AVX segmentation fault on linux [closed]

Question

I am trying to run this code and it says segmentation fault when I run it. It compiles good. Here is the code. (It works fine on windows).

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

const int size = 1000000;

std::vector<float>A(size);
std::vector<float>B(size);
std::vector<float>C(size);

void bar(int i){
    const float a = 2.0f;
    __m256 _a = _mm256_broadcast_ss(&a);
    __m256 _A = _mm256_load_ps(&A[0] + i*8);
    __m256 _B = _mm256_load_ps(&B[0] + i*8);
    __m256 _C = _mm256_add_ps(_B, _mm256_mul_ps(_a,_A));
    _mm256_store_ps(&C[0] + i*8, _C);
}


int main(){
    std::fill(A.begin(), A.end(), 1.0f);
    std::fill(B.begin(), B.end(), 2.0f);
    bar(0);

    return 0;
}

Compiling: g++ -mavx t2.cpp -o t2

It's exiting when it hit the first AVX instruction. I just want someone to review my code.

Here is gdb back trace

(gdb) run
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400aea in bar(int) ()
Missing separate debuginfos, use: debuginfo-install glibc-2.17-78.el7.x86_64 libgcc-4.8.3-9.el7.x86_64 libstdc++-4.8.3-9.el7.x86_64
(gdb) bt
#0  0x0000000000400aea in bar(int) ()
#1  0x0000000000400b95 in main ()
(gdb)

Answer 1

It is probably an data alignment issue. _mm256_load_ps requires 256-bit (32-bytes) aligned memory. The default allocator for std::vector doesn't meet that requirement. You'll need to supply an aligned allocator or use another instruction with less stringent alignment requirement (such as _mm256_loadu_ps).