Valhalla EG notes April 10, 2019

[John Rose]

On Apr 18, 2019, at 5:09 PM, forax at univ-mlv.fr wrote:
> 
> I'm not sure we need a 'G' because NV is a property of the container too.

The flattened layout is not a visible property of the container,
if the container is typed using the legacy L-descxriptor.
If the container is flattened, you need an extra signal ("L*-NV"
not "L-NV").  (It took us four days last month to realize
this and to discard a long list of workarounds.)

The L*-NV descriptor (perhaps a descriptor augmented by a
load-it-now side channel that carries the starts) gives the JVM
permission to do the following extra steps.

1. Load NV.class when it lays out the container (of type L*-NV).
1a. Execute any class loader side effects due to that loading.
2. Throw a class circularity error if loading NV.class needs to lay out the container recursively.

We generally agree to disregard effects in 1a.  But the effect of
2 is a real departure from the standard contract of L-descriptors,
and it needs a new contract.

You could separate the "*" from "L*-NV" and instead put all
the stars into a PreloadClasses attribute, as we once discussed.
This is (a) too fragile to ensure robust flattening, and (b) not
fine-grained enough, since some occurrences of "L*-NV"
need the star, and others need to *omit* the star.

This is why we are moving towards keeping the star in the
descriptor, in effect.  Physically, "L*" should be spelled with
a single letter, of course; the working title was "Q" but is
now "G" (meaning "go and look", since it applies to both
inlines and templates).