linux-kernel - RE: [PATCH 2/3] riscv: optimized memmove

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Date: Tue, 30 Jan 2024 11:51:05 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Jisheng Zhang' <jszhang@...nel.org>
CC: Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
	<palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
	"linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Matteo Croce
	<mcroce@...rosoft.com>, kernel test robot <lkp@...el.com>
Subject: RE: [PATCH 2/3] riscv: optimized memmove

From: Jisheng Zhang
> Sent: 30 January 2024 11:31
> 
> On Sun, Jan 28, 2024 at 12:47:00PM +0000, David Laight wrote:
> > From: Jisheng Zhang
> > > Sent: 28 January 2024 11:10
> > >
> > > When the destination buffer is before the source one, or when the
> > > buffers doesn't overlap, it's safe to use memcpy() instead, which is
> > > optimized to use a bigger data size possible.
> > >
> > ...
> > > + * Simply check if the buffer overlaps an call memcpy() in case,
> > > + * otherwise do a simple one byte at time backward copy.
> >
> > I'd at least do a 64bit copy loop if the addresses are aligned.
> >
> > Thinks a bit more....
> >
> > Put the copy 64 bytes code (the body of the memcpy() loop)
> > into it an inline function and call it with increasing addresses
> > in memcpy() are decrementing addresses in memmove.
> 
> Hi David,
> 
> Besides the 64 bytes copy, there's another optimization in __memcpy:
> word-by-word copy even if s and d are not aligned.
> So if we make the two optimizd copy as inline functions and call them
> in memmove(), we almost duplicate the __memcpy code, so I think
> directly calling __memcpy is a bit better.

If a forwards copy is valid call memcpy() - which I think you do.
If not you can still use the same 'copy 8 register' code
that memcpy() uses - just with a decrementing block address.

	David

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