Should fully stick to C++11 memory model

Created by: wks

Currently the memory ordering primitives are copied from LLVM. As stated by LLVM langref, their memory model is not precisely defined.

These semantics (ordering) are borrowed from Java and C++0x, but are somewhat more colloquial. If these descriptions aren’t precise enough, check those specs (see spec references in the atomics guide).

But the MicroVM should have a precise memory model. The C++11 memory model is the result of a lot of effort and we should stick to it.

Things should be done in the MicroVM:

• Remove the "UNORDERED" memory order. It is designed by LLVM for Java, but the MicroVM will treat data race as undefined behaviour and will leave the security constraints of Java to the client.
  • If JVM can be implemented in C/C++, its memory model should also be implementable on a MicroVM with C++11-like memory model.
• Add "CONSUME" memory order. Define the "carries-a-data-dependency-to" relation in MicroVM. Some hardwares are aware of dependency.
• Define the program order (which is a total order per MicroVM thread because MicroVM does not have unspecified parameter evaluation order), the synchronisation order, the synchronises-with and the happens-before relations.