Home » Fastest way to clamp an integer to the range 0-255

Fastest way to clamp an integer to the range 0-255



 int x=n>255?255:n;
 ... x<0?0:x ...

I’d expect this to produce something like:

 mov     ebx, 255
 mov     eax, n
 cmp     eax, ebx
 cmovg   eax, ebx ; conditional mov instruction
 test    eax, eax
 mov     ebx, 0
 cmovl   eax, ebx

If you are using MSVC SIX, you may not get the conditional move instruction. Try switching to a modern version of visual studio.

Here’s my attempt:

unsigned char clamp(int n){
    int a = 255;
    a -= n;
    a >>= 31;
    a |= n;
    n >>= 31;
    n = ~n;
    n &= a;
    return n;

It compiles to 7 instructions – which is the same as your current version. So it may or may not be faster. I haven’t timed it though. But I think these are all single-cycle instructions.

mov eax, 255
sub eax, ecx
sar eax, 31
or  al , cl
sar ecx, 31
not cl
and al , cl

Conclusion 2011-12-05:

I tried all of the suggestions again with VS 2010 Express. The generated code didn’t change much, but the register assignments did which affected the overall results. A slight modification of the straightforward implementation suggested by Ira Baxter came up the winner.

BYTE Clamp(int n)
    n = n>255 ? 255 : n;
    return n<0 ? 0 : n;

    cmp  ecx, 255
    jle  SHORT $LN8
    mov  ecx, 255
    test ecx, ecx
    sets bl
    dec  bl
    and  bl, cl

I learned a valuable lesson with this. I started with an assumption that bit-twiddling would beat anything that included a branch; I hadn’t really tried any code that included an if statement or ternary operator. That was a mistake, as I hadn’t counted on the power of the branch prediction built into a modern CPU. A ternary solution turned out to be the fastest, especially when the compiler substituted its own bit-twiddling code for one of the cases. The overall timing for this function within my benchmark algorithm went from 0.24 seconds to 0.19. This is very close to the 0.18 seconds that resulted when I removed the clamp entirely.

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