Getting max value in a __m128i vector with SSE?

I have just started using SSE and I am confused how to get the maximum integer value (max) of a __m128i. For instance:

__m128i t = _mm_setr_ps(0,1,2,3);
// max(t) = 3;

Searching around led me to MAXPS instruction but I can't seem to find how to use that with "xmmintrin.h".

Also, is there any documentation for "xmmintrin.h" that you would recommend, rather than looking into the header file itself?

Solution 1:

In case anyone cares and since intrinsics seem to be the way to go these days here is a solution in terms of intrinsics.

int horizontal_max_Vec4i(__m128i x) {
    __m128i max1 = _mm_shuffle_epi32(x, _MM_SHUFFLE(0,0,3,2));
    __m128i max2 = _mm_max_epi32(x,max1);
    __m128i max3 = _mm_shuffle_epi32(max2, _MM_SHUFFLE(0,0,0,1));
    __m128i max4 = _mm_max_epi32(max2,max3);
    return _mm_cvtsi128_si32(max4);
}

I don't know if that's any better than this:

int horizontal_max_Vec4i(__m128i x) {
    int result[4] __attribute__((aligned(16))) = {0};
    _mm_store_si128((__m128i *) result, x);
    return max(max(max(result[0], result[1]), result[2]), result[3]); 
}

Solution 2:

If you find yourself needing to do horizontal operations on vectors, especially if it's inside an inner loop, then it's usually a sign that you are approaching your SIMD implementation in the wrong way. SIMD likes to operate element-wise on vectors - "vertically" if you like, not horizontally.

As for documentation, there is a very useful reference on intel.com which contains all the opcodes and intrinsics for everything from MMX through the various flavours of SSE all the way up to AVX and AVX-512.

Solution 3:

According to this page, there is no horizontal max, and you need to test the elements vertically:

movhlps xmm1,xmm0         ; Move top two floats to lower part of xmm1
maxps   xmm0,xmm1         ; Get maximum of the two sets of floats
pshufd  xmm1,xmm0,$55     ; Move second float to lower part of xmm1
maxps   xmm0,xmm1         ; Get minimum of the two remaining floats

Conversely, getting the minimum:

movhlps xmm1,xmm0
minps   xmm0,xmm1
pshufd  xmm1,xmm0,$55
minps   xmm0,xmm1