Messages in this thread | | | Date | Wed, 23 Nov 2011 21:38:49 +0100 | From | "N. Coesel" <> | Subject | Re: Fast memcpy patch |
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Sasha,
At 14:04 23-11-2011, Sasha Levin wrote: >On Wed, 2011-11-23 at 13:51 +0100, N. Coesel wrote: > > Sasha, > > > > At 13:10 23-11-2011, Sasha Levin wrote: > > >On Wed, 2011-11-23 at 12:25 +0100, N. Coesel wrote: > > > > Dear readers, > > > > I noticed the Linux kernel still uses a byte-by-byte copy method for > > > > memcpy. Since most memory allocations are aligned to the integer size > > > > of a cpu it is often faster to copy by using the CPU's native word > > > > size. The patch below does that. The code is already at work in many > > > > 16 and 32 bit embedded products. It should also work for 64 bit > > > > platforms. So far I only tested 16 and 32 bit platforms. > > > > > >[snip] > > > > > >memcpy (along with other mem* functions) are arch specific - for > > >example, look at arch/x86/lib/memcpy_64.S for the implementation(s) for > > >x86. > > > > > >The code under lib/string.c is simple and should work on all platforms > > >(and is probably not being used anywhere anymore). > > > > Thanks for pointing that out. Currently my primary target is ARM. It > > seems the memcpy for that arch uses byte-by-byte copying as well with > > some loop unrolling. I modified the code so it tries to use > > word-by-word copy if the pointers are aligned on word boundaries, if > > not it reverts to the old method. For clarity: by word I mean the > > CPU's native bus width. In case of ARM that's (still) 32 bit. > >I don't think we're looking at the same file. > >For arm it's arch/arm/lib/copy_template.S, right? Or are you talking >about something else?
I was looking somewhere else indeed. There are a lot of versions of memcpy in the kernel :-) Thanks for pointing me to the right file. The asm stuff looks pretty nifty by using the load/store multiple instructions. Too bad. I was hoping to get some extra speed with a quick fix. I need to copy several MB from a driver.
Nico Coesel
>-- > >Sasha.
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