Horizontal add with __m512 (AVX512)

Question

How does one efficiently perform horizontal addition with floats in a 512-bit AVX register (ie add the items from a single vector together)? For 128 and 256 bit registers this can be done using _mm_hadd_ps and _mm256_hadd_ps but there is no _mm512_hadd_ps. The Intel intrinsics guide documents _mm512_reduce_add_ps. It doesn't actually correspond to a single instruction but its existence suggests there is an optimal method, but it doesn't appear to be defined in the header files that come with the latest snapshot of GCC and I can't find a definition for it with Google.

I figure "hadd" can be emulated with _mm512_shuffle_ps and _mm512_add_ps or I could use _mm512_extractf32x4_ps to break a 512-bit register into four 128-bit registers but I want to make sure I'm not missing something better.

Answer 1

The INTEL compiler has the following intrinsic defined to do horizontal sums

_mm512_reduce_add_ps     //horizontal sum of 16 floats
_mm512_reduce_add_pd     //horizontal sum of 8 doubles
_mm512_reduce_add_epi32  //horizontal sum of 16 32-bit integers
_mm512_reduce_add_epi64  //horizontal sum of 8 64-bit integers

However, as far as I can tell these are broken into multiple instructions anyway so I don't think you gain anything more than doing the horizontal sum of the upper and lower part of the AVX512 register.

__m256 low  = _mm512_castps512_ps256(zmm);
__m256 high = _mm256_castpd_ps(_mm512_extractf64x4_pd(_mm512_castps_pd(zmm),1));

__m256d low  = _mm512_castpd512_pd256(zmm);
__m256d high = _mm512_extractf64x4_pd(zmm,1);

__m256i low  = _mm512_castsi512_si256(zmm);
__m256i high = _mm512_extracti64x4_epi64(zmm,1);

To get the horizontal sum you then do sum = horizontal_add(low + high).

static inline float horizontal_add (__m256 a) {
    __m256 t1 = _mm256_hadd_ps(a,a);
    __m256 t2 = _mm256_hadd_ps(t1,t1);
    __m128 t3 = _mm256_extractf128_ps(t2,1);
    __m128 t4 = _mm_add_ss(_mm256_castps256_ps128(t2),t3);
    return _mm_cvtss_f32(t4);        
}

static inline double horizontal_add (__m256d a) {
    __m256d t1 = _mm256_hadd_pd(a,a);
    __m128d t2 = _mm256_extractf128_pd(t1,1);
    __m128d t3 = _mm_add_sd(_mm256_castpd256_pd128(t1),t2);
    return _mm_cvtsd_f64(t3);        
}

I got all this information and functions from Agner Fog's Vector Class Library and the Intel Instrinsics Guide online.