[lworld] RFR: 8371199: [lworld] Flattening of nullable elements of classes similar to j.l.Long [v4]

[Quan Anh Mai]

> Hi,
> 
> Currently, the layout of a nullable `j.l.Long`, if flattened, must be at least 65 bits. This exceeds the maximum size a GPR can generally hold, as well as the default object alignment of Hotspot. As a result, accessing such elements atomically require 128-bit atomic accesses, as well as mechanism from the GC to allocate overaligned objects. And even then, we will encounter the same issue, just with larger objects.
> 
> We can observe that, a nullable element does not really have an additional bit of information, it just has an additional state (e.g. a nullable `j.l.Long` has `2^64 + 1` states, not `2^65`), and it is just unfortunate that we need a whole bit to be able to represent such an element.  However, we can rely on the fact that all payload bits are irrelevant when the null marker is 0 to make a sequence of more than 1 memory access instructions look atomic.
> 
> C1 has not been implemented yet, we will bailout the compilation when encountering such an access. I will implement this functionality later.
> 
> Please take a look and leave your reviews, thanks a lot.

Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:

  missing barrier

-------------

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1720/files
  - new: https://git.openjdk.org/valhalla/pull/1720/files/e80107b7..217c3670

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1720&range=03
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1720&range=02-03

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1720.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1720/head:pull/1720

PR: https://git.openjdk.org/valhalla/pull/1720