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

[David Desobry]

On 1/21/26 00:49, H. Peter Anvin wrote:
> 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

Actually, the V3 change: 	
if (val < 0) {
-		d++;
-		val = -val;
+		d = 1;
+		v = -val;
+	} else {
+		d = 0;
+		v = val;
 	}
reintroduced the undefined behavior for val == INT_MIN.
So this V3 version is incorrect.
I'm not familiar enough with the rest of the codebase to know if 
changing the function signature to unsigned int is correct here.