Hi Nicolas, On Wed, 2015-10-28 at 19:32 -0400, Nicolas Pitre wrote: > On Thu, 29 Oct 2015, Alexey Brodkin wrote: > > > Fortunately we already have much better __div64_32() for 32-bit ARM. > > There in case of division by constant preprocessor calculates so-called > > "magic number" which is later used in multiplications instead of divisions. > > It's not magic, it is science. :-)
Indeed, but I was under impression that's how people call that value in that particular case. So for me it looks appropriate here. > > It's really nice and very optimal but obviously works only for ARM > > because ARM assembly is involved. > > > > Now why don't we extend the same approach to all other 32-bit arches > > with multiplication part implemented in pure C. With good compiler > > resulting assembly will be quite close to manually written assembly. > > You appear to have left out optimizations where there is no overflow to > carry. That, too, can be determined at compile time. That might be the case - let me look at that a bit more. But that was not the biggest problem. I actually wanted to send it as RFC but due to last minute change I made "git pathc-format -1" and forgot to change topic from PATCH to RFC. > > But there's at least 1 problem which I don't know how to solve. > > Preprocessor magic only happens if __div64_32() is inlined (that's > > obvious - preprocessor has to know if divider is constant or not). > > > > But __div64_32() is already marked as weak function (which in its turn > > is required to allow some architectures to provide its own optimal > > implementations). I.e. addition of "inline" for __div64_32() is not an > > option. > > You can't inline __div64_32(). It should remain as is and used only for > the slow path. > > For the constant based optimization to work, you need to modify do_div() > in include/asm-generic/div64.h directly. I thought about that but if I replace existing implementation of do_div() with proposed here some arches like SH and MIPS won't be able to use their own __div64_32() in do_div() any longer. So how to deal with that then? > > So I do want to hear opinions on how to proceed with that patch. > > Indeed there's the simplest solution - use this implementation only in > > my architecture of preference (read ARC) but IMHO this change may > > benefit other architectures as well. > > > > Signed-off-by: Alexey Brodkin <[email protected]> > > Cc: [email protected] > > Cc: Vineet Gupta <[email protected]> > > Cc: Ingo Molnar <[email protected]> > > Cc: Stephen Hemminger <[email protected]> > > Cc: David S. Miller <[email protected]> > > Cc: Nicolas Pitre <[email protected]> > > This email address has been unused for the last 7 years. Please update > your reference. My bad - I blindly took that email from your prehistoric patch "[ARM] 3611/4: optimize do_div() when divisor is constant". -Alexey
