Getting Illegal Instruction while running a basic Avx512 code

问题

I am trying to learn AVX instructions and while running a basic code I recieve

Illegal instruction (core dumped)

The code is mentioned below and I am compiling it using

g++ -mavx512f 1.cpp

What exactly is the problem and how to overcome it? Thank You!

#include <immintrin.h>
#include<iostream>
using namespace std;

void add(const float a[], const float b[], float res[], int n)
{
    int i = 0;

    for(; i < (n&(~0x31)) ; i+=32 )
    {
        __m512 x = _mm512_loadu_ps( &a[i] );
        __m512 y = _mm512_loadu_ps( &b[i] );

        __m512 z = _mm512_add_ps(x,y);
        _mm512_stream_ps(&res[i],z);
    }

    for(; i<n; i++) res[i] = a[i] + b[i];
}

int main()
{
    int n = 100000;
    float a[n], b[n], res[n];
    for(int i = 0;i < n; i++)
    {
        a[i] = i;
        b[i] = i+10;
    }
    add(a,b,res,n);
    for(int i=0;i<n;i++) cout<<res[i]<<" ";
    cout<<endl;
    return 0;
}

回答1:

Probably your CPU doesn't support AVX512 at all. Mostly only server chips so far support it (like skylake-server, Cascade Lake, and Xeon Phi). Also the very-limited-release Cannon Lake laptop chip that Intel is abandoning; Ice Lake is planned to be the first client CPU that supports AVX512. See also Wikipedia's CPUs with AVX-512 table.

Use g++ -O3 -march=native to enable everything your CPU supports.

If you get compile errors (like undeclared function _mm512_loadu_ps), your CPU does not support AVX512 so g++ didn't enable it, so immintrin.h wouldn't define that intrinsic.

(Or another possible error is error "inlining" a builtin that target options don't allow.)

Only use separate -mavx512f and -mtune= options if you want to make a binary for other CPUs, not just the machine you're compiling on.

Related: How to test AVX-512 instructions w/o supported hardware?

Coding on insufficient hardware
Intel AVX intrinsics: any compatibility library out? (emulate at compile time instead of runtime).

来源：https://stackoverflow.com/questions/56621809/getting-illegal-instruction-while-running-a-basic-avx512-code

标签

c++

avx

avx2

avx512