Remove rust_atomic_increment/decrement and rust_compare_and_swap_ptr

[brson]

There are intrinsics for this purpose now.

[vertexclique]

How to remove usage of rust_atomic_increment in src/libcore/private.rs line 455 ?

let new_count = rustrt::rust_atomic_increment(&mut ptr.count);

[brson]

Here's the list of intrinsics: https://github.com/mozilla/rust/blob/incoming/src/rustc/middle/trans/foreign.rs#L800

And here are the ones used by core::pipes: https://github.com/mozilla/rust/blob/incoming/src/libcore/pipes.rs#L240

rust_atomic_increment needs to be replaced either with atomic_xadd, atomic_xadd_acq or atomic_xadd_rel. Probably atomic_xadd is the safest unless you can determine that one of the other two is appropriate. I'm not familiar with the differences.

[brson]

@bblum will know for sure

[bblum]

atomic_xadd_acq would be plenty safe in the clone_shared_mutable_state case.

The suffixes denote whether the instruction is intended to have "acquire" or "release" semantics, which means that on the processor (or maybe compiler instruction-selection?) level, they might be reordered to be earlier (for acquire) or later (for release). Using atomic_xadd is always safer than ..._acq or ..._rel, which are only for potential performance.