linux-kernel - Re: [PATCH v2] x86/lib: Optimize num_digits() and fix INT

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Message-ID: <ef852c16-3736-4bd5-bc65-69211b05506e@formalgen.com>
Date: Wed, 21 Jan 2026 01:04:39 +0100
From: David Desobry <david.desobry@...malgen.com>
To: "H. Peter Anvin" <hpa@...or.com>,
 David Laight <david.laight.linux@...il.com>
Cc: tglx@...nel.org, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/lib: Optimize num_digits() and fix INT_MIN
 overflow

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.