Re: [PATCH] optimize ktime_divns for constant divisors

From: Nicolas Pitre
Date: Thu Dec 04 2014 - 11:47:27 EST

On Thu, 4 Dec 2014, Arnd Bergmann wrote:

> On Thursday 04 December 2014 08:46:27 Nicolas Pitre wrote:
> > On Thu, 4 Dec 2014, Arnd Bergmann wrote:
> > 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().
> I was trying this in 32-bit mode to see how it would work in x86-32
> kernels. Since that architecture has a 64-by-32 divide instruction,
> that gets used here.
> x86-64 has a 64x64=128 multiply instruction and gcc uses that for
> any 64-bit division by constant, so that's what already happens
> in do_div. I assume for any 64-bit architecture, the result will
> be similar.

OK. In that case x86-64 will also benefit from the patch at the
beginning of this thread.

> I guess the only architectures that would benefit from your implementation
> above are the ones that do not have any optimization for constant
> 64-by-32-bit division and just call do_div.

And then it would be best to optimize do_div() directly so all users
would benefit.

> > > 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.
> Makes sense. I also just checked the gcc sources and it seems that the
> idiv/udiv instructions on ARM are not even used for implementing
> __aeabi_uldivmod there. Not sure if that's intentional, but we probably
> don't need to bother optimizing this in the kernel before user space
> does.

I wouldn't say so. There are many precedents where we optimized those
things in the kernel before gcc caught up. In a few cases I contributed
the same optimized arithmetic routines to both gcc and the kernel.

> Building with -march=armv7ve still sounds helpful to avoid the
> __aeabi_uidiv calls though.


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