An example showing how to trim an array of integers using a boundary in SIMD

simd_boundary_trim.c

// Credit for this algorithm goes to @pervognsen.
// I just took his pseudocode and turned it into C
//
// NOTE: this code is not "production safe" and makes assumptions about
//   the input (like 'count' being divisible by 4).

#include <tmmintrin.h>
#include <emmintrin.h>
#include <popcntintrin.h>

#define SLOT(n) (n*4+0), (n*4+1), (n*4+2), (n*4+3)
#define _ 128, 128, 128, 128

// this table maps a 16-bit 'movmskps' mask into a 'pshufb' mask
char sse_compact_lut[16][16] = {
    /* 0b0000 */ {      _,       _,       _,       _  },
    /* 0b0001 */ { SLOT(0),      _,       _,       _  },
    /* 0b0010 */ { SLOT(1),      _,       _,       _  },
    /* 0b0011 */ { SLOT(0), SLOT(1),      _,       _  },
    /* 0b0100 */ { SLOT(2),      _,       _,       _  },
    /* 0b0101 */ { SLOT(0), SLOT(2),      _,       _  },
    /* 0b0110 */ { SLOT(1), SLOT(2),      _,       _  },
    /* 0b0111 */ { SLOT(0), SLOT(1), SLOT(2),      _  },
    /* 0b1000 */ { SLOT(3),      _,       _,       _  },
    /* 0b1001 */ { SLOT(0), SLOT(3),      _,       _  },
    /* 0b1010 */ { SLOT(1), SLOT(3),      _,       _  },
    /* 0b1011 */ { SLOT(0), SLOT(1), SLOT(3),      _  },
    /* 0b1100 */ { SLOT(2), SLOT(3),      _,       _  },
    /* 0b1101 */ { SLOT(0), SLOT(2), SLOT(3),      _  },
    /* 0b1110 */ { SLOT(1), SLOT(2), SLOT(3),      _  },
    /* 0b1111 */ { SLOT(0), SLOT(1), SLOT(2), SLOT(3) },
};

#undef SLOT
#undef _

void trim_using_lower_bound_sse( int *src, int count, int *dst, int bound )
{
    int *end = src + count;

    __m128i b = _mm_set1_epi32(bound);
    __m128i *lut = (__m128i *) sse_compact_lut;

    while (src < end) {
        __m128i nums = _mm_loadu_si128((__m128i_u *) src);
        int mask = _mm_movemask_ps(_mm_cmplt_epi32(nums, b));
        __m128i res = _mm_shuffle_epi8(nums, lut[mask]);
        _mm_storeu_si128((__m128i_u *) dst, res);
        dst += _mm_popcnt_u32(mask);
        src += 4;
    }
}