# arm "neon"

Torbjorn Granlund tg at gmplib.org
Mon Jan 14 16:01:17 CET 2013

```nisse at lysator.liu.se (Niels Möller) writes:

Torbjorn Granlund <tg at gmplib.org> writes:

> But IIUC, we are thus performing a 32 x 32 -> 64 mul per cycle.
> Can one stick addition here without consuming cycles?

As I understand the manual, operations in the main cpu can be done in
parallel with the simd instructions. But it also warns about transferring
data between core registers and simd registers, with little details.

Doing carry propagation with the simd registers seems awkward. One would
need some comparisons to get carry conditions. And to make it even
worse, it seems the comparison instruction, vcgt, doesn't support 64bit
operations.

IIRC, one can to 32 + 32 -> 64 and 32 + 64 -> 64.  To extract the high
one must use right shift (vshr).

One can do two parallel umlaal using

umlaal?  You probably mean umul, umaal, or umlal...

That avoids carry propagation beyond 64 bits.

Is it possible to arrange an addmul_2 (or any other interesting
function) with two *independent* umaal-like operations? If we have to
accept that we can't do any adds in parallel, addmul_2 would need
something like

vmull.u32,	computing u0*v0 and u1 *v0
vaddl.u32,	lo (chain variable) + r0 (result area)
vaddl.u32,	hi (chain variable) + high half of above sum

Looks like 6 cycles (which is poor, right?), excluding any data
movement. And recurrency latency of four adds, which shouldn't be too

I didn't read carefully, and I miss v1 multiplies.

The parallelism of addmul_(k) for k >= 2 should allow for shallow
recurrency.  One may handle the v1 products many cycles before the v0
products.  For Neon, we should surely accumulate into 64-bit "d" types,
accumulating carry in bits 32, 33, etc.

There's also vmlal (mul and accumulate). One could shave one cycle off
the recurrency chain by using vmlal rather than vmull, to add in r0
earlier, and then deleting one of the low adds.

If you say so.  I'd perhaps use vmlal for accumulating the rp[] operands
as the primary approach.

And one could possibly add in hi (high part of chain variable) with
the same vmlal, but I'm not sure that's very usful. The challenge is
that one still has to add the high part of the low product into the
low part of the high product, and that's serial, not parallel. But one
could potentially reduce the number of instructions

having a non-zero operand in the high part wouldn't work unless we use
nails, since else it would overflow.

vmlal.u32,	compute u0*v0 + r0 and u1*v0 + hi

That would be 4 cycles, but one also needs to somehow extend the values

You might want to take a look at the repo mpn/arm/v6/addmul_2.asm code.
It avoids the long recurrency chain.

L(top): ldr     u0, [up, #4]
umaal   r4, cya, u1, v0
str     r4, [rp, #4]
ldr     r4, [rp, #12]
umaal   r5, cyb, u1, v1
ldr     u1, [up, #8]
umaal   r5, cya, u0, v0
str     r5, [rp, #8]
ldr     r5, [rp, #16]
umaal   r4, cyb, u0, v1
...

Neat with just umaal and ld/st...

(The addmul_1 code is not good for A15, it should be rewritten to use