linux-kernel - [PATCH v4] x86/lib: Optimize num_digits() and fix INT

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Message-ID: <20260121093911.27042-1-david.desobry@formalgen.com>
Date: Wed, 21 Jan 2026 10:39:11 +0100
From: David Desobry <david.desobry@...malgen.com>
To: tglx@...nel.org,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com
Cc: x86@...nel.org,
	hpa@...or.com,
	linux-kernel@...r.kernel.org,
	David Desobry <david.desobry@...malgen.com>
Subject: [PATCH v4] x86/lib: Optimize num_digits() and fix INT_MIN overflow

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@...malgen.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