Re: [PATCH] optimize ktime_divns for constant divisors

[Nicolas Pitre]

On Thu, 4 Dec 2014, Arnd Bergmann wrote:

> 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@xxxxxxxxxx>
> > > 
> > > 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.

Note the above code is for 32-bit architectures that support a 32x32=64 
bit multiply instruction.  And even then, what kills performances is the 
inhability to efficiently deal with carry bits from C code.  Hence the 
far better output from do_div() on ARM.

If x86-64 has a 64x64=128 bit multiply instruction then the above may 
greatly be simplified to a single multiply and a shift.  That would 
possibly outperform do_div().

> 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.

Multiplications will always be faster than divisions.  However the idiv 
instruction would come very handy in the slow path when the divisor is 
not constant.


Nicolas
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