[foreign-memaccess] RFR 8223712: Investigate more fluent LayoutPath API

[Jorn Vernee]

Hi,

I think this is a nice move!

One minor nit: In LayoutPathImpl::sequenceElement there is an 
unspecified throw of IllegalStateException. Should probably be a UOE 
instead?

There's also still a bit of a split between Sequence and Group. For 
instance, it is not possible to create a LayoutPath to an exact element 
of a sequence, and there is no way to make a LayoutPath to a group, 
where you would pass in the name/index of the element you want to access 
to the VarHandle later. Maybe it could be nice to add support for these 
missing cases?

Like you, I'm also a bit skeptical about the inheritance relationship 
between Sequence and Group. e.g. if you look at 
LayoutPathImpl::lookupGroup, any Group is a valid argument, except if 
it's a Sequence. This seems to violate LSP. I think it might be better 
to have Sequence inherit from AbstractLayout directly instead?

Cheers,
Jorn

Maurizio Cimadamore schreef op 2019-05-10 17:24:
> Hi,
> after some discussion with Brian, here's an attempt to make the
> LayoutPath lookup API simpler, by removing the Stream-returning lookup
> method.
> 
> Now, you just ask a path out of a layout (the path you obtain is
> rooted there), and then you proceed from there calling various
> builder-like methods - example:
> 
> Sequence seq = Sequence.of(20,
>                 Sequence.of(10,
>                       Group.struct(
>                             Padding.of(32),
>                             Value.ofUnsignedInt(32).withName("elem")
>                       )
>                 ));
> 
> 
> seq.toPath()
>                 .sequenceElement()
>                 .sequenceElement()
>                 .groupElement("elem")
> 
> 
> I've also added support for 'by-index' lookups, so the above can also
> be written by this:
> 
> seq.toPath()
>                 .sequenceElement()
>                 .sequenceElement()
>                 .groupElement(1)
> 
> 
> Webrev:
> 
> http://cr.openjdk.java.net/~mcimadamore/panama/8223712/
> 
> 
> As a side-bar, I'm growing more and more skeptical about the
> relationship between Sequence and Group; there are sequences that
> cannot be represented as groups:
> 
> * if sequence has more than 2^32 elements (in which case we cannot
> create a big enough group for it)
> * if sequence has an unknown arity (which we know in the case of
> native interop can happen)
> 
> The latter case is important - when you have a pointer, languages are
> typically very loose as to whether you point to ONE element of that
> kind, or MANY. It would be nice to be able to model these different
> cases with different layouts.
> 
> Maurizio