Update Atomics.rst

[morisset]

Summary

I changed various bits of the compilation of atomics recently, and forgot
updating the documentation. This patch just brings it up to date.

[silvas]

Go ahead and commit the 0x->11 changes. I can't review the content changes but since you're the one making the corresponding code changes (I think?), I assume you would know best what is correct, so IMO go ahead and commit them.

[morisset]

Closed by commit rL218937 (authored by @morisset).

docs/Atomics.rst

 rough semantics in the presence of concurrency.  However, this is changing;
 there are now new instructions which are well-defined in the presence of threads
 and asynchronous signals, and the model for existing instructions has been
 clarified in the IR.
 
 The atomic instructions are designed specifically to provide readable IR and
 optimized code generation for the following:
 
-* The new C++0x ``<atomic>`` header.  (`C++0x draft available here
-  <http://www.open-std.org/jtc1/sc22/wg21/>`_.) (`C1x draft available here
+* The new C++11 ``<atomic>`` header.  (`C++11 draft available here
+  <http://www.open-std.org/jtc1/sc22/wg21/>`_.) (`C11 draft available here
   <http://www.open-std.org/jtc1/sc22/wg14/>`_.)
 
 * Proper semantics for Java-style memory, for both ``volatile`` and regular
   shared variables. (`Java Specification
   <http://docs.oracle.com/javase/specs/jls/se8/html/jls-17.html>`_)
 
 * gcc-compatible ``__sync_*`` builtins. (`Description
   <https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html>`_)

 
 ``cmpxchg`` and ``atomicrmw`` are essentially like an atomic load followed by an
 atomic store (where the store is conditional for ``cmpxchg``), but no other
 memory operation can happen on any thread between the load and store.
 
 A ``fence`` provides Acquire and/or Release ordering which is not part of
 another operation; it is normally used along with Monotonic memory operations.
 A Monotonic load followed by an Acquire fence is roughly equivalent to an
-Acquire load.
+Acquire load, and a Monotonic store following a Release fence is roughly
+equivalent to a Release store. SequentiallyConsistent fences behave as both
+an Acquire and a Release fence, and offer some additional complicated
+guarantees, see the C++11 standard for details.
 
 Frontends generating atomic instructions generally need to be aware of the
 target to some degree; atomic instructions are guaranteed to be lock-free, and
 therefore an instruction which is wider than the target natively supports can be
 impossible to generate.
 
 .. _Atomic orderings:
 

 ---------
 
 Monotonic is the weakest level of atomicity that can be used in synchronization
 primitives, although it does not provide any general synchronization. It
 essentially guarantees that if you take all the operations affecting a specific
 address, a consistent ordering exists.
 
 Relevant standard
-  This corresponds to the C++0x/C1x ``memory_order_relaxed``; see those
+  This corresponds to the C++11/C11 ``memory_order_relaxed``; see those
   standards for the exact definition.
 
 Notes for frontends
   If you are writing a frontend which uses this directly, use with caution.  The
   guarantees in terms of synchronization are very weak, so make sure these are
   only used in a pattern which you know is correct.  Generally, these would
   either be used for atomic operations which do not protect other memory (like
   an atomic counter), or along with a ``fence``.

 
 Acquire
 -------
 
 Acquire provides a barrier of the sort necessary to acquire a lock to access
 other memory with normal loads and stores.
 
 Relevant standard
-  This corresponds to the C++0x/C1x ``memory_order_acquire``. It should also be
-  used for C++0x/C1x ``memory_order_consume``.
+  This corresponds to the C++11/C11 ``memory_order_acquire``. It should also be
+  used for C++11/C11 ``memory_order_consume``.
 
 Notes for frontends
   If you are writing a frontend which uses this directly, use with caution.
   Acquire only provides a semantic guarantee when paired with a Release
   operation.
 
 Notes for optimizers
   Optimizers not aware of atomics can treat this like a nothrow call.  It is

 
 Release
 -------
 
 Release is similar to Acquire, but with a barrier of the sort necessary to
 release a lock.
 
 Relevant standard
-  This corresponds to the C++0x/C1x ``memory_order_release``.
+  This corresponds to the C++11/C11 ``memory_order_release``.
 
 Notes for frontends
   If you are writing a frontend which uses this directly, use with caution.
   Release only provides a semantic guarantee when paired with a Acquire
   operation.
 
 Notes for optimizers
   Optimizers not aware of atomics can treat this like a nothrow call.  It is

 
 AcquireRelease
 --------------
 
 AcquireRelease (``acq_rel`` in IR) provides both an Acquire and a Release
 barrier (for fences and operations which both read and write memory).
 
 Relevant standard
-  This corresponds to the C++0x/C1x ``memory_order_acq_rel``.
+  This corresponds to the C++11/C11 ``memory_order_acq_rel``.
 
 Notes for frontends
   If you are writing a frontend which uses this directly, use with caution.
   Acquire only provides a semantic guarantee when paired with a Release
   operation, and vice versa.
 
 Notes for optimizers
   In general, optimizers should treat this like a nothrow call; the possible
   optimizations are usually not interesting.
 
 Notes for code generation
   This operation has Acquire and Release semantics; see the sections on Acquire
   and Release.
 
 SequentiallyConsistent
 ----------------------
 
 SequentiallyConsistent (``seq_cst`` in IR) provides Acquire semantics for loads
 and Release semantics for stores. Additionally, it guarantees that a total
 ordering exists between all SequentiallyConsistent operations.
 
 Relevant standard
-  This corresponds to the C++0x/C1x ``memory_order_seq_cst``, Java volatile, and
+  This corresponds to the C++11/C11 ``memory_order_seq_cst``, Java volatile, and
   the gcc-compatible ``__sync_*`` builtins which do not specify otherwise.
 
 Notes for frontends
   If a frontend is exposing atomic operations, these are much easier to reason
   about for the programmer than other kinds of operations, and using them is
   generally a practical performance tradeoff.
 
 Notes for optimizers

 * ``isUnordered()``: A load or store which is not volatile and at most
   Unordered. This would be checked, for example, by LICM before hoisting an
   operation.
 
 * ``mayReadFromMemory()``/``mayWriteToMemory()``: Existing predicate, but note
   that they return true for any operation which is volatile or at least
   Monotonic.
 
+* ``isAtLeastAcquire()``/``isAtLeastRelease()``: These are predicates on
+  orderings. They can be useful for passes that are aware of atomics, for
+  example to do DSE across a single atomic access, but not across a
+  release-acquire pair (see MemoryDependencyAnalysis for an example of this)
+
 * Alias analysis: Note that AA will return ModRef for anything Acquire or
   Release, and for the address accessed by any Monotonic operation.
 
 To support optimizing around atomic operations, make sure you are using the
 right predicates; everything should work if that is done.  If your pass should
 optimize some atomic operations (Unordered operations in particular), make sure
 it doesn't replace an atomic load or store with a non-atomic operation.
 
 Some examples of how optimizations interact with various kinds of atomic
 operations:
 
 * ``memcpyopt``: An atomic operation cannot be optimized into part of a
   memcpy/memset, including unordered loads/stores.  It can pull operations
   across some atomic operations.
 
 * LICM: Unordered loads/stores can be moved out of a loop.  It just treats
   monotonic operations like a read+write to a memory location, and anything
   stricter than that like a nothrow call.
 
 * DSE: Unordered stores can be DSE'ed like normal stores.  Monotonic stores can
   be DSE'ed in some cases, but it's tricky to reason about, and not especially
-  important.
+  important. It is possible in some case for DSE to operate across a stronger
+  atomic operation, but it is fairly tricky. DSE delegates this reasoning to
+  MemoryDependencyAnalysis (which is also used by other passes like GVN).
 
 * Folding a load: Any atomic load from a constant global can be constant-folded,
   because it cannot be observed.  Similar reasoning allows scalarrepl with
   atomic loads and stores.
 
 Atomics and Codegen
 ===================
 
 Atomic operations are represented in the SelectionDAG with ``ATOMIC_*`` opcodes.
 On architectures which use barrier instructions for all atomic ordering (like
-ARM), appropriate fences are split out as the DAG is built.
+ARM), appropriate fences can be emitted by the AtomicExpand Codegen pass if
+``setInsertFencesForAtomic()`` was used.
 
 The MachineMemOperand for all atomic operations is currently marked as volatile;
 this is not correct in the IR sense of volatile, but CodeGen handles anything
 marked volatile very conservatively.  This should get fixed at some point.
 
 Common architectures have some way of representing at least a pointer-sized
 lock-free ``cmpxchg``; such an operation can be used to implement all the other
 atomic operations which can be represented in IR up to that size.  Backends are
 expected to implement all those operations, but not operations which cannot be
 implemented in a lock-free manner.  It is expected that backends will give an
 error when given an operation which cannot be implemented.  (The LLVM code
 generator is not very helpful here at the moment, but hopefully that will
 change.)
 
-The implementation of atomics on LL/SC architectures (like ARM) is currently a
-bit of a mess; there is a lot of copy-pasted code across targets, and the
-representation is relatively unsuited to optimization (it would be nice to be
-able to optimize loops involving cmpxchg etc.).
-
 On x86, all atomic loads generate a ``MOV``. SequentiallyConsistent stores
 generate an ``XCHG``, other stores generate a ``MOV``. SequentiallyConsistent
 fences generate an ``MFENCE``, other fences do not cause any code to be
 generated.  cmpxchg uses the ``LOCK CMPXCHG`` instruction.  ``atomicrmw xchg``
 uses ``XCHG``, ``atomicrmw add`` and ``atomicrmw sub`` use ``XADD``, and all
 other ``atomicrmw`` operations generate a loop with ``LOCK CMPXCHG``.  Depending
 on the users of the result, some ``atomicrmw`` operations can be translated into
 operations like ``LOCK AND``, but that does not work in general.
 
 On ARM (before v8), MIPS, and many other RISC architectures, Acquire, Release,
 and SequentiallyConsistent semantics require barrier instructions for every such
 operation. Loads and stores generate normal instructions.  ``cmpxchg`` and
 ``atomicrmw`` can be represented using a loop with LL/SC-style instructions
 which take some sort of exclusive lock on a cache line (``LDREX`` and ``STREX``
 on ARM, etc.).
+
+It is often easiest for backends to use AtomicExpandPass to lower some of the
+atomic constructs. Here are some lowerings it can do:
+* cmpxchg -> loop with load-linked/store-conditional
+  by overriding ``hasLoadLinkedStoreConditional()``, ``emitLoadLinked()``,
+  ``emitStoreConditional()``
+* large loads/stores -> ll-sc/cmpxchg
+  by overriding ``shouldExpandAtomicStoreInIR()``/``shouldExpandAtomicLoadInIR()``
+* strong atomic accesses -> monotonic accesses + fences
+  by using ``setInsertFencesForAtomic()`` and overriding ``emitLeadingFence()``
+  and ``emitTrailingFence()``
+* atomic rmw -> loop with cmpxchg or load-linked/store-conditional
+  by overriding ``expandAtomicRMWInIR()``
+For an example of all of these, look at the ARM backend.