RE: [PATCH 1/3] riscv: optimized memcpy

[David Laight <David.Laight@ACULAB.COM>]

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.

	David

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