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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <814A431C-66E0-49F8-9F78-63F17D8415A2@zytor.com>
Date: Tue, 20 Jan 2026 10:03:31 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: David Desobry <david.desobry@...malgen.com>, tglx@...nel.org,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com
CC: x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/lib: Optimize num_digits() and fix INT_MIN overflow

On January 20, 2026 9:47:48 AM PST, David Desobry <david.desobry@...malgen.com> wrote:
>The current implementation of num_digits() uses a loop with repeated
>multiplication, which is inefficient. Furthermore, the negation of
>the input value "val = -val" causes undefined behavior when val is
>INT_MIN, as its absolute value cannot be represented as a 32-bit integer.
>
>Replace the loop with a switch statement using GCC case ranges. This
>allows the compiler to generate a jump table or a series of optimized
>comparisons, providing O(1) performance. By using an unsigned int to
>handle the magnitude, we safely handle the INT_MIN case without
>relying on 64-bit types or undefined signed overflow.
>
>Removed the outdated comment.
>
>Signed-off-by: David Desobry <david.desobry@...malgen.com>
>---
>v2:
> - Replaced loop with switch statement and GCC case ranges.
> - Fixed INT_MIN overflow using unsigned int cast instead of s64/long long.
> - Removed outdated comment regarding mobile submission.
> arch/x86/lib/misc.c | 35 +++++++++++++++++++++++++----------
> 1 file changed, 25 insertions(+), 10 deletions(-)
>
>diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c
>index 40b81c338ae5..03ba028d5326 100644
>--- a/arch/x86/lib/misc.c
>+++ b/arch/x86/lib/misc.c
>@@ -3,22 +3,37 @@
> 
> /*
>  * Count the digits of @val including a possible sign.
>- *
>- * (Typed on and submitted from hpa's mobile phone.)
>  */
> int num_digits(int val)
> {
>-	long long m = 10;
>-	int d = 1;
>+	unsigned int v = val;
>+	int d = 0;
> 
> 	if (val < 0) {
>-		d++;
>-		val = -val;
>+		d = 1;
>+		v = -v;
> 	}
> 
>-	while (val >= m) {
>-		m *= 10;
>-		d++;
>+	switch (v) {
>+	case 0 ... 9:
>+		return d + 1;
>+	case 10 ... 99:
>+		return d + 2;
>+	case 100 ... 999:
>+		return d + 3;
>+	case 1000 ... 9999:
>+		return d + 4;
>+	case 10000 ... 99999:
>+		return d + 5;
>+	case 100000 ... 999999:
>+		return d + 6;
>+	case 1000000 ... 9999999:
>+		return d + 7;
>+	case 10000000 ... 99999999:
>+		return d + 8;
>+	case 100000000 ... 999999999:
>+		return d + 9;
>+	default:
>+		return d + 10;
> 	}
>-	return d;
> }

Looks good to me 

Acked-by: H. Peter Anvin (Intel) <hpa@...or.com>