Messages in this thread | | | Date | Wed, 3 Dec 2014 15:37:39 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [PATCH] optimize ktime_divns for constant divisors |
| |
On Wed, 3 Dec 2014, Robert Jarzmik wrote:
> Nicolas Pitre <nicolas.pitre@linaro.org> 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
| |