Re: volk and alignment

[thomas]

On Wed, 8 Jul 2020 18:09:30 +0100, "Marcus Müller" <mueller@kit.edu> said:
>  > Is there a maximum size that volk_get_alignment could return, a size
>  > that's reasonable?
>
> I'd go with "realistically, yes, but isn't relying on that a bad idea?".

Yes, it does sound like a bad idea. :-)
Really I'm looking to solve the problem, not a specific solution.

> I'm thinking back and forth about how to address that problem.
> Basically, what we'd need is a "worst case of all available machines"
> alignment, that is present in an integer constant expression, so you can
> put it into alignas(), right?

Right.

It's not my field, but surely 4kiB will align everything? On the other
hand, of course, stepping into a new page may incur a page fault,
which could be more than even using `volk::vector` which may incur an
allocation, but usually won't incur a context switch.

I suppose a mere dynamic stack alloc would do just fine:

```
char buf[alignment+bytes_needed];
int adjust = (aligned - (buf % alignment)) % aligned;
char* p = buf + adjust;
```

(except making sure that the pointer arithmetic doesn't cause UB. Off
the top of my head I don't know the right types to use)

Not very nice with two mod ops per time this is needed, though. For
the PR linked to this would happen every sample.

Another option is a thread-local stack, which would make
allocs/deallocs very cheap. Assuming all use cases of this would be
for local variables.

--
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