AMDGPU: Preliminary documentation for named barriers

Author: nhaehnle

llvm/docs/AMDGPUUsage.rst

@@ -1179,6 +1179,53 @@ is conservatively correct for OpenCL.
                              other operations within the same address space.
      ======================= ===================================================
 
+Target Types
+------------
+
+The AMDGPU backend implements some target extension types.
+
+.. _amdgpu-types-named-barriers:
+
+Named Barriers
+~~~~~~~~~~~~~~
+
+Named barriers are represented as memory objects of type
+``target("amdgcn.named.barrier", 0)``. They are allocated as global variables
+in the LDS address space. They do not occupy regular LDS memory, but their
+lifetime and allocation granularity matches that of global variables in LDS.
+
+The following types built from named barriers are supported in global variables,
+defined recursively:
+
+* a standalone ``target("amdgcn.named.barrier", 0)``
+* an array of supported types
+* a struct containing a single element of supported type
+
+.. code-block:: llvm
+
+      @bar = addrspace(3) global target("amdgcn.named.barrier", 0) undef
+      @foo = addrspace(3) global [2 x target("amdgcn.named.barrier", 0)] undef
+      @baz = addrspace(3) global { target("amdgcn.named.barrier", 0) } undef
+
+Barrier types may not be used in ``alloca``.
+
+The integral representation of a pointer to a valid named barrier is in the
+range ``0x0080'0010`` to ``0x0080'0100`` (inclusive). The representation is
+formed by the expression ``0x0080'0000 | (id << 4)``, where ``id`` is the
+hardware barrier ID. The integral representation of the null named barrier is
+``0x0080'0000``.
+
+It is not legal to attempt to form a pointer to any non-named barrier objects.
+
+It is undefined behavior to use a pointer to any part of a named barrier object
+as the pointer operand of a regular memory access instruction or intrinsic.
+Pointers to named barrier objects are intended to be used with dedicated
+intrinsics.
+
+We expand on the semantics of named barriers in
+:ref:`the memory model section <amdgpu-memory-model-named-barriers>`.
+
+
 LLVM IR Intrinsics
 ------------------
 
@@ -6621,6 +6668,138 @@ Multiple tags can be used at the same time to synchronize with more than one add
   better code optimization, at the cost of synchronizing additional address
   spaces.
 
+.. _amdgpu-memory-model-barriers:
+
+Hardware Barriers
++++++++++++++++++
+
+.. note::
+
+  This section is preliminary. The semantics described here are intended to be
+  formalized properly in the future.
+
+Hardware barriers synchronize execution between concurrently running waves using
+fixed function hardware. Intuitively, a set of waves are "members" of a barrier.
+Waves *signal* the barrier and later *wait* for it. Execution only proceeds past
+the *wait* once all member waves have *signaled* the barrier.
+
+Formally, barriers affect semantics in exactly two ways. First, they affect
+forward progress. Waiting on a barrier that never completes (is not signaled
+sufficiently) prevents forward progress and therefore, given the assumption of
+forward progress, is undefined behavior. Second, barrier operations can pair
+with fences to contribute *synchronizes-with* relations in the memory model.
+
+Roughly speaking:
+
+- Release fences pair with barrier signal operations that are later in program
+  order
+- Barrier wait operations pair with acquire fences that are later in program
+  order
+- If a barrier signal operation contributes to allowing a wait operation to
+  complete, then the corresponding paired fences can synchronize-with each
+  other (given compatible sync scopes and memory model relaxation annotations)
+
+Default Barriers
+################
+
+There is a default workgroup barrier and a default cluster barrier. All waves
+of a workgroup and cluster are members of the same default workgroup and
+cluster barriers, respectively.
+
+.. _amdgpu-memory-model-named-barriers:
+
+Named Barriers
+##############
+
+All named barrier operations must occur in wave-uniform control flow. All
+arguments of named barrier intrinsics must be wave-uniform.
+
+Named barriers are allocated as global variables of
+:ref:`a target extension type <amdgpu-types-named-barriers>`.
+
+Named barriers may be signaled by the intrinsics:
+
+.. code-block:: llvm
+
+  declare void @llvm.amdgcn.s.barrier.signal(i32 %barrier_hw_id)
+  declare void @llvm.amdgcn.s.barrier.signal.var(ptr addrspace(3) %barrier_ptr, i32 %member_count)
+
+If the second form is used and ``member_count`` is non-zero, the operation is
+an *initializing* signal, else it is *non*-initializing.
+
+Named barriers may be initialized explicitly using:
+
+.. code-block:: llvm
+
+  declare void @llvm.amdgcn.s.barrier.init(ptr addrspace(3) %barrier_ptr, i32 %member_count)
+
+It is possible to "leave" a named barrier. This decrements the named barrier's
+member count and completes the barrier if all other members have signaled it:
+
+.. code-block:: llvm
+
+  declare void @llvm.amdgcn.s.barrier.leave(i32 %barrier_type)
+
+``barrier_type`` must be set to ``1``.
+
+Note that leaving a named barrier is not exactly the opposite of joining a
+barrier (for example, joining a barrier does not change its member count).
+
+Leaving implicitly *joins* (see below) a null named barrier.
+
+Signal, leave, and initializing operations on the same named barrier must obey
+certain ordering constraints:
+
+* Non-initializing signals must be ordered after some initializing signal or an
+  explicit initializing operation.
+* Explicit initializing operations must not race signal or leave operations.
+* Initializing signal operations must not race leave operations.
+* Initializing signal operations with contradicting member counts must not race
+  each other.
+
+The details of how these orders can be established and races prevented are tbd.
+Using a default workgroup or cluster barrier in the natural way is guaranteed to
+be sufficient.
+
+In order to wait for a named barrier, a wave must first *join* the named barrier
+using:
+
+.. code-block:: llvm
+
+  declare void @llvm.amdgcn.s.barrier.join(ptr addrspace(3) %barrier_ptr)
+
+The named barrier may then be waited for using:
+
+.. code-block:: llvm
+
+  declare void @llvm.amdgcn.s.barrier.wait(i32 %barrier_type)
+
+... with ``barrier_type`` set to ``1``.
+
+Signal, leave, join, and wait operations must obey certain ordering constraints.
+The details are tbd. Satisfying the following rules is guaranteed to be
+sufficient:
+
+* Signal or wait for a named barrier only if it is the most recent to have been
+  joined in program order.
+* Signal or leave a named barrier only if the number of prior signaling
+  operations on that named barrier since the most recent join in program order
+  is equal to the number of prior wait operations on that named barrier since
+  the most recent join in program order.
+* Wait for a named barrier only if the number of prior signaling operations on
+  that named barrier since the most recent join in program order is one larger
+  than the number of prior wait operations on that named barrier since the most
+  recent join in program order.
+* Do not signal a named barrier or wait for it in program order after leaving it.
+
+Additionally, use signal, leave, and wait operations on a named barrier from a
+consistent associated set of waves that is determined at initialization time and
+whose initial size is the member count used at initialization. The set of waves
+may shrink with leave operations. Operations on a named barrier object with
+conflicting sets of waves must not race. The details of this rule and how an
+ordering can be established to prevent a race is tbd. Using a default workgroup
+or cluster barrier in the natural way is guaranteed to be sufficient.
+
 .. _amdgpu-amdhsa-memory-model-gfx6-gfx9:
 
 Memory Model GFX6-GFX9