gcc compilation error: vector shuffling operation will be expanded piecewise

chakka.lokesh · 10-27-2015, 10:02 AM

i have written a piece of code as follows:

Code:

char my_ex_or_function( char *ptr )
{
        char ex_or = 0;
        int i;

        for( i=0; i<10000; i++ )// ptr will be pointing to fixed block of size 10000
                ex_or ^= ptr[i];
        return ex_or;
}

while compiling above code, I am seeing following error:

cc1: error: vector shuffling operation will be expanded piecewise [-Werror=vector-operation-performance]

I referred this link
but my requirement is to do XOR.

As I am treating the warnings also as errors, I am seeing compilation is failing.
I am using gcc version 5.2.1 20151010 (Ubuntu 5.2.1-22ubuntu2)
can some one help me how to fix this warning ?

pan64 · 10-27-2015, 10:28 AM

probably because you passed a pointer and used an array. Try to use pointer instead of index.

Code:

        p = ptr;
        for( i=0; i<10000; i++ ) {// ptr will be pointing to fixed block of size 10000
                ex_or ^= *p;
                p++;
        }
        return ex_or;

or use: char ptr[10000] instead of pointer

chakka.lokesh · 10-28-2015, 10:03 AM

pan64,

thanks for your reply. I tried both the ways you suggested.
still seeing same problem.

NevemTeve · 10-28-2015, 11:20 AM

You might have used some exotic compiler-option having something to do with vector-operations (eg: -Wvector-operation-performance).

Note: hard-coding this number is impractical, try to be more flexible:

Code:

char my_ex_or_function (int n, const char *ptr)
{
    char ex_or = 0;
    int i;

    for (i=0; i<n; i++ ) {
        ex_or ^= ptr[i];
    }
    return ex_or;
}