No segmentation fault with unaligned access

Hi,

I did a quick check with your program on a Raspberry Pi 3 running Ubuntu Mate (info from this article)

https://community.arm.com/tools/b/blog/posts/profiling-alexnet-on-raspberry-pi-and-hikey-960-with-the-compute-library

It segfaulted for me on this system.

$ gcc -g t.c -mfpu=neon
$ ./a.out
Segmentation fault

Some things that may be relevant:

$ cat /proc/cpu/alignment

Check if the user count is increasing when you run your program. 

Also, look at your /proc/config.gz for the alignment related kernel configuration. 

Thanks,

Jason

Hi,

I did a quick check with your program on a Raspberry Pi 3 running Ubuntu Mate (info from this article)

https://community.arm.com/tools/b/blog/posts/profiling-alexnet-on-raspberry-pi-and-hikey-960-with-the-compute-library

It segfaulted for me on this system.

$ gcc -g t.c -mfpu=neon
$ ./a.out
Segmentation fault

Some things that may be relevant:

$ cat /proc/cpu/alignment

Check if the user count is increasing when you run your program. 

Also, look at your /proc/config.gz for the alignment related kernel configuration. 

Thanks,

Jason

Hi Jason and thanks for the reply.

I didn't find the user count increasing when running the program, is that a sign of something?

I am not sure what that indicates. could you point me material about the same OR kindly provide more information about the same?

Also jason there are 2 keypoints you have missed here.

1) The kernel needs an argument to be passed to it, say ./a.out 32
2) I have accidentally have posted the wrong kernel. The loop with i must begin with i = 1. (I am try and edit the original post if possible)

Here is the correct kernel for reference

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "arm_neon.h"

void add (uint32x4_t *data_a,uint32x4_t *data_b)
{
    /* Set each sixteen values of the vector to 3.
     *
     * Remark: a 'q' suffix to intrinsics indicates
     * the instruction run for 128 bits registers.
     */
    
    *data_a = vaddq_u32 (*data_a, *data_b);
}

int main (int argc,char** argv)
{
    unsigned int n = atoi(argv[1]);

    /* Create custom arbitrary data. */
    uint32_t uint32_data_a[n];
    uint32_t uint32_data_b[n];
    uint32_t uint32_data_c[n];
    struct timespec start,end;

    for(uint32_t i = 1; i <= n ; i+=1)
    {
        uint32_data_a[i-1] = i;
        uint32_data_b[i-1] = i;
        uint32_data_c[i-1] = i;
    }

    /* Create the vector with our data. */
    uint32x4_t data_a;
    uint32x4_t data_b;

   
    clock_gettime(CLOCK_MONOTONIC,&start);

    for(int count = 0; count < 10; count++)
    {
        for(int i = 0; i < n ; i+=4)
        {    
            /* Load our custom data into the vector register. */
            data_a  = vld1q_u32 (uint32_data_a + i);
        data_b  = vld1q_u32 (uint32_data_b + i);

            /* Call of the add3 function. */
            add(&data_a,&data_b);

            vst1q_u32(uint32_data_c + i,data_a);
    }    
    }

    clock_gettime(CLOCK_MONOTONIC,&end);

    double time_usec=(((double)end.tv_sec * 1000000 + (double)end.tv_nsec/1000) - ((double)start.tv_sec *1000000 + (double)start.tv_nsec/1000));
    printf("Time taken for aligned load is : %fus and count is %d \n", time_usec/10,n );

    for(uint32_t i = 0; i < n ; i++) printf("%2d ",uint32_data_c[i]);
    printf("\n");
    return 0;

Hi,

The program works for me know, thanks for the corrections. 

I'm interested to find out why you think there are unaligned memory accesses and how you determined this? A little more info on what you think the problem is would be helpful.

Thanks,

Jason

Notice the for loop, it clearly begins with 1, hence

1) if the array was indeed aligned to a 128 bit/16 byte boundary we would see a segmentation fault straight away, since a vector load at an unaligned boundary must cause a segmentation fault.

2) if the array wasn't aligned and by chance always the element 1 was processed okay, we would expect a segmentation fault atleast on any of other values of i (since every element of an array can never be aligned). Note :- The loop has stride 1, (i = 0 ; i < n ; i++).

Hence, I expect a segmentation fault.

Hi,

I recommend to narrow the focus to a specific instruction and load address you think should be causing segfault.

Use objdump -S to see the source code mixed with the disassembly, gdb to single step by source and assembly and print registers, or just printf() statements to print pointer values.

I didn't see any unaligned accesses. The stride of 1 is used for pointer arithmetic which moves to the next vector.

Thanks,

Jason