Generic atomic

text/0000-generic-atomic.md

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+- Feature Name:
+- Start Date: 2016-01-22
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Add compiler support for generic atomic operations.
+
+# Motivation
+[motivation]: #motivation
+
+It is sometimes necessary or useful to use atomic operations in a generic
+context.
+
+```rust
+Atomic<T>
+```
+
+where `T` is any type. This is currently possible but fails to compile if `T` is
+too large for the native atomic operations of the target architecture.
+
+# Detailed design
+[design]: #detailed-design
+
+Four language items are added:
+
+```rust
+#[lang = "atomic_load"]
+fn atomic_load<T>(ptr: *const T, ordering: ...) -> T { ... }
+
+#[lang = "atomic_store"]
+fn atomic_store<T>(ptr: *const T, val: T, ordering: ...) { ... }
+
+#[lang = "atomic_exchange"]
+fn atomic_exchange<T>(ptr: *const T, val: T, ordering: ...) -> T { ... }
+
+#[lang = "atomic_compare_exchange"]
+fn atomic_compare_exchange<T>(ptr: *const T, old: &T, new: T, ordering: ...) -> T { ... }
+```
+
+When a call to one of the corresponding atomic intrinsics cannot be translated
+to a native operation, it is translated to a call to the corresponding language
+item.
+
+`atomic_compare_exchange` performs a bitwise comparison of the current and the
+expected value.
+
+One intrinsic is added:
+
+```rust
+fn has_native_atomic_ops<T>() -> bool;
+```
+
+This intrinsic returns `false` iff a call to one of the aforementioned atomic
+operations is translated to a library call.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+`atomic_compare_exchange` performs a bitwise comparison of the current and the
+expected value. This must be so because atomic operations for `T: PartialEq`
+where `T` has a native operation available will be translated to an operation
+that performs a bitwise comparison.
+
+# Alternatives
+[alternatives]: #alternatives
+
+None.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.