[llvm] [AMDGPU][Doc] GFX12.5 Barrier Execution Model (PR #185632)
Pierre van Houtryve via llvm-commits
llvm-commits at lists.llvm.org
Wed Apr 1 01:37:12 PDT 2026
================
@@ -6950,121 +6975,233 @@ Informally, we can deduce from the above formal model that execution barriers be
* *Barrier-executes-before* relates the dynamic instances of operations from different threads together.
For example, if ``A -> B`` in *barrier-executes-before*, then the execution of ``A`` must complete
before the execution of ``B`` can complete.
-* When a barrier *signal* or *leave* causes the *signal count* of a barrier *object* to be identical to the
- *expected count*, the *signal count* is reset to zero, and threads that have *joined* the barrier *object*
- will:
- * Wake-up if they were sleeping because of a barrier *wait*, **or**
- * Skip the next barrier *wait* operation if they have not previously *waited*.
+ * This property can also be combined with *program-order*. For example, let two (non-barrier) operations
+ ``X`` and ``Y`` where ``X -> A`` and ``B -> Y`` in *program-order*, then we know that the execution
+ of ``X`` completes before the execution of ``Y`` does.
* Barriers do not complete "out-of-thin-air"; a barrier *wait* ``W`` cannot depend on a barrier operation
``X`` to complete if ``W -> X`` in *barrier-executes-before*.
-* It is undefined behavior to operate on an uninitialized barrier.
+* It is undefined behavior to operate on an uninitialized barrier object.
* It is undefined behavior for a barrier *wait* to never complete.
+* It is not mandatory to *drop* a barrier after *joining* it. The operations are not opposites; *drop*
+ affects future barrier operations by decrementing the *expected count* of the barrier *object*, which
+ can only be undone by re-*initializing* the barrier.
+* A thread may not *arrive* at then *drop* a barrier *object* unless the barrier completes before the
+ barrier *drop*. Incrementing the *signal count* and decrementing the *expected count* directly
+ after may cause undefined behavior.
+* *Joining* a barrier is only useful if the thread will *wait* on that same barrier *object* later.
Execution Barrier GFX6-11
+++++++++++++++++++++++++
Targets from GFX6 through GFX11 included do not have the split barrier feature.
-The barrier *signal* and barrier *wait* operations cannot be performed independently.
+The barrier *arrive* and barrier *wait* operations **cannot** be performed independently.
There is only one *workgroup barrier* object of ``workgroup`` scope that is implicitly used
by all barrier operations.
- .. table:: AMDHSA Execution Barriers Code Sequences GFX6-GFX11
- :name: amdgpu-amdhsa-execution-barriers-code-sequences-gfx6-gfx11-table
-
- ===================== ====================== ===========================================================
- Barrier Operation(s) Barrier *Object* AMDGPU Machine Code
- ===================== ====================== ===========================================================
- **Init, Join and Leave**
- --------------------------------------------------------------------------------------------------------
- *init* - *Workgroup barrier* See barrier *init* in
- :ref:`amdgpu-amdhsa-execution-barriers-workgroup-barriers`.
-
- *join* - *Workgroup barrier* See barrier *join* in
- :ref:`amdgpu-amdhsa-execution-barriers-workgroup-barriers`.
-
- *leave* - *Workgroup barrier* See barrier *leave* in
- :ref:`amdgpu-amdhsa-execution-barriers-workgroup-barriers`.
-
- **Signal and Wait**
- --------------------------------------------------------------------------------------------------------
- *signal* then *wait* - *Workgroup barrier* | **BackOffBarrier**
- | ``s_barrier``
- | **No BackOffBarrier**
- | ``s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)``
- | ``s_waitcnt_vscnt null, 0x0``
- | ``s_barrier``
-
- - If the target does not have the BackOffBarrier feature,
- then there cannot be any outstanding memory operations
- before issuing the ``s_barrier`` instruction.
- - The waitcnts can independently be moved earlier, or
- removed entirely as long as the associated
- counter remains at zero before issuing the
- ``s_barrier`` instruction.
-
- *signal* - *Workgroup barrier* Not available separately, see *signal* then *wait*
-
- *wait* - *Workgroup barrier* Not available separately, see *signal* then *wait*
- ===================== ====================== ===========================================================
+The following code sequences can be used to implement the barrier operations described by the above specification:
+
+.. table:: AMDHSA Execution Barriers Code Sequences GFX6-GFX11
+ :name: amdgpu-amdhsa-execution-barriers-code-sequences-gfx6-gfx11-table
+ :widths: 15 15 70
+
+ ===================== ====================== ===========================================================
+ Barrier Operation(s) Barrier *Object* AMDGPU Machine Code
+ ===================== ====================== ===========================================================
+ **Init, Join and Drop**
+ --------------------------------------------------------------------------------------------------------
+ *init* - *Workgroup barrier* Automatically initialized by the hardware when a workgroup
+ is launched. The *expected count* of this barrier is set
+ to the number of waves in the workgroup.
+
+ *join* - *Workgroup barrier* Any thread launched within a workgroup automatically *joins*
+ this barrier *object*.
+
+ *drop* - *Workgroup barrier* When a thread ends, it automatically *drops* this barrier
+ *object* if it had previously *joined* it.
+
+ **Arrive and Wait**
+ --------------------------------------------------------------------------------------------------------
+ *arrive* then *wait* - *Workgroup barrier* | **BackOffBarrier**
+ | ``s_barrier``
+ | **No BackOffBarrier**
+ | ``s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)``
+ | ``s_waitcnt_vscnt null, 0x0``
+ | ``s_barrier``
+
+ - If the target does not have the BackOffBarrier feature,
+ then there cannot be any outstanding memory operations
+ before issuing the ``s_barrier`` instruction.
+ - The waitcnts can independently be moved earlier, or
+ removed entirely as long as the associated
+ counter remains at zero before issuing the
+ ``s_barrier`` instruction.
+ - The ``s_barrier`` instruction cannot complete
+ before all waves of the workgroup have launched.
+
+ *arrive* - *Workgroup barrier* Not available separately, see *arrive* then *wait*
+
+ *wait* - *Workgroup barrier* Not available separately, see *arrive* then *wait*
+ ===================== ====================== ===========================================================
Execution Barrier GFX12
+++++++++++++++++++++++
-.. note::
-
- This is incomplete for GFX12.5.
-
GFX12 targets have the split-barrier feature, and also offer multiple barrier *objects* per workgroup
-(see :ref:`amdgpu-amdhsa-execution-barriers-ids-gfx12-table`).
-
- .. table:: AMDHSA Execution Barriers Code Sequences GFX12
- :name: amdgpu-amdhsa-execution-barriers-code-sequences-gfx12-table
-
- ===================== =========================== ===========================================================
- Barrier Operation(s) Barrier *Object* AMDGPU Machine Code
- ===================== =========================== ===========================================================
- **Init, Join and Leave**
- -------------------------------------------------------------------------------------------------------------
- *init* - *Workgroup barrier* See barrier *init* in
- - *Workgroup trap barrier* :ref:`amdgpu-amdhsa-execution-barriers-workgroup-barriers`.
-
- *join* - *Workgroup barrier* See barrier *join* in
- - *Workgroup trap barrier* :ref:`amdgpu-amdhsa-execution-barriers-workgroup-barriers`.
-
- *leave* - *Workgroup barrier* See barrier *leave* in
- - *Workgroup trap barrier* :ref:`amdgpu-amdhsa-execution-barriers-workgroup-barriers`.
-
- **Signal and Wait**
- -------------------------------------------------------------------------------------------------------------
-
- *signal* - *Workgroup barrier* | ``s_barrier_signal -1``
- | Or
- | ``s_barrier_signal_isfirst -1``
+(see :ref:`amdgpu-amdhsa-execution-barriers-ids-gfx12-table`). Each barrier *object* has a unique barrier ID that
+instructions use to operate on them.
+GFX12.5 additionally introduces new barrier *objects* that offer more flexibility for synchronizing the execution
+of a subset of waves of a workgroup, or synchronizing execution across workgroups within a workgroup cluster.
- *wait* - *Workgroup barrier* ``s_barrier_wait -1``.
-
- *signal* - *Workgroup trap barrier* Not available to the shader.
-
- *wait* - *Workgroup trap barrier* Not available to the shader.
- ===================== =========================== ===========================================================
-
-
- .. table:: AMDHSA Execution Barriers IDs GFX12
- :name: amdgpu-amdhsa-execution-barriers-ids-gfx12-table
-
- =========== ============== ==============================================================
- Barrier ID Scope Description
- =========== ============== ==============================================================
- ``-2`` ``workgroup`` *Workgroup trap barrier*, dedicated for the trap handler and
- only available in privileged execution mode
- (not accessible by the shader).
+.. note::
- ``-1`` ``workgroup`` *Workgroup barrier*.
- =========== ============== ==============================================================
+ Check the :ref:`the table below<amdgpu-amdhsa-execution-barriers-ids-gfx12-table>` to determine which barrier IDs are
+ available to the shader on a given target.
+
+
+The following code sequences can be used to implement the barrier operations described by the above specification:
+
+.. table:: AMDHSA Execution Barriers Code Sequences GFX12
+ :name: amdgpu-amdhsa-execution-barriers-code-sequences-gfx12-table
+ :widths: 15 15 70
+
+ ===================== =========================== ===========================================================
+ Barrier Operation(s) Barrier ID AMDGPU Machine Code
+ ===================== =========================== ===========================================================
+ **Init, Join and Drop**
+ -------------------------------------------------------------------------------------------------------------
+ *init* - ``-2``, ``-1`` Automatically initialized by the hardware when a workgroup
+ is launched. The *expected count* of this barrier is set
+ to the number of waves in the workgroup.
+
+ *init* - ``-4``, ``-3`` Automatically initialized by the hardware when a workgroup
+ is launched as part of a workgroup cluster.
+ The *expected count* of this barrier is set to the number
+ of workgroups in the workgroup cluster.
+
+ *init* - ``0`` Automatically initialized by the hardware and always
+ available. This barrier *object* is opaque and immutable
+ as all operations other than barrier *join* are no-ops.
+
+ *init* - ``[1, 16]`` | ``s_barrier_init <N>``
+
+ - ``<N>`` is an immediate constant, or stored in the lower
+ half of ``m0``.
+ - The value to set as the *expected count* of the barrier
+ is stored in the upper half of ``m0``.
+
+ *join* - ``-2``, ``-1`` Any thread launched within a workgroup automatically *joins*
+ this barrier *object*.
+
+ *join* - ``-4``, ``-3`` Any thread launched within a workgroup cluster
+ automatically *joins* this barrier *object*.
+
+ *join* - ``0`` | ``s_barrier_join <N>``
+ - ``[1, 16]``
+ - ``<N>`` is an immediate constant, or stored in the lower
+ half of ``m0``.
+
+ *drop* - ``0`` | ``s_barrier_leave``
+ - ``[1, 16]``
+ - ``s_barrier_leave`` takes no operand. It can only be used
+ to *drop* a barrier *object* ``BO`` if ``BO`` was
+ previously *joined* using ``s_barrier_join``.
+ - *Drops* the barrier *object* ``BO`` if and only if
+ there is a barrier *join* ``J`` such that ``J`` is
+ *barrier-joined-before* this barrier
+ *drop* operation.
+
+ *drop* - ``-2``, ``-1`` When a thread ends, it automatically *drops* this barrier
+ - ``-4``, ``-3`` *object* if it had previously *joined* it.
+
+ **Arrive and Wait**
+ -------------------------------------------------------------------------------------------------------------
+
+ *arrive* - ``-4``, ``-3`` | ``s_barrier_signal <N>``
+ - ``-2``, ``-1`` | Or
+ - ``0`` | ``s_barrier_signal_isfirst <N>``
+ - ``[1, 16]``
+ - ``<N>`` is an immediate constant, or stored in bits ``[4:0]`` of ``m0``.
+ - The ``_isfirst`` variant sets ``SCC=1`` if this wave is the first
+ to signal the barrier, otherwise ``SCC=0``.
+ - For barrier *objects* ``[1, 16]``: When using ``m0`` as an operand,
+ if there is a non-zero value contained in the bits ``[22:16]`` of ``m0``,
+ the *expected count* of the barrier *object* is set to that value before
+ the *arrive count* of the barrier *object* is incremented.
+ The new *expected count* value must be greater than or equal to the old
+ value, otherwise the behavior is undefined.
----------------
Pierre-vh wrote:
Done
https://github.com/llvm/llvm-project/pull/185632
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