WIP: Memory Model

[nspark]

This is a very WIP draft for changes to define a memory model. It is in no way complete and almost certainly incorrect in the bit it attempts to specify. But, I invite and welcome all your feedback (You can see the remains of many previous attempts as commented-out LaTeX.)

The style of wording is heavily based on C11/C17 and cppreference.com (e.g., atomic_thread_fence).

Part of the approach here is to add the notion of PE-happens-before and PE-synchronizes-with, rather than overload/extend C's definitions of happens-before and synchronizes-with. What has so far made this a complicated endeavor (both in wording and correctness) is that OpenSHMEM has blocking and nonblocking operations, fence vs. quiet.

[nspark]

Most of what's currently drafted... (see bullet 7 for one challenging area)

[nspark]

Some feedback points from 10/4:

• shmem_fence is really just a release fence; it doesn't order nonblocking gets and fetching AMOs. A standalone acquire fence is likely necessary for completeness.
• shmem_fence is a release fence for both blocking and nonblocking operations.
• Examples or litmus tests are likely to be very helpful and illustrative.
• Likely needs a complete definition of transitivity for PE-synchronizes-with.