Re: [RFC PATCH for-10.0] include/hw/boards: Optimize the booleans in MachineClass

[Daniel P . Berrangé]

On Mon, Nov 25, 2024 at 12:47:33PM +0100, Philippe Mathieu-Daudé wrote:
> On 25/11/24 12:03, Daniel P. Berrangé wrote:
> > On Fri, Nov 22, 2024 at 09:49:23AM +0100, Thomas Huth wrote:
> > > While looking at the QEMU binary with "pahole", I noticed that we
> > > could optimize the size of MachineClass a little bit: So far we
> > > are using a mixture of a bitfield and single "bool" members here
> > > for the boolean flags. Declaring all flags as part of the bitfield
> > > helps to shrink the size of the struct a little bit.
> > > 
> > > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > > ---
> > >   It's just a micro-optimization (the size of the struct decreases by
> > >   eight bytes), so I'm not sure whether it's worth the effort...?
> > 
> > Given that this is a QOM class, rather than an instance, we'll
> > only ever save memory once. That's an unmeasurably small real
> > world improvement. We also have no ABI reasons to use bitfields
> > for this.
> > 
> > So if anything I'd suggest we take the opposite approach, and
> > eliminate that bitfields in favour of just using 'bool' for
> > everything, on the basis that a bitfield has no reason to
> > exist.
> 
> While I agree with you on this, and the patch would take less
> than 1 minute, all of these fields are legacy options. Maybe
> a good opportunity to tackle this technical debt.
> 
> I.e. as of 2024 having to set no_parallel/no_floppy/no_cdrom
> to true to every new machines seems counter intuitive.

We don't set them for all machines though, they are all set
selectively, and this is machine ABI controlled. So we could
only remove this flags, once all versioned machines that have
them active, are deleted.

With regards,
Daniel
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