)
    {
        xmax0 = xmax2;
        ret = 2;
    }
    if (xmax3 > xmax0)
        ret = 3;
    return ret;
}