Re: [PATCH v6 3/4] riscv: Add checksum library

[Charlie Jenkins <charlie@rivosinc.com>]

On Sat, Sep 16, 2023 at 09:32:40AM +0000, David Laight wrote:
> From: Charlie Jenkins
> > Sent: 15 September 2023 18:01
> > 
> > Provide a 32 and 64 bit version of do_csum. When compiled for 32-bit
> > will load from the buffer in groups of 32 bits, and when compiled for
> > 64-bit will load in groups of 64 bits.
> > 
> ...
> > +	/*
> > +	 * Do 32-bit reads on RV32 and 64-bit reads otherwise. This should be
> > +	 * faster than doing 32-bit reads on architectures that support larger
> > +	 * reads.
> > +	 */
> > +	while (len > 0) {
> > +		csum += data;
> > +		csum += csum < data;
> > +		len -= sizeof(unsigned long);
> > +		ptr += 1;
> > +		data = *ptr;
> > +	}
> 
> I think you'd be better adding the 'carry' bits in a separate
> variable.
> It reduces the register dependency chain length in the loop.
> (Helps if the cpu can execute two instructions in one clock.)
> 
> The masked misaligned data values are max 24 bits
> (if 
> 
> You'll also almost certainly remove at least one instruction
> from the loop by comparing against the end address rather than
> changing 'len'.
> 
> So ending up with (something like):
> 	end = buff + length;
> 	...
> 	while (++ptr < end) {
> 		csum += data;
> 		carry += csum < data;
> 		data = ptr[-1];
> 	}
> (Although a do-while loop tends to generate better code
> and gcc will pretty much always make that transformation.)
> 
> I think that is 4 instructions per word (load, add, cmp+set, add).
> In principle they could be completely pipelined and all
> execute (for different loop iterations) in the same clock.
> (But that is pretty unlikely to happen - even x86 isn't that good.)
> But taking two clocks is quite plausible.
> Plus 2 instructions per loop (inc, cmp+jmp).
> They might execute in parallel, but unrolling once
> may be required.
> 
It looks like GCC actually ends up generating 7 total instructions:
ffffffff808d2acc:	97b6                	add	a5,a5,a3
ffffffff808d2ace:	00d7b533          	sltu	a0,a5,a3
ffffffff808d2ad2:	0721                	add	a4,a4,8
ffffffff808d2ad4:	86be                	mv	a3,a5
ffffffff808d2ad6:	962a                	add	a2,a2,a0
ffffffff808d2ad8:	ff873783          	ld	a5,-8(a4)
ffffffff808d2adc:	feb768e3          	bltu	a4,a1,ffffffff808d2acc <do_csum+0x34>

This mv instruction could be avoided if the registers were shuffled
around, but perhaps this way reduces some dependency chains.
> ...
> > +	if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) &&
> > +	    riscv_has_extension_likely(RISCV_ISA_EXT_ZBB)) {
> ...
> > +		}
> > +end:
> > +		return csum >> 16;
> > +	}
> 
> Is it really worth doing all that to save (I think) 4 instructions?
> (shift, shift, or with rotate twice).
> There is much more to be gained by careful inspection
> of the loop (even leaving it in C).
> 

The main benefit was from using rev8 to replace swab32. However, now
that I am looking at the assembly in the kernel it is not outputting the
asm that matches what I have from an out of kernel test case, so rev8
might not be beneficial. I am going to have to look at this more to
figure out what is happening.

> > +
> > +#ifndef CONFIG_32BIT
> > +	csum += (csum >> 32) | (csum << 32);
> > +	csum >>= 32;
> > +#endif
> > +	csum = (unsigned int)csum + (((unsigned int)csum >> 16) | ((unsigned int)csum << 16));
> 
> Use ror64() and ror32().
> 
> 	David
> 

Good idea.

- Charlie

> > +	if (offset & 1)
> > +		return (unsigned short)swab32(csum);
> > +	return csum >> 16;
> > +}
> > 
> > --
> > 2.42.0
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)