Re: [PATCH v2] x86/lib: Optimize num_digits() and fix INT_MIN overflow

[H. Peter Anvin]

On 2026-01-20 15:32, David Desobry wrote:
> 
> Good catch. I have replaced the switch statement with a linear if-chain in v3
> to ensure better code generation for both GCC and Clang.
> 

I think a bigger deal is just to change it to unsigned.

Now, for really silly optimization:

int num_digits(unsigned int x)
{
    int n = 0;
    asm("cmp %2,%1; sbb $-2,%0" : "+r" (n) : "r" (x), "g" (10));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (100));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (1000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (10000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (100000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (1000000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (10000000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (100000000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (1000000000));

    return n;
}

No branches at all!

	-hpa