[PATCH] lib/bcd: optimize _bin2bcd() for improved performance

[Kuan-Wei Chiu]

[ Upstream commit cbf164cd44e06c78938b4a4a4479d3541779c319 ]

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.

Link: https://lkml.kernel.org/r/20240812170229.229380-1-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw at gmail.com>
Cc: Ching-Chun (Jim) Huang <jserv at ccns.ncku.edu.tw>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
[visitorckw at gmail.com: Adapt to bin2bcd() in include/bcd.h]
Signed-off-by: Kuan-Wei Chiu <visitorckw at gmail.com>
---
 include/bcd.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/bcd.h b/include/bcd.h
index 9ecd328284e..289810b99b7 100644
--- a/include/bcd.h
+++ b/include/bcd.h
@@ -17,7 +17,9 @@ static inline unsigned int bcd2bin(unsigned int val)
 
 static inline unsigned int bin2bcd(unsigned int val)
 {
-	return (((val / 10) << 4) | (val % 10));
+	const unsigned int t = (val * 103) >> 10;
+
+	return (t << 4) | (val - t * 10);
 }
 
 #endif /* _BCD_H */
-- 
2.52.0.239.gd5f0c6e74e-goog