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.

Link: https://lkml.kernel.org/r/20240812170229.229380-1-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/lib/bcd.c b/lib/bcd.c
index 7e4750b6e801425dd1c3a68402607b189ca6b99d..c5e79ba9cd7b722d782c990d5f9f61b1cd661524 100644 (file)
--- 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);