[PATCH] Implement low-level ARM ldrex/strex intrinsics

[Tim Northover]

Hi Eli,

> The obvious objection is that the definition of these intrinsics is
> basically "this intrinsic has undefined behavior".

I think that's rather pessimistic, though I can see how my warning
could have given that impression.

In practice I believe CPUs implement the exclusive monitors at the
level of the cache line, and "unrelated" loads and stores are not
allowed to interfere by the specification. On a CPU like this, the
only way LLVM could interfere would be with a spill to a stack slot on
the same cache line as the atomic variable being accessed.

I think that leaves a large segment of uses which are guaranteed to
work: put your ldrexed variables somewhere other than the stack and
either make sure they're alone on their cache line or don't store to
any others nearby between the ldrex and strex.

Navigating that space might be tricky, but given that the alternative
in use now is inline assembly, we can probably assume some degree of
competence for the users of these intrinsics.

Also, these already exist in other compilers (and ours!) which make no
more guarantees than we do as far as I'm aware. That doesn't
necessarily mean we should follow, but it might suggest the issues are
surmountable.

Cheers.

Tim.