Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH] optimize ktime_divns for constant divisors | Date | Thu, 04 Dec 2014 13:02:33 +0100 |
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On Thursday 04 December 2014 02:23:37 Nicolas Pitre wrote: > On Wed, 3 Dec 2014, Arnd Bergmann wrote: > > > On Wednesday 03 December 2014 14:43:06 Nicolas Pitre wrote: > > > At least on ARM, do_div() is optimized to turn constant divisors into > > > an inline multiplication by the reciprocal value at compile time. > > > However this optimization is missed entirely whenever ktime_divns() is > > > used and the slow out-of-line division code is used all the time. > > > > > > 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. > > > > > > Signed-off-by: Nicolas Pitre <nico@linaro.org> > > > > Very cool. I've been thinking about doing something similar for the > > general case but couldn't get the math to work. > > > > Can you think of an architecture-independent way to ktime_to_sec, > > ktime_to_ms, and ktime_to_us efficiently based on what you did for > > the ARM do_div implementation? > > Sure. gcc generates rather shitty code on ARM compared to the output > from my do_div() implementation. But here it is: > > u64 ktime_to_us(ktime_t kt) > { > u64 ns = ktime_to_ns(kt); > u32 x_lo, x_hi, y_lo, y_hi; > u64 res, carry; > > x_hi = ns >> 32; > x_lo = ns; > y_hi = 0x83126e97; > y_lo = 0x8d4fdf3b; > > res = (u64)x_lo * y_lo; > carry = (u64)(u32)res + y_lo; > res = (res >> 32) + (carry >> 32); > > res += (u64)x_lo * y_hi; > carry = (u64)(u32)res + (u64)x_hi * y_lo; > res = (res >> 32) + (carry >> 32); > > res += (u64)x_hi * y_hi; > return res >> 9; > }
Ok, I see, thanks for the example. I also tried this on x86, and it takes about twice as long as do_div on my Opteron, so it wouldn't be as helpful as I hoped.
On a related note, I wonder if we can come up with a more efficient implementation for do_div on ARMv7ve, and I think we should add the Makefile logic to build with -march=armv7ve when we know that we do not need to support processors without idiv.
Arnd
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