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Commit d03bd045 authored by Martin Schwidefsky's avatar Martin Schwidefsky
Browse files

s390/timex: micro optimization for tod_to_ns 



The conversion of a TOD value to nano-seconds currently uses a 32/32 bit
split with the calculation for "nsecs = (TOD * 125) >> 9". Using a
55/9 bit split saves an instruction.

Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
parent e53051e7

arch/s390/include/asm/timex.h

+4 −8
Original line number Diff line number Diff line
@@ -206,20 +206,16 @@ static inline unsigned long long get_tod_clock_monotonic(void) 
 *    ns = (todval * 125) >> 9;

 *

 * In order to avoid an overflow with the multiplication we can rewrite this.

 * With a split todval == 2^32 * th + tl (th upper 32 bits, tl lower 32 bits)

 * With a split todval == 2^9 * th + tl (th upper 55 bits, tl lower 9 bits)

 * we end up with

 *

 *    ns = ((2^32 * th + tl) * 125 ) >> 9;

 * -> ns = (2^23 * th * 125) + ((tl * 125) >> 9);

 *    ns = ((2^9 * th + tl) * 125 ) >> 9;

 * -> ns = (th * 125) + ((tl * 125) >> 9);

 *

 */

static inline unsigned long long tod_to_ns(unsigned long long todval)

{

	unsigned long long ns;



	ns = ((todval >> 32) << 23) * 125;

	ns += ((todval & 0xffffffff) * 125) >> 9;

	return ns;

	return ((todval >> 9) * 125) + (((todval & 0x1ff) * 125) >> 9);

}



#endif