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

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Date: Tue, 30 Jan 2024 19:30:43 +0800
From: Jisheng Zhang <jszhang@...nel.org>
To: David Laight <David.Laight@...lab.com>
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

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.

Thanks
> 
> So memcpy() contains:
> 	src_lim = src_lim + count;
> 	... alignment copy
> 	for (; src + 64 <= src_lim; src += 64; dest += 64)
> 		copy_64_bytes(dest, src);
> 	... tail copy
> 
> Then you can do something very similar for backwards copies.
> 
> 	David
> 
> -
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