EXP(x) for AVX2

exp_avx.cpp

// Approximation for EXP(x), only valid for -126.0f < x < 0.0f.
static inline __m256 _mm256_expfast_ps(const __m256 &q) {

    const __m256 INVLOG_2 = _mm256_set1_ps(1.442695040f);
    const __m256 BIT_SHIFT = _mm256_set1_ps(8388608);
    const __m256 ONE = _mm256_set1_ps(1.0f);

    const __m256 C1 = _mm256_set1_ps(121.2740838f);
    const __m256 C2 = _mm256_set1_ps(27.7280233f);
    const __m256 C3 = _mm256_set1_ps(4.84252568f);
    const __m256 C4 = _mm256_set1_ps(1.49012907f);

    const __m256 p = _mm256_mul_ps(INVLOG_2, q);
    const __m256i w = _mm256_cvttps_epi32(p);
    const __m256 z = _mm256_sub_ps(_mm256_add_ps(p, ONE), _mm256_cvtepi32_ps(w));

    __m256 rcp = _mm256_rcp_ps(_mm256_sub_ps(C3, z));
    rcp = _mm256_fmadd_ps(rcp, C2, _mm256_add_ps(C1, p));
    rcp = _mm256_fnmadd_ps(C4, z, rcp);

    return _mm256_castsi256_ps(_mm256_cvttps_epi32(_mm256_mul_ps(BIT_SHIFT, rcp)));

}