Question

I’d like a shortcut for the following little function, where
performance is very important (the function is called more than 10.000.000 times):

inline int len(uint32 val)
{
    if(val <= 0x000000ff) return 1;
    if(val <= 0x0000ffff) return 2;
    if(val <= 0x00ffffff) return 3;
    return 4;
} 

Does anyone have any idea… a cool bitoperation trick?
Thanks for your help in advance!

Answer 1

How about this one?

inline int len(uint32 val)
{
    return 4
        - ((val & 0xff000000) == 0)
        - ((val & 0xffff0000) == 0)
        - ((val & 0xffffff00) == 0)
    ;
}

Removing the inline keyword, g++ -O2 compiles this to the following branchless code:

movl    8(%ebp), %edx
movl    %edx, %eax
andl    $-16777216, %eax
cmpl    $1, %eax
sbbl    %eax, %eax
addl    $4, %eax
xorl    %ecx, %ecx
testl   $-65536, %edx
sete    %cl
subl    %ecx, %eax
andl    $-256, %edx
sete    %dl
movzbl  %dl, %edx
subl    %edx, %eax

If you don’t mind machine-specific solutions, you can use the bsr instruction which searches for the first 1 bit. Then you simply divide by 8 to convert bits to bytes and add 1 to shift the range 0..3 to 1..4:

int len(uint32 val)
{
    asm("mov 8(%ebp), %eax");
    asm("or  $255, %eax");
    asm("bsr %eax, %eax");
    asm("shr $3, %eax");
    asm("inc %eax");
    asm("mov %eax, 8(%ebp)");
    return val;
}

Note that I am not an inline assembly god, so maybe there’s a better to solution to access val instead of addressing the stack explicitly. But you should get the basic idea.

The GNU compiler also has an interesting built-in function called __builtin_clz:

inline int len(uint32 val)
{
    return ((__builtin_clz(val | 255) ^ 31) >> 3) + 1;
}

This looks much better than the inline assembly version to me 🙂