[v4] x86/lib: Optimize num_digits() and fix INT_MIN overflow

[PATCH v4] x86/lib: Optimize num_digits() and fix INT_MIN overflow

Posted by David Desobry 2 weeks, 3 days ago

The current implementation of num_digits() uses a loop with repeated
multiplication, which is inefficient. Furthermore, it takes a signed
integer which leads to undefined behavior when negating INT_MIN.

As this function is only used for unsigned magnitudes in the kernel code,
convert the interface to take an unsigned int. This naturally resolves
the INT_MIN overflow issue.

Replace the loop with a branchless sequence using inline assembly. By
using the 'sbb' instruction against the carry flag after comparisons,
we eliminate all conditional branches. This provides constant-time
performance and avoids CPU branch misprediction penalties.

Update the function comment to reflect that signs are no longer handled.

Signed-off-by: David Desobry <david.desobry@formalgen.com>
---
 v4:
 - Switched to branchless inline assembly. 
 - Changed function signature to unsigned int.
 - Updated prototype in arch/x86/include/asm/misc.h. 

 arch/x86/include/asm/misc.h |  2 +-
 arch/x86/lib/misc.c         | 28 +++++++++++++---------------
 2 files changed, 14 insertions(+), 16 deletions(-)

diff --git a/arch/x86/include/asm/misc.h b/arch/x86/include/asm/misc.h
index bb049cca3729..48b6bd7c08b9 100644
--- a/arch/x86/include/asm/misc.h
+++ b/arch/x86/include/asm/misc.h
@@ -2,6 +2,6 @@
 #ifndef _ASM_X86_MISC_H
 #define _ASM_X86_MISC_H
 
-int num_digits(int val);
+int num_digits(unsigned int val);
 
 #endif /* _ASM_X86_MISC_H */
diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c
index 40b81c338ae5..9623795b059f 100644
--- a/arch/x86/lib/misc.c
+++ b/arch/x86/lib/misc.c
@@ -2,23 +2,21 @@
 #include <asm/misc.h>
 
 /*
- * Count the digits of @val including a possible sign.
- *
- * (Typed on and submitted from hpa's mobile phone.)
+ * Count the decimal digits of an unsigned integer.
  */
-int num_digits(int val)
+int num_digits(unsigned int x)
 {
-	long long m = 10;
-	int d = 1;
+	int n = 0;
 
-	if (val < 0) {
-		d++;
-		val = -val;
-	}
+	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));
 
-	while (val >= m) {
-		m *= 10;
-		d++;
-	}
-	return d;
+	return n;
 }
-- 
2.43.0

Re: [PATCH v4] x86/lib: Optimize num_digits() and fix INT_MIN overflow

Posted by David Laight 2 weeks, 3 days ago

On Wed, 21 Jan 2026 10:39:11 +0100
David Desobry <david.desobry@formalgen.com> wrote:

> The current implementation of num_digits() uses a loop with repeated
> multiplication, which is inefficient. Furthermore, it takes a signed
> integer which leads to undefined behavior when negating INT_MIN.
> 
> As this function is only used for unsigned magnitudes in the kernel code,
> convert the interface to take an unsigned int. This naturally resolves
> the INT_MIN overflow issue.
> 
> Replace the loop with a branchless sequence using inline assembly. By
> using the 'sbb' instruction against the carry flag after comparisons,
> we eliminate all conditional branches. This provides constant-time
> performance and avoids CPU branch misprediction penalties.

Don't do this version, it isn't worth the effort.

	David

> 
> Update the function comment to reflect that signs are no longer handled.
> 
> Signed-off-by: David Desobry <david.desobry@formalgen.com>
> ---
>  v4:
>  - Switched to branchless inline assembly. 
>  - Changed function signature to unsigned int.
>  - Updated prototype in arch/x86/include/asm/misc.h. 
> 
>  arch/x86/include/asm/misc.h |  2 +-
>  arch/x86/lib/misc.c         | 28 +++++++++++++---------------
>  2 files changed, 14 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/include/asm/misc.h b/arch/x86/include/asm/misc.h
> index bb049cca3729..48b6bd7c08b9 100644
> --- a/arch/x86/include/asm/misc.h
> +++ b/arch/x86/include/asm/misc.h
> @@ -2,6 +2,6 @@
>  #ifndef _ASM_X86_MISC_H
>  #define _ASM_X86_MISC_H
>  
> -int num_digits(int val);
> +int num_digits(unsigned int val);
>  
>  #endif /* _ASM_X86_MISC_H */
> diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c
> index 40b81c338ae5..9623795b059f 100644
> --- a/arch/x86/lib/misc.c
> +++ b/arch/x86/lib/misc.c
> @@ -2,23 +2,21 @@
>  #include <asm/misc.h>
>  
>  /*
> - * Count the digits of @val including a possible sign.
> - *
> - * (Typed on and submitted from hpa's mobile phone.)
> + * Count the decimal digits of an unsigned integer.
>   */
> -int num_digits(int val)
> +int num_digits(unsigned int x)
>  {
> -	long long m = 10;
> -	int d = 1;
> +	int n = 0;
>  
> -	if (val < 0) {
> -		d++;
> -		val = -val;
> -	}
> +	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));
>  
> -	while (val >= m) {
> -		m *= 10;
> -		d++;
> -	}
> -	return d;
> +	return n;
>  }

Re: [PATCH v4] x86/lib: Optimize num_digits() and fix INT_MIN overflow

Posted by H. Peter Anvin 2 weeks, 2 days ago

On January 21, 2026 3:36:43 AM PST, David Laight <david.laight.linux@gmail.com> wrote:
>On Wed, 21 Jan 2026 10:39:11 +0100
>David Desobry <david.desobry@formalgen.com> wrote:
>
>> The current implementation of num_digits() uses a loop with repeated
>> multiplication, which is inefficient. Furthermore, it takes a signed
>> integer which leads to undefined behavior when negating INT_MIN.
>> 
>> As this function is only used for unsigned magnitudes in the kernel code,
>> convert the interface to take an unsigned int. This naturally resolves
>> the INT_MIN overflow issue.
>> 
>> Replace the loop with a branchless sequence using inline assembly. By
>> using the 'sbb' instruction against the carry flag after comparisons,
>> we eliminate all conditional branches. This provides constant-time
>> performance and avoids CPU branch misprediction penalties.
>
>Don't do this version, it isn't worth the effort.
>
>	David
>
>> 
>> Update the function comment to reflect that signs are no longer handled.
>> 
>> Signed-off-by: David Desobry <david.desobry@formalgen.com>
>> ---
>>  v4:
>>  - Switched to branchless inline assembly. 
>>  - Changed function signature to unsigned int.
>>  - Updated prototype in arch/x86/include/asm/misc.h. 
>> 
>>  arch/x86/include/asm/misc.h |  2 +-
>>  arch/x86/lib/misc.c         | 28 +++++++++++++---------------
>>  2 files changed, 14 insertions(+), 16 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/misc.h b/arch/x86/include/asm/misc.h
>> index bb049cca3729..48b6bd7c08b9 100644
>> --- a/arch/x86/include/asm/misc.h
>> +++ b/arch/x86/include/asm/misc.h
>> @@ -2,6 +2,6 @@
>>  #ifndef _ASM_X86_MISC_H
>>  #define _ASM_X86_MISC_H
>>  
>> -int num_digits(int val);
>> +int num_digits(unsigned int val);
>>  
>>  #endif /* _ASM_X86_MISC_H */
>> diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c
>> index 40b81c338ae5..9623795b059f 100644
>> --- a/arch/x86/lib/misc.c
>> +++ b/arch/x86/lib/misc.c
>> @@ -2,23 +2,21 @@
>>  #include <asm/misc.h>
>>  
>>  /*
>> - * Count the digits of @val including a possible sign.
>> - *
>> - * (Typed on and submitted from hpa's mobile phone.)
>> + * Count the decimal digits of an unsigned integer.
>>   */
>> -int num_digits(int val)
>> +int num_digits(unsigned int x)
>>  {
>> -	long long m = 10;
>> -	int d = 1;
>> +	int n = 0;
>>  
>> -	if (val < 0) {
>> -		d++;
>> -		val = -val;
>> -	}
>> +	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));
>>  
>> -	while (val >= m) {
>> -		m *= 10;
>> -		d++;
>> -	}
>> -	return d;
>> +	return n;
>>  }
>

No, please, it was a joke.