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

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

From: Jisheng Zhang
> Sent: 28 January 2024 11:10
> 
> From: Matteo Croce <mcroce@microsoft.com>
> 
> Write a C version of memcpy() which uses the biggest data size allowed,
> without generating unaligned accesses.
> 
> The procedure is made of three steps:
> First copy data one byte at time until the destination buffer is aligned
> to a long boundary.
> Then copy the data one long at time shifting the current and the next u8
> to compose a long at every cycle.
> Finally, copy the remainder one byte at time.
> 
> On a BeagleV, the TCP RX throughput increased by 45%:
...
> +static void __memcpy_aligned(unsigned long *dest, const unsigned long *src, size_t count)
> +{

You should be able to remove an instruction from the loop by using:
	const unsigned long *src_lim = src + count;
	for (; src < src_lim; ) {

> +	for (; count > 0; count -= BYTES_LONG * 8) {
> +		register unsigned long d0, d1, d2, d3, d4, d5, d6, d7;

register is completely ignored and pointless.
(More annoyingly auto is also ignored.)

> +		d0 = src[0];
> +		d1 = src[1];
> +		d2 = src[2];
> +		d3 = src[3];
> +		d4 = src[4];
> +		d5 = src[5];
> +		d6 = src[6];
> +		d7 = src[7];
> +		dest[0] = d0;
> +		dest[1] = d1;
> +		dest[2] = d2;
> +		dest[3] = d3;
> +		dest[4] = d4;
> +		dest[5] = d5;
> +		dest[6] = d6;
> +		dest[7] = d7;
> +		dest += 8;
> +		src += 8;

There two lines belong in the for (...) statement.

> +	}
> +}

If you __always_inline the function you can pass &src and &dest
and use the updated pointers following the loop.

I don't believe that risc-v supports 'reg+reg+(imm5<<3)' addressing
(although there is probably space in the instruction for it.
Actually 'reg+reg' addressing could be supported for loads but
not stores - since the latter would require 3 registers be read.

We use the Nios-II cpu in some fpgas. Intel are removing support
in favour of Risc-V - we are thinking of re-implementing Nios-II
ourselves!
I don't think they understand what the cpu get used for!

	David

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Registration No: 1397386 (Wales)


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