Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH] optimize ktime_divns for constant divisors | Date | Thu, 04 Dec 2014 15:56:47 +0100 |
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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.
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.
> > 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. Building with -march=armv7ve still sounds helpful to avoid the __aeabi_uidiv calls though.
Arnd
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