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SubjectRe: [PATCH 1/3] riscv: optimized memcpy
On Thu, Jun 17, 2021 at 11:30 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Matteo Croce
> > Sent: 16 June 2021 19:52
> > To: Guo Ren <guoren@kernel.org>
> >
> > On Wed, Jun 16, 2021 at 1:46 PM Guo Ren <guoren@kernel.org> wrote:
> > >
> > > Hi Matteo,
> > >
> > > Have you tried Glibc generic implementation code?
> > > ref: https://lore.kernel.org/linux-arch/20190629053641.3iBfk9-
> > I_D29cDp9yJnIdIg7oMtHNZlDmhLQPTumhEc@z/#t
> > >
> > > If Glibc codes have the same performance in your hardware, then you
> > > could give a generic implementation first.
>
> Isn't that a byte copy loop - the performance of that ought to be terrible.
> ...
>
> > I had a look, it seems that it's a C unrolled version with the
> > 'register' keyword.
> > The same one was already merged in nios2:
> > https://elixir.bootlin.com/linux/latest/source/arch/nios2/lib/memcpy.c#L68
>
> I know a lot about the nios2 instruction timings.
> (I've looked at code execution in the fpga's intel 'logic analiser.)
> It is a very simple 4-clock pipeline cpu with a 2-clock delay
> before a value read from 'tightly coupled memory' (aka cache)
> can be used in another instruction.
> There is also a subtle pipeline stall if a read follows a write
> to the same memory block because the write is executed one
> clock later - and would collide with the read.
> Since it only ever executes one instruction per clock loop
> unrolling does help - since you never get the loop control 'for free'.
> OTOH you don't need to use that many registers.
> But an unrolled loop should approach 2 bytes/clock (32bit cpu).
>
> > I copied _wordcopy_fwd_aligned() from Glibc, and I have a very similar
> > result of the other versions:
> >
> > [ 563.359126] Strings selftest: memcpy(src+7, dst+7): 257 Mb/s
>
> What clock speed is that running at?
> It seems very slow for a 64bit cpu (that isn't an fpga soft-cpu).
>
> While the small riscv cpu might be similar to the nios2 (and mips
> for that matter), there are also bigger/faster cpu.
> I'm sure these can execute multiple instructions/clock
> and possible even read and write at the same time.
> Unless they also support significant instruction re-ordering
> the trivial copy loops are going to be slow on such cpu.
>

It's running at 1 GHz.

I get 257 Mb/s with a memcpy, a bit more with a memset,
but I get 1200 Mb/s with a cyle which just reads memory with 64 bit addressing.

--
per aspera ad upstream

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