Re: [PATCH v4 2/5] riscv: Add checksum library

[Charlie Jenkins <charlie@rivosinc.com>]

On Tue, Sep 12, 2023 at 08:45:38AM +0000, David Laight wrote:
> From: Charlie Jenkins
> > Sent: 11 September 2023 23:57
> > 
> > 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. Benchmarking by proxy compiling
> > csum_ipv6_magic (64-bit version) for an x86 chip as well as running
> > the riscv generated code in QEMU, discovered that summing in a
> > tree-like structure is about 4% faster than doing 64-bit reads.
> > 
> ...
> > +	sum   = saddr->s6_addr32[0];
> > +	sum  += saddr->s6_addr32[1];
> > +	sum1  = saddr->s6_addr32[2];
> > +	sum1 += saddr->s6_addr32[3];
> > +
> > +	sum2  = daddr->s6_addr32[0];
> > +	sum2 += daddr->s6_addr32[1];
> > +	sum3  = daddr->s6_addr32[2];
> > +	sum3 += daddr->s6_addr32[3];
> > +
> > +	sum4  = csum;
> > +	sum4 += ulen;
> > +	sum4 += uproto;
> > +
> > +	sum  += sum1;
> > +	sum2 += sum3;
> > +
> > +	sum += sum2;
> > +	sum += sum4;
> 
> Have you got gcc to compile that as-is?
> 
> Whenever I've tried to get a 'tree add' compiled so that the
> early adds can be executed in parallel gcc always pessimises
> it to a linear sequence of adds.
> 
> But I agree that adding 32bit values to a 64bit register
> may be no slower than trying to do an 'add carry' sequence
> that is guaranteed to only do one add/clock.
> (And on Intel cpu from core-2 until IIRC Haswell adc took 2 clocks!)
> 
> IIRC RISCV doesn't have a carry flag, so the adc sequence
> is hard - probably takes two extra instructions per value.
> Although with parallel execute it may not matter.
> Consider:
> 	val = buf[offset];
> 	sum += val;
> 	carry += sum < val;
> 	val = buf[offset1];
> 	sum += val;
> 	...
> the compare and 'carry +=' can be executed at the same time
> as the following two instructions.
> You do then a final sum += carry; sum += sum < carry;
> 
> Assuming all instructions are 1 clock and any read delays
> get filled with other instructions (by source or hardware
> instruction re-ordering) even without parallel execute
> that is 4 clocks for 64 bits, which is much the same as the
> 2 clocks for 32 bits.
> 
> Remember that all the 32bit values can summed first as
> they won't overflow.
> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
Yeah it does seem like the tree-add does just do a linear add. All three
of them were pretty much the same on riscv so I used the version that
did best on x86 with the knowledge that my QEMU setup does not
accurately represent real hardware.

I don't quite understand how doing the carry in the middle of each
stage, even though it can be executed at the same time, would be faster
than just doing a single overflow check at the end. I can just revert
back to the non-tree add version since there is no improvement on riscv.
I can also revert back to the default version that uses carry += sum < val
as well.

- Charlie