Bytewise remapping

gistfile1.txt

#include <stdint.h>

typedef unsigned char U8;
typedef unsigned short U16;
typedef unsigned int U32;
typedef unsigned long long U64;
typedef intptr_t SINTa;

struct KernelState
{
    U8 *output;
    const U8 *input;
    const U8 *lut;

    SINTa count;
};

// Simple reference version
static bool remap_ref(KernelState *s)
{
    U8 * __restrict outp = s->output;
    const U8 *inp = s->input;
    const U8 *lut = s->lut;
    SINTa count = s->count;

    for (SINTa i = 0; i < count; i++)
        outp[i] = lut[inp[i]];

    return true;
}

static bool remap_avx2(KernelState *s)
{
    // Set up remapping table
    __m128i cur0 = _mm_setzero_si128();
    __m128i cur8 = _mm_setzero_si128();
    __m128i remap_tab[16];

    for (int i = 0; i < 8; i++)
    {
        __m128i b0 = _mm_loadu_si128((const __m128i *) (s->lut +   0 + i*16));
        __m128i b8 = _mm_loadu_si128((const __m128i *) (s->lut + 128 + i*16));
        remap_tab[i + 0] = _mm_xor_si128(cur0, b0);
        remap_tab[i + 8] = _mm_xor_si128(cur8, b8);
        cur0 = b0;
        cur8 = b8;
    }

    // Perform the remap
    U8 * __restrict outp = s->output;
    const U8 *inp = s->input;
    SINTa count = s->count;

    // NOTE: doesn't have tail handling yet
    for (SINTa i = 0; i < count; i += 32)
    {
        __m256i inds0 = _mm256_loadu_si256((const __m256i *) (inp + i));
        __m256i inds8 = _mm256_xor_si256(inds0, _mm256_set1_epi8(-0x80));

        // First pass
        __m256i out0 = _mm256_shuffle_epi8(_mm256_broadcastsi128_si256(remap_tab[0]), inds0);
        __m256i out8 = _mm256_shuffle_epi8(_mm256_broadcastsi128_si256(remap_tab[8]), inds8);

    #define PASS(n) \
        inds0 = _mm256_subs_epi8(inds0, _mm256_set1_epi8(0x10)); \
        inds8 = _mm256_subs_epi8(inds8, _mm256_set1_epi8(0x10)); \
        out0 = _mm256_xor_si256(out0, _mm256_shuffle_epi8(_mm256_broadcastsi128_si256(remap_tab[n+0]), inds0)); \
        out8 = _mm256_xor_si256(out8, _mm256_shuffle_epi8(_mm256_broadcastsi128_si256(remap_tab[n+8]), inds8))

        // Remaining passes
        PASS(1);
        PASS(2);
        PASS(3);
        PASS(4);
        PASS(5);
        PASS(6);
        PASS(7);

    #undef PASS

        __m256i result = _mm256_or_si256(out0, out8);
        _mm256_storeu_si256((__m256i *) (outp + i), result);
    }

    return true;
}