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

If your program is x86 only or x64 only, you can easily optimize using inline assambler. The REPE SCASD instruction will scan a buffer until a non EAX dword is found.

Since there is no equivalent standard library function, no compiler/optimizer will probably be able to use these instructions (as Confirmed by Sufian's code).

From the head, something like this would do if your buffer length is 4-bytes aligned (MASM syntax):

_asm {
   CLD                ; search forward
   XOR EAX, EAX       ; search for non-zero
   LEA EDI, [buf]     ; search in buf
   MOV ECX, [buflen]  ; search buflen bytes
   SHR ECX, 2         ; using dwords so len/=4
   REPE SCASD         ; perform scan
   JCXZ bufferEmpty:  ; completes? then buffer is 0
}

Tomas

EDIT: updated with Tony D's fixes