SIMD Intrinsics AVX. Tried to use _mm256_mullo_epi64. But got 0xC000001D: Illegal Instruction exception

Question

I want to multiply two NxN matrices using SIMD. I want to do matrix multiplication for 64-bit integers, and multiply one element of a matrix with another element with the same index. For example:

c[1][1] = a[1][1] * b[1][1]

An error occurs when multiplying with the _mm256_mullo_epi64 operation. I can't figure out why this is happening. Can I write the resulting value into a 256-bit register?

#include <iostream>
#include <immintrin.h>

using namespace std;

int avx_mult(__int64** A, __int64** B, __int64** C, int N) {
    cout << "AVX mult:" << endl;
    if (N < 4 || N % 4 != 0) return 0;

    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j += 4) {
            // filling the resulting AMX register with zeros
            __m256i c_line = _mm256_setzero_si256();
            // load 4 long long elements from array A into AMX register
            __m256i a_line = _mm256_loadu_si256((__m256i*) & A[i][j]);
            // load 4 long long elements from array B into AMX register
            __m256i b_line = _mm256_loadu_si256((__m256i*) & B[i][j]);

            // multiplying two AVX registers
            c_line = _mm256_mullo_epi64(a_line, b_line);
        }
    }
}

int main() {
    const unsigned int N = 4; // array size

    __int64** A = new __int64* [N]; 
    __int64** B = new __int64* [N];
    __int64** C = new __int64* [N]; 

    for (int i = 0; i < N; i++) {
        A[i] = new __int64[N];
        B[i] = new __int64[N];
        C[i] = new __int64[N];
    }

    for (int i = 0; i < N; i++) { // filling arrays
        for (int j = 0; j < N; j++) {
            A[i][j] = __int64(rand() % 10);
            B[i][j] = __int64(rand() % 10);
            C[i][j] = __int64(0);
        }
    }

    avx_mult(A, B, C, N);

    for (int i = 0; i < N; i++) {
        delete[] A[i];
        delete[] B[i];
        delete[] C[i];

    }
    delete[] A;
    delete[] B;
    delete[] C;
}

The code compiles, but the program stops on this line:

c_line = _mm256_mullo_epi64(a_line, b_line);

... with exit code 0xC000001D: Illegal Instruction exception.

The Intel Intrinsics Documentation for _mm256_mullo_epi64 says:

Synopsis

__m256i _mm256_mullo_epi64 (__m256i a, __m256i b)
#include <immintrin.h>
Instruction: vpmullq ymm, ymm, ymm
CPUID Flags: AVX512DQ + AVX512VL

Description

Multiply the packed 64-bit integers in a and b, producing intermediate 128-bit integers, and store the low 64 bits of the intermediate integers in dst.

My function arguments fit the description. Or is there some mistake?

Answer 1

Not every x86_64 processor supports every instruction. Namely, _mm256_mullo_epi64 requires AVX-512 extensions, and if the rest of your code works, but running this intrinsic result in executing an illegal instruction, then you're most likely running this code on a processor without AVX-512.

You can implement packed 64-bit multiplication with just AVX2 as well:

__m256i mul64_haswell (__m256i a, __m256i b) {
    __m256i bswap   = _mm256_shuffle_epi32(b,0xB1);
    __m256i prodlh  = _mm256_mullo_epi32(a,bswap);

    __m256i prodlh2 = _mm256_srli_epi64(prodlh, 32);
    __m256i prodlh3 = _mm256_add_epi32(prodlh2, prodlh);
    __m256i prodlh4 = _mm256_and_si256(prodlh3, _mm256_set1_epi64x(0x00000000FFFFFFFF));

    __m256i prodll  = _mm256_mul_epu32(a,b);
    __m256i prod    = _mm256_add_epi64(prodll,prodlh4);
    return  prod;
}

This code is taken from @PeterCordes' answer to Fastest way to multiply an array of int64_t?