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

For something so simple, you'll need to see what code the compiler is generating.

$ gcc -S -O3 -o empty.s empty.c

And the contents of the assembly:

        .text
        .align 4,0x90
.globl _is_empty
_is_empty:
        pushl       %ebp
        movl        %esp, %ebp
        movl        12(%ebp), %edx  ; edx = pointer to buffer
        movl        8(%ebp), %ecx   ; ecx = size
        testl       %edx, %edx
        jle L3
        xorl        %eax, %eax
        cmpb        $0, (%ecx)
        jne L5
        .align 4,0x90
L6:
        incl        %eax            ; real guts of the loop are in here
        cmpl        %eax, %edx
        je  L3
        cmpb        $0, (%ecx,%eax) ; compare byte-by-byte of buffer
        je  L6
L5:
        leave
        xorl        %eax, %eax
        ret
        .align 4,0x90
L3:
        leave
        movl        $1, %eax
        ret
        .subsections_via_symbols

This is very optimized. The loop does three things:

• Increase the offset
• Compare the offset to the size
• Compare the byte-data in memory at base+offset to 0

It could be optimized slightly more by comparing at a word-by-word basis, but then you'd need to worry about alignment and such.

When all else fails, measure first, don't guess.