On Wed, 3 Dec 2014, Robert Jarzmik wrote: > Nicolas Pitre <[email protected]> writes: > > > Let ktime_divns() use do_div() inline whenever the divisor is constant > > and small enough. This will make things like ktime_to_us() and > > ktime_to_ms() much faster. > > Hi Nicolas, > > I suppose the "small enough" is linked to the "!(div >> 32)" in your patch. > Can > I have the rationale which brought up this value, and if that value is > universal > across architectures (ie. x86/ppc/arm/...) ?
Yes. The do_div() function is defined to accept a 32-bit divisor only. The out-of-line ktime_divns code does scale down both the dividend and the divisor until the divisor is within 32 bits of magnitude before calling do_div(). However the constness of the divisor is lost and the optimised do_div (on ARM at least) doesn't get involved. > And when you say "much faster", do you have figures to add to your commit > message ? No actual figure. But a wild guess would be around an order of magnitude. See commit fa4adc6149 for an example of generated code. Nicolas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
