#include <immintrin.h>

static inline void gfmulby2(__m128i a,__m128i* res)
{
    *res = _mm_srai_epi32(a,31);
    *res = _mm_shuffle_epi32(*res,0x57);
    *res = _mm_and_si128(*res,_mm_set_epi32(0x00,0x01,0x00,0x87));
    *res = _mm_xor_si128(*res,_mm_slli_epi64(a,0x01));
}