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/[email protected]
Signed-off-by: Kuan-Wei Chiu <[email protected]>
Cc: Ching-Chun (Jim) Huang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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);