Faster approach to checking for an all-zero buffer in C?

Question

I am searching for a faster method of accomplishing this:

int is_empty(char * buf, int size) 
{
    int i;
    for(i = 0; i < size; i++) {
        if(buf[         


        

Answer 1

With x86 you can use SSE to test 16 bytes at a time:

#include "smmintrin.h" // note: requires SSE 4.1

int is_empty(const char *buf, const size_t size) 
{
    size_t i;
    for (i = 0; i + 16 <= size; i += 16)
    {
        __m128i v = _mm_loadu_si128((m128i *)&buf[i]);
        if (!_mm_testz_si128(v, v))
            return 0;
    }
    for ( ; i < size; ++i)
    {
        if (buf[i] != 0)
            return 0;
    }
    return 1;
}

This can probably be further improved with loop unrolling.

On modern x86 CPUs with AVX you can even use 256 bit SIMD and test 32 bytes at a time.