[foreign-memaccess] RFR 8223978: Add alignment support to layouts

[Maurizio Cimadamore]

Hi,
this patch adds support to the API to express alignment constraints on 
layouts. Following the model John put forward in [1], I did the following:

* there's a way to compute the 'natural alignment' of a layout

* if no alignment is specified, alignment is the natural alignment

* you can align a layout using Layout::alignTo(long) - this sets the 
additional constraint (provided is power of two, non-negative, and >=8)

* When we obtain a VarHandle out of a Layout, at that point we are in 
the process of computing offsets; here we can catch e.g. if the offset 
of a given path doesn't match the specified alignment - in this case we 
fail fast, even before memory is accessed; this occurs e.g. if the 
layout path offset is not a multiple of its alignment, or if the 
alignment of the nested element is stricter than the one of the 
enclosing element.

(in principle we could enforce these checks on layout creation, but 
given we have unresolved layouts in our radar, I think it's best to do 
minimal checks on layout creation and let the check fully kick in on 
path creation).

* If VarHandle can be constructed, a dynamic check verifies that 
alignment of address passed to VH matches the layout requirements

* MemoryScope::allocate also honors the alignment requirements - this 
works by up-allocating memory (to make sure a pointer with desired 
alignment exists in the allocated area) and then adjusting it after the 
fact. For alignments < 16bytes, nothing is done given that malloc, at 
least on x64 is guaranteed to respect that.


I think this is powerful and yet relatively simple to understand, 
overall I quite like it.

Webrev:
http://cr.openjdk.java.net/~mcimadamore/panama/8223978/

Maurizio