linux-kernel - [PATCH v3] x86/lib: Optimize num_digits() and fix INT

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Message-ID: <20260120231940.17557-1-david.desobry@formalgen.com>
Date: Wed, 21 Jan 2026 00:19:40 +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 v3] 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, 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 linear chain of comparisons. This allows the
compiler to generate optimized branch sequences, providing better
performance than the original loop and more efficient assembly than a
switch statement on compilers like Clang. By using an internal unsigned
int to handle the magnitude, we safely handle the INT_MIN case without
relying on 64-bit types or triggering undefined signed overflow.

Removed the outdated comment.

Signed-off-by: David Desobry <david.desobry@...malgen.com>
---
v3:
 - Replaced switch statement with a linear if-chain for better compiler 
   optimization.
 - Simplified sign handling logic to a single if/else block.
 - (v2) Fixed INT_MIN overflow using unsigned magnitude and removed 
   outdated comment.

 arch/x86/lib/misc.c | 38 +++++++++++++++++++++++++++-----------
 1 file changed, 27 insertions(+), 11 deletions(-)

diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c
index 40b81c338ae5..3d56583411cf 100644
--- a/arch/x86/lib/misc.c
+++ b/arch/x86/lib/misc.c
@@ -3,22 +3,38 @@
 
 /*
  * 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;
+	int d;
 
 	if (val < 0) {
-		d++;
-		val = -val;
+		d = 1;
+		v = -val;
+	} else {
+		d = 0;
+		v = val;
 	}
 
-	while (val >= m) {
-		m *= 10;
-		d++;
-	}
-	return d;
+	if (v <= 9)
+		return d + 1;
+	if (v <= 99)
+		return d + 2;
+	if (v <= 999)
+		return d + 3;
+	if (v <= 9999)
+		return d + 4;
+	if (v <= 99999)
+		return d + 5;
+	if (v <= 999999)
+		return d + 6;
+	if (v <= 9999999)
+		return d + 7;
+	if (v <= 99999999)
+		return d + 8;
+	if (v <= 999999999)
+		return d + 9;
+
+	return d + 10;
 }
-- 
2.43.0