Re: vectorized CRC on ARM64

On Wed, May 14, 2025 I wrote:
>
> We did something similar for x86 for v18, and here is some progress
> towards Arm support.

Coming back to this, since there's been recent interest in Arm support.

v2 is a rebase, with a few changes.

- I simplified it by leaving out the inlining for "assume CRC" builds,
since I wanted to avoid alignment considerations if I can. I think
always indirecting through a pointer will have less risk of
regressions in a realistic setting than for x86 since Arm chips
typically have low latency for carryless multiplication instructions.
With just a bit of code we can still use the direct call for small
constant inputs, so I did that to avoid regressions under WAL insert
lock.

- One coding idiom for a vector literal in the generated code was
giving pgindent indigestion, I so rewrote it using Neon intrinsics and
verified it in Godbolt.

> 0002: Like 3c6e8c12389 and in fact uses the same program to generate
> the code, by specifying Neon instructions with the Arm "crypto"
> extension instead. There are some interesting differences from x86
> here as well:
> - The upstream implementation chose to use inline assembly instead of
> intrinsics for some reason. I initially thought that was a way to get
> broader compiler support, but it turns out you still need to pass the
> relevant flags to get the assembly to link.

To follow-up for curiosity's sake, [1] says that Apple chips can issue
PMULL + EOR as a single uop if they are next to each other in the
instruction stream.

> - I only have Meson support for now, since I used MacOS on CI to test.
> That OS and compiler combination apparently targets the CRC extension,
> but the PMULL instruction runtime check uses Linux-only headers, I
> believe, so previously I hacked the choose function to return true for
> testing. The choose function in 0002 is untested in this form.

This is still true, but now the CI hack lives in a separate
not-for-commit patch for clarity.

autoconf support is a WIP, and I will share that after I do some
testing on an Arm Linux instance.

[1] https://dougallj.github.io/applecpu/firestorm.html

--
John Naylor
Amazon Web Services