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Date: Tue, 6 Dec 2022 16:01:26 +0100
From: Wolfgang Bumiller <w.bumiller@proxmox.com>
To: Lukas Wagner <l.wagner@proxmox.com>
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Subject: Re: [pbs-devel] [PATCH v2 proxmox-backup 2/3] debug cli: add
 'compare-content' flag to `diff archive` command
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On Tue, Dec 06, 2022 at 03:44:07PM +0100, Lukas Wagner wrote:
> Hi,
> 
> On 12/6/22 12:39, Wolfgang Bumiller wrote:
> > > +{
> > > +    let mut buf_a = vec![0u8; BUFFERSIZE];
> > > +    let mut buf_b = vec![0u8; BUFFERSIZE];
> > 
> > Maybe use a Box instead of a Vec, we don't ever resize this atm.
> > 
> 
> Do you mean like this?
>   let mut buf_a = Box::new([0u8; BUFFERSIZE]);
> 
> AFAIU this would construct the array on the stack first before it is moved into the Box,
> leading to some unnecessary memcpys. Or am I missing something here?

With these sizes - and this is only on the CLI AFAICT - this is not an
issue, and in --release builds this will be optimized out.
I prefer not to do things weirdly because "the compiler does stupid
things" if it can be avoided. (Some kind of "placement-new"-like
construct is *really* missing in rust...)
And none of `Box::new_zeroed*`, `Box::new_uninit*` are stable yet, and
even if they were, there's no ergonomic POD version that doesn't imply a
`MaybeUninit`.

Perhaps we should start a proxmox_bytes crate where we move
proxmox_io::vec to, and add a box module for box::zeroed() and
box::uninit()...