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

[Leonardo Bras Soares Passos]

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 ?
I mean, if performance-wise they should be the same, then a new
variable would be just a
bother for the programmer.

Best regards,
Leo






>
> With regards,
> Daniel
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