Re: [PATCH 0/2] arm64: csum: Add fused copy and Internet checksum

[David Laight <david.laight.linux@gmail.com>]

On Mon, 2 Nov 2026 16:14:06 +0100
Demian Shulhan <demyansh@gmail.com> wrote:

> > The scalar path seems like generic C code and not restricted to arm64,
> > curious how that performs against existing (e.g. x86) assembly
> > optimized implementation.  
> Depends. I think __builtin_addcll() is needed to get roughly the same
> performance as the asm version. In my tests GCC on arm64 turns it
> into adds + cset instead of adcs like on x86.
> But I will look at it, thanks!

The C version I posted earlier has the loop:
    buf += count;
    count = -count;
    sum = carry = 0;
    do {
        val = *(u64 *)(buf + count);
        sum += val;
        carry += sum < val;
    } while ((count += 8));

for aarch64 you get:
.L4:
        ldr     x1, [x0, x4]
        adds    x2, x2, x1
        cinc    x3, x3, cs	# adc $0,%rxx on x86
        adds    x4, x4, 8
        bne     .L4

I suspect that is close to optimal - especially for fast cpu with
out of order execution.

https://godbolt.org/z/eso79K7Wq

David

> 
> пн, 2 лист. 2026 р. о 12:44 saeed bishara <saeed.bishara.os@gmail.com> пише:
> >
> > On Sun, Nov 1, 2026 at 3:34 PM Demian Shulhan <demyansh@gmail.com> wrote:  
> > >
> > > I would prefer to keep only the scalar version of the fused copy+csum.
> > > It reads the data once instead of twice, and on real Armv9 hardware it
> > > cuts end-to-end cycles/byte for bulk TCP over WireGuard by 3-12%
> > > depending on the core (+6-8% throughput). It uses only general-purpose
> > > registers, so there is no FPSIMD state to save and it is safe in any
> > > context.
> > >
> > > If you are OK with the scalar path, I can send v2 with NEON dropped,
> > > the scalar path only, and the KUnit fix for the x86 zero-length case
> > > Jakub reported.
> > >  
> > The scalar path seems like generic C code and not restricted to arm64,
> > curious how that performs against existing (e.g. x86) assembly
> > optimized implementation.
> > saeed