Re: [PATCH v2 1/2] QIOChannelSocket: Reduce ifdefs to improve readability

[Daniel P . Berrangé]

On Mon, Jun 13, 2022 at 06:21:18PM -0300, Leonardo Bras Soares Passos wrote:
> On Fri, Jun 10, 2022 at 5:25 AM Daniel P. Berrangé <berrange@redhat.com> 
> wrote:
> >
> 
> [...]
> 
> > Ok, so if it is checked earlier then we merely need an assert.
> >
> >      if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> >  #ifdef QEMU_MSG_ZEROCOPY
> >          sflags = MSG_ZEROCOPY;
> >          zero_copy_enabled = true;
> >  #else
> >          g_assert_unreachable();
> >  #endif
> > >     }
> 
> Ok, I will add that in the next version.
> 
> >
> >
> >
> > > > > @@ -592,15 +594,13 @@ static ssize_t 
> > > > > qio_channel_socket_writev(QIOChannel *ioc,
> > > > >              return QIO_CHANNEL_ERR_BLOCK;
> > > > >          case EINTR:
> > > > >              goto retry;
> > > > > -#ifdef QEMU_MSG_ZEROCOPY
> > > > >          case ENOBUFS:
> > > > > -            if (sflags & MSG_ZEROCOPY) {
> > > > > +            if (zero_copy_enabled) {
> > > >
> > > > if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY)
> > > >
> > > > avoids the #ifdef without needing to add yet another
> > > > variable expressing what's already expressed in both
> > > > 'flags' and 'sflags'.
> > >
> > > Yes, it does, but at the cost of not compiling-out the zero-copy part
> > > when it's not supported,
> > > since the QIO_CHANNEL_WRITE_FLAG_ZERO_COPY comes as a parameter. This 
> > > ends up
> > > meaning there will be at least one extra test for every time this
> > > function is called (the one in the next patch).
> >
> > The cost of a simple bit test is between negligible-and-non-existant
> > with branch prediction. I doubt it would be possible to even measure
> > it.
> 
> Yeah, you are probably right on that.
> So the main learning point here is that it's not worth creating a new
> boolean for compiling-out
> code that should not impact performance ?

As ever "it depends" so there's no hard rule, and sometimes it can
verge on bikeshed colouring :-)

I didn't like the variable in this case, because it introduces a 3rd
variable to the method for representing whether zero copy is need,
which is excessive. I'm not a fan of redundancy as it can often then
lead to inconsistency. So it would need a compelling reason why it is
better, which is difficult for such a simple method. If the code was
more complex, a variable might have benefit of clarity, but in this
case IMHO it was just overkill.

With regards,
Daniel
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