linux-kernel - [PATCH] lib/bcd: Optimize _bin2bcd() for improved performance

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Message-Id: <20240812170229.229380-1-visitorckw@gmail.com>
Date: Tue, 13 Aug 2024 01:02:29 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: akpm@...ux-foundation.org
Cc: jserv@...s.ncku.edu.tw,
	linux-kernel@...r.kernel.org,
	Kuan-Wei Chiu <visitorckw@...il.com>
Subject: [PATCH] lib/bcd: Optimize _bin2bcd() for improved performance

The original _bin2bcd() function used / 10 and % 10 operations for
conversion. Although GCC optimizes these operations and does not
generate division or modulus instructions, the new implementation
reduces the number of mov instructions in the generated code for both
x86-64 and ARM architectures.

This optimization calculates the tens digit using (val * 103) >> 10,
which is accurate for values of 'val' in the range [0, 178]. Given that
the valid input range is [0, 99], this method ensures correctness while
simplifying the generated code.

Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
---
Use a unit test to confirm that the new implementation produces the
same results as the old one for values in the range [0, 99].

 lib/bcd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/bcd.c b/lib/bcd.c
index 7e4750b6e801..c5e79ba9cd7b 100644
--- a/lib/bcd.c
+++ b/lib/bcd.c
@@ -10,6 +10,8 @@ EXPORT_SYMBOL(_bcd2bin);

 unsigned char _bin2bcd(unsigned val)
 {
-	return ((val / 10) << 4) + val % 10;
+	const unsigned int t = (val * 103) >> 10;
+
+	return (t << 4) | (val - t * 10);
 }
 EXPORT_SYMBOL(_bin2bcd);
-- 
2.34.1