Re: [PATCH v5 11/14] software node: move small properties inline when copying

Andy Shevchenko
2019-10-16T07:49:02Z

On Tue, Oct 15, 2019 at 11:25:53AM -0700, Dmitry Torokhov wrote:
> On Tue, Oct 15, 2019 at 03:20:28PM +0300, Andy Shevchenko wrote:
> > On Fri, Oct 11, 2019 at 04:07:18PM -0700, Dmitry Torokhov wrote:
> > > When copying/duplicating set of properties, move smaller properties that
> > > were stored separately directly inside property entry structures. We can
> > > move:
> > >
> > > - up to 8 bytes from U8 arrays
> > > - up to 4 words
> > > - up to 2 double words
> > > - one U64 value
> > > - one or 2 strings.
> >
> > Can you show where you extract such values?
>
> the "value" union's largest member is u64, which is 8 bytes. Strings are
> pointers, so on 32-bit arches you can stuff 2 pointers into 8 bytes,
> while on 64-bits you have space for only one.
>
> >
> > > + if (!dst->is_inline && dst->length <= sizeof(dst->value)) {
> > > + /* We have an opportunity to move the data inline */
> > > + const void *tmp = dst->pointer;
> > > +
> >
> > > + memcpy(&dst->value, tmp, dst->length);
> >
> > ...because this is strange trick.
>
> Not sure what is so strange about it. You just take data that is stored
> separately and move it into the structure, provided that it is not too
> big (i.e. it does not exceed sizeof(value union) size).

You store a value as union, but going to read as a member of union?
I'm pretty sure it breaks standard rules.

--
With Best Regards,
Andy Shevchenko