[llvm] [RFC] Memory Model Relaxation Annotations (PR #78569)

Pierre van Houtryve via llvm-commits llvm-commits at lists.llvm.org
Tue Feb 6 00:59:12 PST 2024


https://github.com/Pierre-vh updated https://github.com/llvm/llvm-project/pull/78569

>From 731c300c8f7b684ab5c61b785b375c61a2d334c5 Mon Sep 17 00:00:00 2001
From: pvanhout <pierre.vanhoutryve at amd.com>
Date: Wed, 17 Jan 2024 11:20:09 +0100
Subject: [PATCH 1/3] [RFC] Memory Model Relaxation Annotations

Work-in-progress patch to implement the core/target-agnostic components of Memory Model Relaxation Annotations.

This diff is mostly complete but likely has a few holes, especially in codegen (MMRAs aren't always carried over to MI layer I believe).
Most of the work needs to be done in adding tests and analyzing the outputs to find what's missing.
---
 .../docs/MemoryModelRelaxationAnnotations.rst | 476 ++++++++++++++++++
 llvm/docs/Reference.rst                       |   4 +
 llvm/include/llvm/Analysis/VectorUtils.h      |   2 +-
 .../CodeGen/GlobalISel/MachineIRBuilder.h     |   9 +
 llvm/include/llvm/CodeGen/MachineFunction.h   |   3 +-
 llvm/include/llvm/CodeGen/MachineInstr.h      |  47 +-
 .../llvm/CodeGen/MachineInstrBuilder.h        |  46 +-
 llvm/include/llvm/CodeGen/SelectionDAG.h      |  12 +
 llvm/include/llvm/IR/FixedMetadataKinds.def   |   1 +
 .../IR/MemoryModelRelaxationAnnotations.h     | 101 ++++
 llvm/lib/Analysis/VectorUtils.cpp             |  11 +-
 llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp  |   1 +
 .../CodeGen/GlobalISel/MachineIRBuilder.cpp   |   4 +-
 llvm/lib/CodeGen/MIRPrinter.cpp               |   7 +
 llvm/lib/CodeGen/MachineFunction.cpp          |   4 +-
 llvm/lib/CodeGen/MachineInstr.cpp             |  49 +-
 .../SelectionDAG/ScheduleDAGSDNodes.cpp       |   9 +
 .../lib/CodeGen/SelectionDAG/SelectionDAG.cpp |   2 +-
 .../SelectionDAG/SelectionDAGBuilder.cpp      |  17 +-
 .../SelectionDAG/SelectionDAGDumper.cpp       |   7 +
 .../CodeGen/SelectionDAG/SelectionDAGISel.cpp |   2 +
 llvm/lib/IR/CMakeLists.txt                    |   1 +
 llvm/lib/IR/Instruction.cpp                   |  14 +-
 .../IR/MemoryModelRelaxationAnnotations.cpp   | 201 ++++++++
 llvm/lib/IR/Verifier.cpp                      |  27 +
 llvm/lib/Transforms/Scalar/GVNHoist.cpp       |   3 +-
 .../Transforms/Utils/FunctionComparator.cpp   |   5 +
 llvm/lib/Transforms/Utils/Local.cpp           |  15 +-
 llvm/test/CodeGen/AMDGPU/GlobalISel/mmra.ll   |  49 ++
 llvm/test/CodeGen/AMDGPU/mmra.ll              | 254 ++++++++++
 llvm/test/Verifier/mmra-allowed.ll            |  33 ++
 llvm/test/Verifier/mmra.ll                    |  37 ++
 llvm/unittests/CodeGen/MachineInstrTest.cpp   |  51 ++
 llvm/unittests/IR/CMakeLists.txt              |   1 +
 .../MemoryModelRelaxationAnnotationsTest.cpp  |  34 ++
 35 files changed, 1487 insertions(+), 52 deletions(-)
 create mode 100644 llvm/docs/MemoryModelRelaxationAnnotations.rst
 create mode 100644 llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h
 create mode 100644 llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp
 create mode 100644 llvm/test/CodeGen/AMDGPU/GlobalISel/mmra.ll
 create mode 100644 llvm/test/CodeGen/AMDGPU/mmra.ll
 create mode 100644 llvm/test/Verifier/mmra-allowed.ll
 create mode 100644 llvm/test/Verifier/mmra.ll
 create mode 100644 llvm/unittests/IR/MemoryModelRelaxationAnnotationsTest.cpp

diff --git a/llvm/docs/MemoryModelRelaxationAnnotations.rst b/llvm/docs/MemoryModelRelaxationAnnotations.rst
new file mode 100644
index 0000000000000..e83e365a3a57d
--- /dev/null
+++ b/llvm/docs/MemoryModelRelaxationAnnotations.rst
@@ -0,0 +1,476 @@
+===================================
+Memory Model Relaxation Annotations
+===================================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+Memory Model Relaxation Annotations (MMRAs) are target-defined properties
+on instructions that can be used to selectively relax constraints placed
+by the memory model. For example:
+
+* The use of ``VulkanMemoryModel`` in a SPIRV program allows certain
+  memory operations to be reordered across ``acquire`` or ``release``
+  operations.
+* OpenCL APIs expose primitives to only fence a specific set of address
+  spaces, carrying that information to the backend can enable the
+  use of faster synchronization instructions, rather than fencing all
+  address spaces.
+
+MMRAs offer an opt-in system for targets to relax the default LLVM
+memory model.
+As such, they are attached to an operation using LLVM metadata which
+can always be dropped without affecting correctness.
+
+Definitions
+===========
+
+memory operation
+    A load, a store, an atomic, or a function call that is marked as
+    accessing memory.
+
+synchronizing operation
+    An instruction that synchronizes memory with other threads (e.g.
+    an atomic or a fence).
+
+tag
+    Metadata attached to a memory or synchronizing operation
+    that represents some target-defined property regarding memory
+    synchronization.
+
+    An operation may have multiple tags that each represent a different
+    property.
+
+    A tag is composed of a pair of metadata nodes:
+
+    * a *prefix* string.
+    * a *suffix* integer or string.
+
+    In LLVM IR, the pair is represented using a metadata tuple.
+    In other cases (comments, documentation, etc.), we may use the
+    ``prefix:suffix`` notation.
+    For example:
+
+    .. code-block::
+      :caption: Example: Tags in Metadata
+
+      !0 = !{!"scope", !"workgroup"}  # scope:workgroup
+      !1 = !{!"scope", !"device"}     # scope:device
+      !2 = !{!"scope", !"system"}     # scope:system
+      !3 = !{!"sync-as", i32 2}  # sync-as:2
+      !4 = !{!"sync-as", i32 1}  # sync-as:1
+      !5 = !{!"sync-as", i32 0}  # sync-as:0
+
+    .. note::
+
+      The only semantics relevant to the optimizer is the
+      "compatibility" relation defined below. All other
+      semantics are target defined.
+
+    Tags can also be organised in lists to allow operations
+    to specify all of the tags they belong to. Such a list
+    is referred to as a "set of tags".
+
+    .. code-block::
+      :caption: Example: Set of Tags in Metadata
+
+      !0 = !{!"scope", !"workgroup"}
+      !1 = !{!"sync-as", !"private"}
+      !2 = !{!0, !2}
+
+    .. note::
+
+      If an operation does not have MMRA metadata, it's treated as if
+      it has an empty list (``!{}``) of tags.
+
+    Note that it is not an error if a tag is not recognized by the
+    instruction it is applied to, or by the current target.
+    Such tags are simply ignored.
+
+    Both synchronizing operations and memory operations can have
+    zero or more tags attached to them using the ``!mmra`` syntax.
+
+    For the sake of readability in examples below,
+    we use a (non-functional) short syntax to represent MMMRA metadata:
+
+    .. code-block::
+      :caption: Short Syntax Example
+
+      store %ptr1 # foo:bar
+      store %ptr1 !mmra !{!"foo", !"bar"}
+
+    These two notations can be used in this document and are strictly
+    equivalent. However, only the second version is functional.
+
+compatibility
+    Two sets of tags are said to be *compatible* iff, for every unique
+    tag prefix P present in at least one set:
+
+    - the other set contains no tag with prefix P, or
+    - at least one tag with prefix P is common to both sets.
+
+    The above definition implies that an empty set is always compatible
+    with any other set. This is an important property as it ensures that
+    if a transform drops the metadata on an operation, it can never affect
+    correctness. In other words, the memory model cannot be relaxed further
+    by deleting metadata from instructions.
+
+.. _HappensBefore:
+
+The *happens-before* Relation
+==============================
+
+Compatibility checks can be used to opt out of the *happens-before* relation
+established between two instructions.
+
+Ordering
+    When two instructions' metadata are not compatible, any program order
+    between them are not in *happens-before*.
+
+    For example, consider two tags ``foo:bar`` and
+    ``foo:baz`` exposed by a target:
+
+    .. code-block::
+
+       A: store %ptr1                 # foo:bar
+       B: store %ptr2                 # foo:baz
+       X: store atomic release %ptr3  # foo:bar
+
+    In the above figure, ``A`` is compatible with ``X``, and hence ``A``
+    happens-before ``X``. But ``B`` is not compatible with
+    ``X``, and hence it is not happens-before ``X``.
+
+Synchronization
+    If an synchronizing operation has one or more tags, then whether it
+    participate in the  ``seq_cst`` order with other operations is target
+    dependent.
+
+    .. code-block::
+
+       ; Depending on the semantics of foo:bar & foo:bux, this may not
+       ; synchronize with another sequence.
+       fence release               # foo:bar
+       store atomic %ptr1          # foo:bux
+
+Examples
+--------
+
+.. code-block:: text
+  :caption: Example 1
+
+   A: store ptr addrspace(1) %ptr2                  # sync-as:1 vulkan:nonprivate
+   B: store atomic release ptr addrspace(1) %ptr3   # sync-as:0 vulkan:nonprivate
+
+A and B are not ordered relative to each other
+(no *happens-before*) because their sets of tags are not compatible.
+
+Note that the ``sync-as`` value does not have to match the ``addrspace`` value.
+e.g. In Example 1, a store-release to a location in ``addrspace(1)`` wants to
+only synchronize with operations happening in ``addrspace(0)``.
+
+.. code-block:: text
+  :caption: Example 2
+
+   A: store ptr addrspace(1) %ptr2                 # sync-as:1 vulkan:nonprivate
+   B: store atomic release ptr addrspace(1) %ptr3  # sync-as:1 vulkan:nonprivate
+
+The ordering of A and B is unaffected because their set of tags are
+compatible.
+
+Note that A and B may or may not be in *happens-before* due to other reasons.
+
+.. code-block:: text
+  :caption: Example 3
+
+   A: store ptr addrspace(1) %ptr2                 # sync-as:1 vulkan:nonprivate
+   B: store atomic release ptr addrspace(1) %ptr3  # vulkan:nonprivate
+
+The ordering of A and B is unaffected because their set of tags are
+compatible.
+
+.. code-block:: text
+  :caption: Example 3
+
+   A: store ptr addrspace(1) %ptr2                 # sync-as:1
+   B: store atomic release ptr addrspace(1) %ptr3  # sync-as:2
+
+A and B do not have to be ordered relative to each other
+(no *happens-before*) because their sets of tags are not compatible.
+
+Use-cases
+=========
+
+SPIRV ``NonPrivatePointer``
+---------------------------
+
+MMRAs can support the SPIRV capability
+``VulkanMemoryModel``, where synchronizing operations only affect
+memory operations that specify ``NonPrivatePointer`` semantics.
+
+The example below is generated from a SPIRV program using the
+following recipe:
+
+- Add ``vulkan:nonprivate`` to every synchronizing operation.
+- Add ``vulkan:nonprivate`` to every non-atomic memory operation
+  that is marked ``NonPrivatePointer``.
+- Add ``vulkan:private`` to tags of every non-atomic memory operation
+  that is not marked ``NonPrivatePointer``.
+
+.. code-block::
+
+   Thread T1:
+    A: store %ptr1                 # vulkan:nonprivate
+    B: store %ptr2                 # vulkan:private
+    X: store atomic release %ptr3  # vulkan:nonprivate
+
+   Thread T2:
+    Y: load atomic acquire %ptr3   # vulkan:nonprivate
+    C: load %ptr2                  # vulkan:private
+    D: load %ptr1                  # vulkan:nonprivate
+
+Compatibility ensures that operation ``A`` is ordered
+relative to ``X`` while operation ``D`` is ordered relative to ``Y``.
+If ``X`` synchronizes with ``Y``, then ``A`` happens-before ``D``.
+No such relation can be inferred about operations ``B`` and ``C``.
+
+.. note::
+   The `Vulkan Memory Model <https://registry.khronos.org/vulkan/specs/1.3-extensions/html/vkspec.html#memory-model-non-private>`_
+   considers all atomic operation non-private.
+
+   Whether ``vulkan:nonprivate`` would be specified on atomic operations is
+   an implementation detail, as an atomic operation is always ``nonprivate``.
+   The implementation may choose to be explicit and emit IR with
+   ``vulkan:nonprivate`` on every atomic operation, or it could choose to
+   only emit ``vulkan::private`` and assume ``vulkan:nonprivate``
+   by default.
+
+Operations marked with ``vulkan:private`` effectively opt out of the
+happens-before order in a SPIRV program since they are incompatible
+with every synchronizing operation. Note that SPIRV operations that
+are not marked ``NonPrivatePointer`` are not entirely private to the
+thread --- they are implicitly synchronized at the start or end of a
+thread by the Vulkan *system-synchronizes-with* relationship. This
+example assumes that the target-defined semantics of
+``vulkan:private`` correctly implements this property.
+
+This scheme is general enough to express the interoperability of SPIRV
+programs with other environments.
+
+.. code-block::
+
+   Thread T1:
+   A: store %ptr1                 # vulkan:nonprivate
+   X: store atomic release %ptr2  # vulkan:nonprivate
+
+   Thread T2:
+   Y: load atomic acquire %ptr2   # foo:bar
+   B: load %ptr1
+
+In the above example, thread ``T1`` originates from a SPIRV program
+while thread ``T2`` originates from a non-SPIRV program. Whether ``X``
+can synchronize with ``Y`` is target defined.  If ``X`` synchronizes
+with ``Y``, then ``A`` happens before ``B`` (because A/X and
+Y/B are compatible).
+
+Implementation Example
+~~~~~~~~~~~~~~~~~~~~~~
+
+Consider the implementation of SPIRV ``NonPrivatePointer`` on a target
+where all memory operations are cached, and the entire cache is
+flushed or invalidated at a ``release`` or ``acquire`` respectively. A
+possible scheme is that when translating a SPIRV program, memory
+operations marked ``NonPrivatePointer`` should not be cached, and the
+cache contents should not be touched during an ``acquire`` and
+``release`` operation.
+
+This could be implemented using the tags that share the ``vulkan:`` prefix,
+as follows:
+
+- For memory operations:
+
+  - Operations with ``vulkan:nonprivate`` should bypass the cache.
+  - Operations with ``vulkan:private`` should be cached.
+  - Operations that specify neither or both should conservatively
+    bypass the cache to ensure correctness.
+
+- For synchronizing operations:
+
+  - Operations with ``vulkan:nonprivate`` should not flush or
+    invalidate the cache.
+  - Operations with ``vulkan:private`` should flush or invalidate the cache.
+  - Operations that specify neither or both should conservatively
+    flush or invalidate the cache to ensure correctness.
+
+.. note::
+   In such an implementation, dropping the metadata on an operation, while
+   not affecting correctness, may have big performance implications.
+   e.g. an operation bypasses the cache when it shouldn't.
+
+Memory Types
+------------
+
+MMRAs may express the selective synchronization of
+different memory types.
+
+As an example, a target may expose an ``sync-as:<N>`` tag to
+pass information about which address spaces are synchronized by the
+execution of a synchronizing operation.
+
+.. note::
+  Address spaces are used here as a common example, but this concept isn't
+  can apply for other "memory types". What "memory types" means here is
+  up to the target.
+
+.. code-block::
+
+   # let 1 = global address space
+   # let 3 = local address space
+
+   Thread T1:
+   A: store %ptr1                                  # sync-as:1
+   B: store %ptr2                                  # sync-as:3
+   X: store atomic release ptr addrspace(0) %ptr3  # sync-as:3
+
+   Thread T2:
+   Y: load atomic acquire ptr addrspace(0) %ptr3   # sync-as:3
+   C: load %ptr2                                   # sync-as:3
+   D: load %ptr1                                   # sync-as:1
+
+In the above figure, ``X`` and ``Y`` are atomic operations on a
+location in the ``global``  address space. If ``X`` synchronizes with
+``Y``, then ``B`` happens-before ``C`` in the ``local`` address
+space. But no such statement can be made about operations ``A`` and
+``D``, although they are peformed on a location in the ``global``
+address space.
+
+Implementation Example: Adding Address Space Information to Fences
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Languages such as OpenCL C provide fence operations such as
+``atomic_work_item_fence`` that can take an explicit address
+space to fence.
+
+By default, LLVM has no means to carry that information in the IR, so
+the information is lost during lowering to LLVM IR. This means that
+targets such as AMDGPU have to conservatively emit instructions to
+fence all address spaces in all cases, which can have a noticeable
+performance impact in high-performance applications.
+
+MMRAs may be used to preserve that information at the IR level, all the
+way through code generation. For example, a fence that only affects the
+global address space ``addrspace(1)`` may be lowered as
+
+.. code-block::
+
+    fence release # sync-as:1
+
+and the target may use the presence of ``sync-as:1`` to infer that it
+must only emit instruction to fence the global address space.
+
+Note that as MMRAs are opt in, a fence that does not have MMRA metadata
+could still be lowered conservatively, so this optimization would only
+apply if the front-end emits the MMRA metadata on the fence instructions.
+
+Additional Topics
+=================
+
+.. note::
+
+  The following sections are informational.
+
+Performance Impact
+------------------
+
+MMRAs are a way to capture optimization opportunities in the program.
+But when an operation mentions no tags or conflicting tags,
+the target may need to produce conservative code to ensure correctness
+at the cost of performance. This can happen in the following situations:
+
+1. When a target first introduces MMRAs, the
+   frontend might not have been updated to emit them.
+2. An optimization may drop MMRA metadata.
+3. An optimization may add arbitrary tags to an operation.
+
+Note that targets can always choose to ignore (or even drop) MMRAs
+and revert to the default behavior/codegen heuristics without
+affecting correctness.
+
+Consequences of the Absence of *happens-before*
+-----------------------------------------------
+
+In the :ref:`happens-before<HappensBefore>` section, we defined how an
+*happens-before* relation between two instruction can be broken
+by leveraging compatibility between MMRAs. When the instructions
+are incompatible and there is no *happens-before* relation, we say
+that the instructions "do not have to be ordered relative to each
+other".
+
+"Ordering" in this context is a very broad term which covers both
+static and runtime aspects.
+
+When there is no ordering constraint, we *could* statically reorder
+the instructions in an optimizer transform if the reordering does
+not break other constraints as single location coherence.
+Static reordering is one consequence of breaking *happens-before*,
+but is not the most interesting one.
+
+Run-time consequences are more interesting. When there is an
+*happens-before* relation between instructions, the target has to emit
+synchronization code to ensure other threads will observe the effects of
+the instructions in the right order.
+
+For instance, the target may have to wait for previous loads & stores to
+finish before starting a fence-release, or there may be a need to flush a
+memory cache before executing the next instruction.
+In the absence of *happens-before*, there is no such requirement and
+no waiting or flushing is required. This may noticeably speed up
+execution in some cases.
+
+Combining Operations
+--------------------
+
+If a pass can combine multiple memory or synchronizing operations
+into one, then the metadata of the new instruction(s) shall be a
+prefix-wise union of the metadata of the source instructions.
+
+Let A and B be two tags set, and U be the prefix-wise union of A and B.
+For every unique tag prefix P present in A or B:
+
+* If either A or B has no tags with prefix P, no tags with prefix
+  P are added to U.
+* If both A and B have at least one tag with prefix P, only the tags
+  common to A and B are added to U.
+
+Examples:
+
+.. code-block::
+
+    A: store release %ptr1  # foo:x, foo:y, bar:x
+    B: store release %ptr2  # foo:x, bar:y
+
+    # Unique prefixes P = [foo, bar]
+    # "foo:x" is common to A and B so it's added to U.
+    # "bar:x" != "bar:y" so it's not added to U.
+    U: store release %ptr3  # foo:x
+
+.. code-block::
+
+    A: store release %ptr1  # foo:x, foo:y
+    B: store release %ptr2  # foo:x, bux:y
+
+    # Unique prefixes P = [foo, bux]
+    # "foo:x" is common to A and B so it's added to U.
+    # No tags have the prefix "bux" in A.
+    U: store release %ptr3  # foo:x
+
+.. code-block::
+
+    A: store release %ptr1
+    B: store release %ptr2  # foo:x, bar:y
+
+    # Unique prefixes P = [foo, bar]
+    # No tags with "foo" or "bar" in A, so no tags added.
+    U: store release %ptr3
diff --git a/llvm/docs/Reference.rst b/llvm/docs/Reference.rst
index 3a1d1665be439..1661c8c533db1 100644
--- a/llvm/docs/Reference.rst
+++ b/llvm/docs/Reference.rst
@@ -39,6 +39,7 @@ LLVM and API reference documentation.
    PDB/index
    PointerAuth
    ScudoHardenedAllocator
+   MemoryModelRelaxationAnnotations
    MemTagSanitizer
    Security
    SecurityTransparencyReports
@@ -194,6 +195,9 @@ Additional Topics
 :doc:`ScudoHardenedAllocator`
   A library that implements a security-hardened `malloc()`.
 
+:doc:`MemoryModelRelaxationAnnotations`
+  Target-defined relaxation to LLVM's concurrency model.
+
 :doc:`MemTagSanitizer`
   Security hardening for production code aiming to mitigate memory
   related vulnerabilities. Based on the Armv8.5-A Memory Tagging Extension.
diff --git a/llvm/include/llvm/Analysis/VectorUtils.h b/llvm/include/llvm/Analysis/VectorUtils.h
index 7a92e62b53c53..d1a16fcd7c5eb 100644
--- a/llvm/include/llvm/Analysis/VectorUtils.h
+++ b/llvm/include/llvm/Analysis/VectorUtils.h
@@ -301,7 +301,7 @@ MDNode *intersectAccessGroups(const Instruction *Inst1,
                               const Instruction *Inst2);
 
 /// Specifically, let Kinds = [MD_tbaa, MD_alias_scope, MD_noalias, MD_fpmath,
-/// MD_nontemporal, MD_access_group].
+/// MD_nontemporal, MD_access_group, MD_MMRA].
 /// For K in Kinds, we get the MDNode for K from each of the
 /// elements of VL, compute their "intersection" (i.e., the most generic
 /// metadata value that covers all of the individual values), and set I's
diff --git a/llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h b/llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
index 1387a0a37561c..2dc23e8643765 100644
--- a/llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
+++ b/llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
@@ -52,6 +52,8 @@ struct MachineIRBuilderState {
   DebugLoc DL;
   /// PC sections metadata to be set to any instruction we create.
   MDNode *PCSections = nullptr;
+  /// MMRA Metadata to be set on any instruction we create.
+  MDNode *MMRA = nullptr;
 
   /// \name Fields describing the insertion point.
   /// @{
@@ -353,6 +355,7 @@ class MachineIRBuilder {
     setMBB(*MI.getParent());
     State.II = MI.getIterator();
     setPCSections(MI.getPCSections());
+    setMMRAMetadata(MI.getMMRAMetadata());
   }
   /// @}
 
@@ -383,9 +386,15 @@ class MachineIRBuilder {
   /// Set the PC sections metadata to \p MD for all the next build instructions.
   void setPCSections(MDNode *MD) { State.PCSections = MD; }
 
+  /// Set the PC sections metadata to \p MD for all the next build instructions.
+  void setMMRAMetadata(MDNode *MMRA) { State.MMRA = MMRA; }
+
   /// Get the current instruction's PC sections metadata.
   MDNode *getPCSections() { return State.PCSections; }
 
+  /// Get the current instruction's MMRA metadata.
+  MDNode *getMMRAMetadata() { return State.MMRA; }
+
   /// Build and insert <empty> = \p Opcode <empty>.
   /// The insertion point is the one set by the last call of either
   /// setBasicBlock or setMI.
diff --git a/llvm/include/llvm/CodeGen/MachineFunction.h b/llvm/include/llvm/CodeGen/MachineFunction.h
index 05c9b14a423cd..991016d8808bf 100644
--- a/llvm/include/llvm/CodeGen/MachineFunction.h
+++ b/llvm/include/llvm/CodeGen/MachineFunction.h
@@ -1101,7 +1101,8 @@ class LLVM_EXTERNAL_VISIBILITY MachineFunction {
   MachineInstr::ExtraInfo *createMIExtraInfo(
       ArrayRef<MachineMemOperand *> MMOs, MCSymbol *PreInstrSymbol = nullptr,
       MCSymbol *PostInstrSymbol = nullptr, MDNode *HeapAllocMarker = nullptr,
-      MDNode *PCSections = nullptr, uint32_t CFIType = 0);
+      MDNode *PCSections = nullptr, uint32_t CFIType = 0,
+      MDNode *MMRAs = nullptr);
 
   /// Allocate a string and populate it with the given external symbol name.
   const char *createExternalSymbolName(StringRef Name);
diff --git a/llvm/include/llvm/CodeGen/MachineInstr.h b/llvm/include/llvm/CodeGen/MachineInstr.h
index bd72ac23fc9c0..e7285a150b58a 100644
--- a/llvm/include/llvm/CodeGen/MachineInstr.h
+++ b/llvm/include/llvm/CodeGen/MachineInstr.h
@@ -25,6 +25,7 @@
 #include "llvm/CodeGen/TargetOpcodes.h"
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/IR/InlineAsm.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/MC/MCInstrDesc.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/ArrayRecycler.h"
@@ -157,37 +158,41 @@ class MachineInstr
                              MCSymbol *PreInstrSymbol = nullptr,
                              MCSymbol *PostInstrSymbol = nullptr,
                              MDNode *HeapAllocMarker = nullptr,
-                             MDNode *PCSections = nullptr,
-                             uint32_t CFIType = 0) {
+                             MDNode *PCSections = nullptr, uint32_t CFIType = 0,
+                             MDNode *MMRAs = nullptr) {
       bool HasPreInstrSymbol = PreInstrSymbol != nullptr;
       bool HasPostInstrSymbol = PostInstrSymbol != nullptr;
       bool HasHeapAllocMarker = HeapAllocMarker != nullptr;
+      bool HasMMRAs = MMRAs != nullptr;
       bool HasCFIType = CFIType != 0;
       bool HasPCSections = PCSections != nullptr;
       auto *Result = new (Allocator.Allocate(
           totalSizeToAlloc<MachineMemOperand *, MCSymbol *, MDNode *, uint32_t>(
               MMOs.size(), HasPreInstrSymbol + HasPostInstrSymbol,
-              HasHeapAllocMarker + HasPCSections, HasCFIType),
+              HasHeapAllocMarker + HasPCSections + HasMMRAs, HasCFIType),
           alignof(ExtraInfo)))
           ExtraInfo(MMOs.size(), HasPreInstrSymbol, HasPostInstrSymbol,
-                    HasHeapAllocMarker, HasPCSections, HasCFIType);
+                    HasHeapAllocMarker, HasPCSections, HasCFIType, HasMMRAs);
 
       // Copy the actual data into the trailing objects.
       std::copy(MMOs.begin(), MMOs.end(),
                 Result->getTrailingObjects<MachineMemOperand *>());
 
+      unsigned MDNodeIdx = 0;
+
       if (HasPreInstrSymbol)
         Result->getTrailingObjects<MCSymbol *>()[0] = PreInstrSymbol;
       if (HasPostInstrSymbol)
         Result->getTrailingObjects<MCSymbol *>()[HasPreInstrSymbol] =
             PostInstrSymbol;
       if (HasHeapAllocMarker)
-        Result->getTrailingObjects<MDNode *>()[0] = HeapAllocMarker;
+        Result->getTrailingObjects<MDNode *>()[MDNodeIdx++] = HeapAllocMarker;
       if (HasPCSections)
-        Result->getTrailingObjects<MDNode *>()[HasHeapAllocMarker] =
-            PCSections;
+        Result->getTrailingObjects<MDNode *>()[MDNodeIdx++] = PCSections;
       if (HasCFIType)
         Result->getTrailingObjects<uint32_t>()[0] = CFIType;
+      if (HasMMRAs)
+        Result->getTrailingObjects<MDNode *>()[MDNodeIdx++] = MMRAs;
 
       return Result;
     }
@@ -220,6 +225,12 @@ class MachineInstr
       return HasCFIType ? getTrailingObjects<uint32_t>()[0] : 0;
     }
 
+    MDNode *getMMRAMetadata() const {
+      return HasMMRAs ? getTrailingObjects<MDNode *>()[HasHeapAllocMarker +
+                                                       HasPCSections]
+                      : nullptr;
+    }
+
   private:
     friend TrailingObjects;
 
@@ -234,6 +245,7 @@ class MachineInstr
     const bool HasHeapAllocMarker;
     const bool HasPCSections;
     const bool HasCFIType;
+    const bool HasMMRAs;
 
     // Implement the `TrailingObjects` internal API.
     size_t numTrailingObjects(OverloadToken<MachineMemOperand *>) const {
@@ -248,15 +260,19 @@ class MachineInstr
     size_t numTrailingObjects(OverloadToken<uint32_t>) const {
       return HasCFIType;
     }
+    size_t numTrailingObjects(OverloadToken<MMRAMetadata>) const {
+      return HasMMRAs;
+    }
 
     // Just a boring constructor to allow us to initialize the sizes. Always use
     // the `create` routine above.
     ExtraInfo(int NumMMOs, bool HasPreInstrSymbol, bool HasPostInstrSymbol,
-              bool HasHeapAllocMarker, bool HasPCSections, bool HasCFIType)
+              bool HasHeapAllocMarker, bool HasPCSections, bool HasCFIType,
+              bool HasMMRAs)
         : NumMMOs(NumMMOs), HasPreInstrSymbol(HasPreInstrSymbol),
           HasPostInstrSymbol(HasPostInstrSymbol),
           HasHeapAllocMarker(HasHeapAllocMarker), HasPCSections(HasPCSections),
-          HasCFIType(HasCFIType) {}
+          HasCFIType(HasCFIType), HasMMRAs(HasMMRAs) {}
   };
 
   /// Enumeration of the kinds of inline extra info available. It is important
@@ -835,6 +851,15 @@ class MachineInstr
     return nullptr;
   }
 
+  /// Helper to extract mmra.op metadata.
+  MDNode *getMMRAMetadata() const {
+    if (!Info)
+      return nullptr;
+    if (ExtraInfo *EI = Info.get<EIIK_OutOfLine>())
+      return EI->getMMRAMetadata();
+    return nullptr;
+  }
+
   /// Helper to extract a CFI type hash if one has been added.
   uint32_t getCFIType() const {
     if (!Info)
@@ -1898,6 +1923,8 @@ class MachineInstr
   // addresses into.
   void setPCSections(MachineFunction &MF, MDNode *MD);
 
+  void setMMRAMetadata(MachineFunction &MF, MDNode *MMRAs);
+
   /// Set the CFI type for the instruction.
   void setCFIType(MachineFunction &MF, uint32_t Type);
 
@@ -2010,7 +2037,7 @@ class MachineInstr
   void setExtraInfo(MachineFunction &MF, ArrayRef<MachineMemOperand *> MMOs,
                     MCSymbol *PreInstrSymbol, MCSymbol *PostInstrSymbol,
                     MDNode *HeapAllocMarker, MDNode *PCSections,
-                    uint32_t CFIType);
+                    uint32_t CFIType, MDNode *MMRAs);
 };
 
 /// Special DenseMapInfo traits to compare MachineInstr* by *value* of the
diff --git a/llvm/include/llvm/CodeGen/MachineInstrBuilder.h b/llvm/include/llvm/CodeGen/MachineInstrBuilder.h
index 954d8e6770a29..4fcb99aa410a9 100644
--- a/llvm/include/llvm/CodeGen/MachineInstrBuilder.h
+++ b/llvm/include/llvm/CodeGen/MachineInstrBuilder.h
@@ -29,6 +29,7 @@
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cassert>
 #include <cstdint>
@@ -322,6 +323,12 @@ class MachineInstrBuilder {
     return *this;
   }
 
+  const MachineInstrBuilder &setMMRAMetadata(MDNode *MMRA) const {
+    if (MMRA)
+      MI->setMMRAMetadata(*MF, MMRA);
+    return *this;
+  }
+
   /// Copy all the implicit operands from OtherMI onto this one.
   const MachineInstrBuilder &
   copyImplicitOps(const MachineInstr &OtherMI) const {
@@ -337,14 +344,15 @@ class MachineInstrBuilder {
 };
 
 /// Set of metadata that should be preserved when using BuildMI(). This provides
-/// a more convenient way of preserving DebugLoc and PCSections.
+/// a more convenient way of preserving DebugLoc, PCSections and MMRA.
 class MIMetadata {
 public:
   MIMetadata() = default;
-  MIMetadata(DebugLoc DL, MDNode *PCSections = nullptr)
-      : DL(std::move(DL)), PCSections(PCSections) {}
-  MIMetadata(const DILocation *DI, MDNode *PCSections = nullptr)
-      : DL(DI), PCSections(PCSections) {}
+  MIMetadata(DebugLoc DL, MDNode *PCSections = nullptr, MDNode *MMRA = nullptr)
+      : DL(std::move(DL)), PCSections(PCSections), MMRA(MMRA) {}
+  MIMetadata(const DILocation *DI, MDNode *PCSections = nullptr,
+             MDNode *MMRA = nullptr)
+      : DL(DI), PCSections(PCSections), MMRA(MMRA) {}
   explicit MIMetadata(const Instruction &From)
       : DL(From.getDebugLoc()),
         PCSections(From.getMetadata(LLVMContext::MD_pcsections)) {}
@@ -353,17 +361,20 @@ class MIMetadata {
 
   const DebugLoc &getDL() const { return DL; }
   MDNode *getPCSections() const { return PCSections; }
+  MDNode *getMMRAMetadata() const { return MMRA; }
 
 private:
   DebugLoc DL;
   MDNode *PCSections = nullptr;
+  MDNode *MMRA = nullptr;
 };
 
 /// Builder interface. Specify how to create the initial instruction itself.
 inline MachineInstrBuilder BuildMI(MachineFunction &MF, const MIMetadata &MIMD,
                                    const MCInstrDesc &MCID) {
   return MachineInstrBuilder(MF, MF.CreateMachineInstr(MCID, MIMD.getDL()))
-           .setPCSections(MIMD.getPCSections());
+      .setPCSections(MIMD.getPCSections())
+      .setMMRAMetadata(MIMD.getMMRAMetadata());
 }
 
 /// This version of the builder sets up the first operand as a
@@ -371,8 +382,9 @@ inline MachineInstrBuilder BuildMI(MachineFunction &MF, const MIMetadata &MIMD,
 inline MachineInstrBuilder BuildMI(MachineFunction &MF, const MIMetadata &MIMD,
                                    const MCInstrDesc &MCID, Register DestReg) {
   return MachineInstrBuilder(MF, MF.CreateMachineInstr(MCID, MIMD.getDL()))
-           .setPCSections(MIMD.getPCSections())
-           .addReg(DestReg, RegState::Define);
+      .setPCSections(MIMD.getPCSections())
+      .setMMRAMetadata(MIMD.getMMRAMetadata())
+      .addReg(DestReg, RegState::Define);
 }
 
 /// This version of the builder inserts the newly-built instruction before
@@ -386,8 +398,9 @@ inline MachineInstrBuilder BuildMI(MachineBasicBlock &BB,
   MachineInstr *MI = MF.CreateMachineInstr(MCID, MIMD.getDL());
   BB.insert(I, MI);
   return MachineInstrBuilder(MF, MI)
-           .setPCSections(MIMD.getPCSections())
-           .addReg(DestReg, RegState::Define);
+      .setPCSections(MIMD.getPCSections())
+      .setMMRAMetadata(MIMD.getMMRAMetadata())
+      .addReg(DestReg, RegState::Define);
 }
 
 /// This version of the builder inserts the newly-built instruction before
@@ -404,8 +417,9 @@ inline MachineInstrBuilder BuildMI(MachineBasicBlock &BB,
   MachineInstr *MI = MF.CreateMachineInstr(MCID, MIMD.getDL());
   BB.insert(I, MI);
   return MachineInstrBuilder(MF, MI)
-           .setPCSections(MIMD.getPCSections())
-           .addReg(DestReg, RegState::Define);
+      .setPCSections(MIMD.getPCSections())
+      .setMMRAMetadata(MIMD.getMMRAMetadata())
+      .addReg(DestReg, RegState::Define);
 }
 
 inline MachineInstrBuilder BuildMI(MachineBasicBlock &BB, MachineInstr &I,
@@ -435,7 +449,9 @@ inline MachineInstrBuilder BuildMI(MachineBasicBlock &BB,
   MachineFunction &MF = *BB.getParent();
   MachineInstr *MI = MF.CreateMachineInstr(MCID, MIMD.getDL());
   BB.insert(I, MI);
-  return MachineInstrBuilder(MF, MI).setPCSections(MIMD.getPCSections());
+  return MachineInstrBuilder(MF, MI)
+      .setPCSections(MIMD.getPCSections())
+      .setMMRAMetadata(MIMD.getMMRAMetadata());
 }
 
 inline MachineInstrBuilder BuildMI(MachineBasicBlock &BB,
@@ -445,7 +461,9 @@ inline MachineInstrBuilder BuildMI(MachineBasicBlock &BB,
   MachineFunction &MF = *BB.getParent();
   MachineInstr *MI = MF.CreateMachineInstr(MCID, MIMD.getDL());
   BB.insert(I, MI);
-  return MachineInstrBuilder(MF, MI).setPCSections(MIMD.getPCSections());
+  return MachineInstrBuilder(MF, MI)
+      .setPCSections(MIMD.getPCSections())
+      .setMMRAMetadata(MIMD.getMMRAMetadata());
 }
 
 inline MachineInstrBuilder BuildMI(MachineBasicBlock &BB, MachineInstr &I,
diff --git a/llvm/include/llvm/CodeGen/SelectionDAG.h b/llvm/include/llvm/CodeGen/SelectionDAG.h
index e867448b9d551..8d7b02cc613b8 100644
--- a/llvm/include/llvm/CodeGen/SelectionDAG.h
+++ b/llvm/include/llvm/CodeGen/SelectionDAG.h
@@ -33,6 +33,7 @@
 #include "llvm/CodeGen/SelectionDAGNodes.h"
 #include "llvm/CodeGen/ValueTypes.h"
 #include "llvm/IR/DebugLoc.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/ArrayRecycler.h"
@@ -285,6 +286,7 @@ class SelectionDAG {
     CallSiteInfo CSInfo;
     MDNode *HeapAllocSite = nullptr;
     MDNode *PCSections = nullptr;
+    MDNode *MMRA = nullptr;
     bool NoMerge = false;
   };
   /// Out-of-line extra information for SDNodes.
@@ -2326,11 +2328,21 @@ class SelectionDAG {
   void addPCSections(const SDNode *Node, MDNode *MD) {
     SDEI[Node].PCSections = MD;
   }
+  /// Set MMRAMetadata to be associated with Node.
+  void addMMRAMetadata(const SDNode *Node, MDNode *MMRA) {
+    SDEI[Node].MMRA = MMRA;
+  }
   /// Return PCSections associated with Node, or nullptr if none exists.
   MDNode *getPCSections(const SDNode *Node) const {
     auto It = SDEI.find(Node);
     return It != SDEI.end() ? It->second.PCSections : nullptr;
   }
+  /// Return the MMRA MDNode associated with Node, or nullptr if none
+  /// exists.
+  MDNode *getMMRAMetadata(const SDNode *Node) const {
+    auto It = SDEI.find(Node);
+    return It != SDEI.end() ? It->second.MMRA : nullptr;
+  }
   /// Set NoMergeSiteInfo to be associated with Node if NoMerge is true.
   void addNoMergeSiteInfo(const SDNode *Node, bool NoMerge) {
     if (NoMerge)
diff --git a/llvm/include/llvm/IR/FixedMetadataKinds.def b/llvm/include/llvm/IR/FixedMetadataKinds.def
index b375d0f091206..ee052e97eced5 100644
--- a/llvm/include/llvm/IR/FixedMetadataKinds.def
+++ b/llvm/include/llvm/IR/FixedMetadataKinds.def
@@ -51,3 +51,4 @@ LLVM_FIXED_MD_KIND(MD_kcfi_type, "kcfi_type", 36)
 LLVM_FIXED_MD_KIND(MD_pcsections, "pcsections", 37)
 LLVM_FIXED_MD_KIND(MD_DIAssignID, "DIAssignID", 38)
 LLVM_FIXED_MD_KIND(MD_coro_outside_frame, "coro.outside.frame", 39)
+LLVM_FIXED_MD_KIND(MD_MMRA, "mmra", 40)
diff --git a/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h b/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h
new file mode 100644
index 0000000000000..8daf491cc89ce
--- /dev/null
+++ b/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h
@@ -0,0 +1,101 @@
+//===- MemoryModelRelaxationAnnotations.h -----------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+/// \file
+/// This file provides utility for Memory Model Relaxation Annotations (MMRAs).
+/// Those annotations are represented using Metadata. The MMRATagSet class
+/// offers a simple API to parse the metadata and perform common operations on
+/// it. The MMRAMetadata class is a simple tuple of MDNode that provides easy
+/// access to all MMRA annotations on an instruction.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_IR_MEMORYMODELRELAXATIONANNOTATIONS_H
+#define LLVM_IR_MEMORYMODELRELAXATIONANNOTATIONS_H
+
+#include "llvm/ADT/STLExtras.h"
+#include <string>
+#include <tuple>
+#include <unordered_set>
+
+namespace llvm {
+
+class MDNode;
+class MDTuple;
+class StringRef;
+class raw_ostream;
+class LLVMContext;
+class Instruction;
+
+/// Helper class for `!mmra` metadata nodes which can both build MMRA MDNodes,
+/// and parse them.
+///
+/// This can be visualized as a set of "tags", with each tag
+/// representing a particular property of an instruction, as
+/// explained in the MemoryModelRelaxationAnnotations docs.
+///
+/// This class (and the optimizer in general) does not reason
+/// about the exact nature of the tags and the properties they
+/// imply. It just sees the metadata as a collection of tags, which
+/// are a prefix/suffix pair of strings.
+class MMRAMetadata {
+public:
+  using TagT = std::pair<std::string, std::string>;
+  using SetT = std::unordered_set<TagT, pair_hash<std::string, std::string>>;
+  using const_iterator = SetT::const_iterator;
+
+  MMRAMetadata() = default;
+  MMRAMetadata(const Instruction &I);
+  MMRAMetadata(MDNode *MD);
+
+  /// Checks another set of tag for compatibility with this set of tags.
+  // TODO: Unit test this
+  bool isCompatibleWith(const MMRAMetadata &Other) const;
+
+  // TODO: Unit test this
+  MMRAMetadata combine(const MMRAMetadata &Other) const;
+
+  MMRAMetadata &addTag(StringRef Prefix, StringRef Suffix);
+  MMRAMetadata &addTag(const TagT &Tag) {
+    Tags.insert(Tag);
+    return *this;
+  }
+
+  bool hasTag(StringRef Prefix, StringRef Suffix) const;
+  bool hasTag(const TagT &Tag) const { return Tags.count(Tag); }
+
+  std::vector<TagT> getAllTagsWithPrefix(StringRef Prefix) const;
+
+  bool hasTagWithPrefix(StringRef Prefix) const;
+
+  MDTuple *getAsMD(LLVMContext &Ctx) const;
+
+  const_iterator begin() const;
+  const_iterator end() const;
+  bool empty() const;
+  unsigned size() const;
+
+  void print(raw_ostream &OS) const;
+  void dump() const;
+
+  operator bool() const { return !Tags.empty(); }
+  bool operator==(const MMRAMetadata &Other) const {
+    return Tags == Other.Tags;
+  }
+  bool operator!=(const MMRAMetadata &Other) const {
+    return Tags != Other.Tags;
+  }
+
+private:
+  SetT Tags;
+};
+
+bool canInstructionHaveMMRAs(const Instruction &I);
+
+} // namespace llvm
+
+#endif
diff --git a/llvm/lib/Analysis/VectorUtils.cpp b/llvm/lib/Analysis/VectorUtils.cpp
index 73facc76a92b2..b991adac6e064 100644
--- a/llvm/lib/Analysis/VectorUtils.cpp
+++ b/llvm/lib/Analysis/VectorUtils.cpp
@@ -23,6 +23,7 @@
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/Value.h"
 #include "llvm/Support/CommandLine.h"
@@ -793,13 +794,21 @@ Instruction *llvm::propagateMetadata(Instruction *Inst, ArrayRef<Value *> VL) {
   for (auto Kind : {LLVMContext::MD_tbaa, LLVMContext::MD_alias_scope,
                     LLVMContext::MD_noalias, LLVMContext::MD_fpmath,
                     LLVMContext::MD_nontemporal, LLVMContext::MD_invariant_load,
-                    LLVMContext::MD_access_group}) {
+                    LLVMContext::MD_access_group, LLVMContext::MD_MMRA}) {
     MDNode *MD = I0->getMetadata(Kind);
+    if (Kind == LLVMContext::MD_MMRA && !MD)
+      continue;
 
     for (int J = 1, E = VL.size(); MD && J != E; ++J) {
       const Instruction *IJ = cast<Instruction>(VL[J]);
       MDNode *IMD = IJ->getMetadata(Kind);
       switch (Kind) {
+      case LLVMContext::MD_MMRA: {
+        auto Tags = MMRAMetadata(dyn_cast_or_null<MDTuple>(MD));
+        auto ITags = MMRAMetadata(dyn_cast_or_null<MDTuple>(IMD));
+        MD = Tags.combine(ITags).getAsMD(Inst->getContext());
+        break;
+      }
       case LLVMContext::MD_tbaa:
         MD = MDNode::getMostGenericTBAA(MD, IMD);
         break;
diff --git a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
index 662de0f3fe0e5..c56f1037e0a7c 100644
--- a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -3253,6 +3253,7 @@ void IRTranslator::finishPendingPhis() {
 bool IRTranslator::translate(const Instruction &Inst) {
   CurBuilder->setDebugLoc(Inst.getDebugLoc());
   CurBuilder->setPCSections(Inst.getMetadata(LLVMContext::MD_pcsections));
+  CurBuilder->setMMRAMetadata(Inst.getMetadata(LLVMContext::MD_MMRA));
 
   auto &TLI = *MF->getSubtarget().getTargetLowering();
   if (TLI.fallBackToDAGISel(Inst))
diff --git a/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp b/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
index a5827c26c04f4..5bf6c639c0934 100644
--- a/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
@@ -28,6 +28,7 @@ void MachineIRBuilder::setMF(MachineFunction &MF) {
   State.TII = MF.getSubtarget().getInstrInfo();
   State.DL = DebugLoc();
   State.PCSections = nullptr;
+  State.MMRA = nullptr;
   State.II = MachineBasicBlock::iterator();
   State.Observer = nullptr;
 }
@@ -37,7 +38,8 @@ void MachineIRBuilder::setMF(MachineFunction &MF) {
 //------------------------------------------------------------------------------
 
 MachineInstrBuilder MachineIRBuilder::buildInstrNoInsert(unsigned Opcode) {
-  return BuildMI(getMF(), {getDL(), getPCSections()}, getTII().get(Opcode));
+  return BuildMI(getMF(), {getDL(), getPCSections(), getMMRAMetadata()},
+                 getTII().get(Opcode));
 }
 
 MachineInstrBuilder MachineIRBuilder::insertInstr(MachineInstrBuilder MIB) {
diff --git a/llvm/lib/CodeGen/MIRPrinter.cpp b/llvm/lib/CodeGen/MIRPrinter.cpp
index fee237104022e..0b0eafe434b95 100644
--- a/llvm/lib/CodeGen/MIRPrinter.cpp
+++ b/llvm/lib/CodeGen/MIRPrinter.cpp
@@ -849,6 +849,13 @@ void MIPrinter::print(const MachineInstr &MI) {
     PCSections->printAsOperand(OS, MST);
     NeedComma = true;
   }
+  if (MDNode *MMRA = MI.getMMRAMetadata()) {
+    if (NeedComma)
+      OS << ',';
+    OS << " mmra ";
+    MMRA->printAsOperand(OS, MST);
+    NeedComma = true;
+  }
   if (uint32_t CFIType = MI.getCFIType()) {
     if (NeedComma)
       OS << ',';
diff --git a/llvm/lib/CodeGen/MachineFunction.cpp b/llvm/lib/CodeGen/MachineFunction.cpp
index 57af571ed9bfd..ebdbfcc22d5f6 100644
--- a/llvm/lib/CodeGen/MachineFunction.cpp
+++ b/llvm/lib/CodeGen/MachineFunction.cpp
@@ -562,10 +562,10 @@ MachineFunction::getMachineMemOperand(const MachineMemOperand *MMO,
 MachineInstr::ExtraInfo *MachineFunction::createMIExtraInfo(
     ArrayRef<MachineMemOperand *> MMOs, MCSymbol *PreInstrSymbol,
     MCSymbol *PostInstrSymbol, MDNode *HeapAllocMarker, MDNode *PCSections,
-    uint32_t CFIType) {
+    uint32_t CFIType, MDNode *MMRAs) {
   return MachineInstr::ExtraInfo::create(Allocator, MMOs, PreInstrSymbol,
                                          PostInstrSymbol, HeapAllocMarker,
-                                         PCSections, CFIType);
+                                         PCSections, CFIType, MMRAs);
 }
 
 const char *MachineFunction::createExternalSymbolName(StringRef Name) {
diff --git a/llvm/lib/CodeGen/MachineInstr.cpp b/llvm/lib/CodeGen/MachineInstr.cpp
index 27eae372f8ad7..6aea88ccd3820 100644
--- a/llvm/lib/CodeGen/MachineInstr.cpp
+++ b/llvm/lib/CodeGen/MachineInstr.cpp
@@ -317,14 +317,15 @@ void MachineInstr::setExtraInfo(MachineFunction &MF,
                                 MCSymbol *PreInstrSymbol,
                                 MCSymbol *PostInstrSymbol,
                                 MDNode *HeapAllocMarker, MDNode *PCSections,
-                                uint32_t CFIType) {
+                                uint32_t CFIType, MDNode *MMRAs) {
   bool HasPreInstrSymbol = PreInstrSymbol != nullptr;
   bool HasPostInstrSymbol = PostInstrSymbol != nullptr;
   bool HasHeapAllocMarker = HeapAllocMarker != nullptr;
   bool HasPCSections = PCSections != nullptr;
   bool HasCFIType = CFIType != 0;
+  bool HasMMRAs = MMRAs != nullptr;
   int NumPointers = MMOs.size() + HasPreInstrSymbol + HasPostInstrSymbol +
-                    HasHeapAllocMarker + HasPCSections + HasCFIType;
+                    HasHeapAllocMarker + HasPCSections + HasCFIType + HasMMRAs;
 
   // Drop all extra info if there is none.
   if (NumPointers <= 0) {
@@ -336,11 +337,11 @@ void MachineInstr::setExtraInfo(MachineFunction &MF,
   // out of line because PointerSumType cannot hold more than 4 tag types with
   // 32-bit pointers.
   // FIXME: Maybe we should make the symbols in the extra info mutable?
-  else if (NumPointers > 1 || HasHeapAllocMarker || HasPCSections ||
+  else if (NumPointers > 1 || HasMMRAs || HasHeapAllocMarker || HasPCSections ||
            HasCFIType) {
     Info.set<EIIK_OutOfLine>(
         MF.createMIExtraInfo(MMOs, PreInstrSymbol, PostInstrSymbol,
-                             HeapAllocMarker, PCSections, CFIType));
+                             HeapAllocMarker, PCSections, CFIType, MMRAs));
     return;
   }
 
@@ -358,7 +359,8 @@ void MachineInstr::dropMemRefs(MachineFunction &MF) {
     return;
 
   setExtraInfo(MF, {}, getPreInstrSymbol(), getPostInstrSymbol(),
-               getHeapAllocMarker(), getPCSections(), getCFIType());
+               getHeapAllocMarker(), getPCSections(), getCFIType(),
+               getMMRAMetadata());
 }
 
 void MachineInstr::setMemRefs(MachineFunction &MF,
@@ -369,7 +371,8 @@ void MachineInstr::setMemRefs(MachineFunction &MF,
   }
 
   setExtraInfo(MF, MMOs, getPreInstrSymbol(), getPostInstrSymbol(),
-               getHeapAllocMarker(), getPCSections(), getCFIType());
+               getHeapAllocMarker(), getPCSections(), getCFIType(),
+               getMMRAMetadata());
 }
 
 void MachineInstr::addMemOperand(MachineFunction &MF,
@@ -393,7 +396,8 @@ void MachineInstr::cloneMemRefs(MachineFunction &MF, const MachineInstr &MI) {
   if (getPreInstrSymbol() == MI.getPreInstrSymbol() &&
       getPostInstrSymbol() == MI.getPostInstrSymbol() &&
       getHeapAllocMarker() == MI.getHeapAllocMarker() &&
-      getPCSections() == MI.getPCSections()) {
+      getPCSections() == MI.getPCSections() && getMMRAMetadata() &&
+      MI.getMMRAMetadata()) {
     Info = MI.Info;
     return;
   }
@@ -478,7 +482,8 @@ void MachineInstr::setPreInstrSymbol(MachineFunction &MF, MCSymbol *Symbol) {
   }
 
   setExtraInfo(MF, memoperands(), Symbol, getPostInstrSymbol(),
-               getHeapAllocMarker(), getPCSections(), getCFIType());
+               getHeapAllocMarker(), getPCSections(), getCFIType(),
+               getMMRAMetadata());
 }
 
 void MachineInstr::setPostInstrSymbol(MachineFunction &MF, MCSymbol *Symbol) {
@@ -493,7 +498,8 @@ void MachineInstr::setPostInstrSymbol(MachineFunction &MF, MCSymbol *Symbol) {
   }
 
   setExtraInfo(MF, memoperands(), getPreInstrSymbol(), Symbol,
-               getHeapAllocMarker(), getPCSections(), getCFIType());
+               getHeapAllocMarker(), getPCSections(), getCFIType(),
+               getMMRAMetadata());
 }
 
 void MachineInstr::setHeapAllocMarker(MachineFunction &MF, MDNode *Marker) {
@@ -502,7 +508,7 @@ void MachineInstr::setHeapAllocMarker(MachineFunction &MF, MDNode *Marker) {
     return;
 
   setExtraInfo(MF, memoperands(), getPreInstrSymbol(), getPostInstrSymbol(),
-               Marker, getPCSections(), getCFIType());
+               Marker, getPCSections(), getCFIType(), getMMRAMetadata());
 }
 
 void MachineInstr::setPCSections(MachineFunction &MF, MDNode *PCSections) {
@@ -511,7 +517,8 @@ void MachineInstr::setPCSections(MachineFunction &MF, MDNode *PCSections) {
     return;
 
   setExtraInfo(MF, memoperands(), getPreInstrSymbol(), getPostInstrSymbol(),
-               getHeapAllocMarker(), PCSections, getCFIType());
+               getHeapAllocMarker(), PCSections, getCFIType(),
+               getMMRAMetadata());
 }
 
 void MachineInstr::setCFIType(MachineFunction &MF, uint32_t Type) {
@@ -520,7 +527,16 @@ void MachineInstr::setCFIType(MachineFunction &MF, uint32_t Type) {
     return;
 
   setExtraInfo(MF, memoperands(), getPreInstrSymbol(), getPostInstrSymbol(),
-               getHeapAllocMarker(), getPCSections(), Type);
+               getHeapAllocMarker(), getPCSections(), Type, getMMRAMetadata());
+}
+
+void MachineInstr::setMMRAMetadata(MachineFunction &MF, MDNode *MMRAs) {
+  // Do nothing if old and new symbols are the same.
+  if (MMRAs == getMMRAMetadata())
+    return;
+
+  setExtraInfo(MF, memoperands(), getPreInstrSymbol(), getPostInstrSymbol(),
+               getHeapAllocMarker(), getPCSections(), getCFIType(), MMRAs);
 }
 
 void MachineInstr::cloneInstrSymbols(MachineFunction &MF,
@@ -536,6 +552,7 @@ void MachineInstr::cloneInstrSymbols(MachineFunction &MF,
   setPostInstrSymbol(MF, MI.getPostInstrSymbol());
   setHeapAllocMarker(MF, MI.getHeapAllocMarker());
   setPCSections(MF, MI.getPCSections());
+  setMMRAMetadata(MF, MI.getMMRAMetadata());
 }
 
 uint32_t MachineInstr::mergeFlagsWith(const MachineInstr &Other) const {
@@ -1843,6 +1860,14 @@ void MachineInstr::print(raw_ostream &OS, ModuleSlotTracker &MST,
     OS << " pcsections ";
     PCSections->printAsOperand(OS, MST);
   }
+  if (MDNode *MMRA = getMMRAMetadata()) {
+    if (!FirstOp) {
+      FirstOp = false;
+      OS << ',';
+    }
+    OS << " mmra ";
+    MMRA->printAsOperand(OS, MST);
+  }
   if (uint32_t CFIType = getCFIType()) {
     if (!FirstOp)
       OS << ',';
diff --git a/llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp b/llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp
index c9e2745f00c95..6a683720e5a04 100644
--- a/llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp
@@ -27,6 +27,7 @@
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/Config/llvm-config.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/MC/MCInstrItineraries.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
@@ -898,6 +899,14 @@ EmitSchedule(MachineBasicBlock::iterator &InsertPos) {
     if (MDNode *MD = DAG->getPCSections(Node))
       MI->setPCSections(MF, MD);
 
+    // Set MMRAs on _all_ added instructions.
+    if (MDNode *MMRA = DAG->getMMRAMetadata(Node)) {
+      for (MachineBasicBlock::iterator It = MI->getIterator(),
+                                       End = std::next(After);
+           It != End; ++It)
+        It->setMMRAMetadata(MF, MMRA);
+    }
+
     return MI;
   };
 
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 2123f3d3b6885..69126513b2cf6 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -12917,7 +12917,7 @@ void SelectionDAG::copyExtraInfo(SDNode *From, SDNode *To) {
   // Use of operator[] on the DenseMap may cause an insertion, which invalidates
   // the iterator, hence the need to make a copy to prevent a use-after-free.
   NodeExtraInfo NEI = I->second;
-  if (LLVM_LIKELY(!NEI.PCSections)) {
+  if (LLVM_LIKELY(!NEI.PCSections) && LLVM_LIKELY(!NEI.MMRA)) {
     // No deep copy required for the types of extra info set.
     //
     // FIXME: Investigate if other types of extra info also need deep copy. This
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 2c477b9474300..069e345b7c350 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -79,6 +79,7 @@
 #include "llvm/IR/IntrinsicsAMDGPU.h"
 #include "llvm/IR/IntrinsicsWebAssembly.h"
 #include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
@@ -1289,7 +1290,8 @@ void SelectionDAGBuilder::visit(const Instruction &I) {
   bool NodeInserted = false;
   std::unique_ptr<SelectionDAG::DAGNodeInsertedListener> InsertedListener;
   MDNode *PCSectionsMD = I.getMetadata(LLVMContext::MD_pcsections);
-  if (PCSectionsMD) {
+  MDNode *MMRA = I.getMetadata(LLVMContext::MD_MMRA);
+  if (PCSectionsMD || MMRA) {
     InsertedListener = std::make_unique<SelectionDAG::DAGNodeInsertedListener>(
         DAG, [&](SDNode *) { NodeInserted = true; });
   }
@@ -1301,14 +1303,17 @@ void SelectionDAGBuilder::visit(const Instruction &I) {
     CopyToExportRegsIfNeeded(&I);
 
   // Handle metadata.
-  if (PCSectionsMD) {
+  if (PCSectionsMD || MMRA) {
     auto It = NodeMap.find(&I);
     if (It != NodeMap.end()) {
-      DAG.addPCSections(It->second.getNode(), PCSectionsMD);
+      if (PCSectionsMD)
+        DAG.addPCSections(It->second.getNode(), PCSectionsMD);
+      if (MMRA)
+        DAG.addMMRAMetadata(It->second.getNode(), MMRA);
     } else if (NodeInserted) {
       // This should not happen; if it does, don't let it go unnoticed so we can
       // fix it. Relevant visit*() function is probably missing a setValue().
-      errs() << "warning: loosing !pcsections metadata ["
+      errs() << "warning: loosing !pcsections and/or !mmra metadata ["
              << I.getModule()->getName() << "]\n";
       LLVM_DEBUG(I.dump());
       assert(false);
@@ -5082,9 +5087,9 @@ void SelectionDAGBuilder::visitTargetIntrinsic(const CallInst &I,
       Result =
           DAG.getAssertAlign(getCurSDLoc(), Result, Alignment.valueOrOne());
     }
-
-    setValue(&I, Result);
   }
+
+  setValue(&I, Result);
 }
 
 /// GetSignificand - Get the significand and build it into a floating-point
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
index 9ebef642e4232..de8b29a88da41 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
@@ -879,6 +879,13 @@ void SDNode::print_details(raw_ostream &OS, const SelectionDAG *G) const {
       MD->printAsOperand(OS, G->getMachineFunction().getFunction().getParent());
       OS << ']';
     }
+
+    if (MDNode *MMRA = G ? G->getMMRAMetadata(this) : nullptr) {
+      OS << " [mmra ";
+      MMRA->printAsOperand(OS,
+                           G->getMachineFunction().getFunction().getParent());
+      OS << ']';
+    }
   }
 }
 
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index 359d738d2ca09..a897715dfa488 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -1059,6 +1059,8 @@ class ISelUpdater : public SelectionDAG::DAGUpdateListener {
     SDNode *CurNode = &*ISelPosition;
     if (MDNode *MD = DAG.getPCSections(CurNode))
       DAG.addPCSections(N, MD);
+    if (MDNode *MMRA = DAG.getMMRAMetadata(CurNode))
+      DAG.addMMRAMetadata(N, MMRA);
   }
 };
 
diff --git a/llvm/lib/IR/CMakeLists.txt b/llvm/lib/IR/CMakeLists.txt
index f1668ee3be63b..b5fb7409d8e88 100644
--- a/llvm/lib/IR/CMakeLists.txt
+++ b/llvm/lib/IR/CMakeLists.txt
@@ -41,6 +41,7 @@ add_llvm_component_library(LLVMCore
   LLVMRemarkStreamer.cpp
   LegacyPassManager.cpp
   MDBuilder.cpp
+  MemoryModelRelaxationAnnotations.cpp
   Mangler.cpp
   Metadata.cpp
   Module.cpp
diff --git a/llvm/lib/IR/Instruction.cpp b/llvm/lib/IR/Instruction.cpp
index 717e33f1857b8..f7703ad8923d9 100644
--- a/llvm/lib/IR/Instruction.cpp
+++ b/llvm/lib/IR/Instruction.cpp
@@ -17,6 +17,7 @@
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Operator.h"
 #include "llvm/IR/ProfDataUtils.h"
 #include "llvm/IR/Type.h"
@@ -433,7 +434,8 @@ void Instruction::dropUBImplyingAttrsAndMetadata() {
   // !noundef and various AA metadata must be dropped, as it generally produces
   // immediate undefined behavior.
   unsigned KnownIDs[] = {LLVMContext::MD_annotation, LLVMContext::MD_range,
-                         LLVMContext::MD_nonnull, LLVMContext::MD_align};
+                         LLVMContext::MD_nonnull, LLVMContext::MD_align,
+                         LLVMContext::MD_MMRA};
   dropUBImplyingAttrsAndUnknownMetadata(KnownIDs);
 }
 
@@ -698,6 +700,16 @@ bool Instruction::hasSameSpecialState(const Instruction *I2,
   assert(I1->getOpcode() == I2->getOpcode() &&
          "Can not compare special state of different instructions");
 
+  // MMRAs may change semantics of an operation, e.g. make a fence only
+  // affect a given address space.
+  //
+  // FIXME: Not sure if this stinks or not. Maybe we should just look at
+  // all callers and make them check MMRAs.
+  // OTOH, MMRAs can really alter semantics so this is technically correct
+  // (the best kind of correct).
+  if (MMRAMetadata(*this) != MMRAMetadata(*I2))
+    return false;
+
   if (const AllocaInst *AI = dyn_cast<AllocaInst>(I1))
     return AI->getAllocatedType() == cast<AllocaInst>(I2)->getAllocatedType() &&
            (AI->getAlign() == cast<AllocaInst>(I2)->getAlign() ||
diff --git a/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp b/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp
new file mode 100644
index 0000000000000..40ec80a1b70ad
--- /dev/null
+++ b/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp
@@ -0,0 +1,201 @@
+//===- MemoryModelRelaxationAnnotations.cpp ---------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
+#include "llvm/ADT/StringSet.h"
+#include "llvm/IR/Metadata.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+
+// FIXME: Only needed for canInstructionHaveMMRAs, should it move to another
+// file?
+#include "llvm/IR/Instructions.h"
+
+using namespace llvm;
+
+static bool isTagMD(const MDNode *MD) {
+  return isa<MDTuple>(MD) && MD->getNumOperands() == 2 &&
+         isa<MDString>(MD->getOperand(0)) && isa<MDString>(MD->getOperand(1));
+}
+
+MMRAMetadata::MMRAMetadata(const Instruction &I)
+    : MMRAMetadata(I.getMetadata(LLVMContext::MD_MMRA)) {}
+
+MMRAMetadata::MMRAMetadata(MDNode *MD) {
+  if (!MD)
+    return;
+
+  // TODO: Split this into a "tryParse" function that can return an err.
+  // CTor can use the tryParse & just fatal on err.
+
+  MDTuple *Tuple = dyn_cast<MDTuple>(MD);
+  if (!Tuple)
+    report_fatal_error("MMRAs should always be MDTuples!");
+
+  const auto HandleTagMD = [this](MDNode *TagMD) {
+    addTag(cast<MDString>(TagMD->getOperand(0))->getString(),
+           cast<MDString>(TagMD->getOperand(1))->getString());
+  };
+
+  if (isTagMD(Tuple)) {
+    HandleTagMD(Tuple);
+    return;
+  }
+
+  for (const MDOperand &Op : Tuple->operands()) {
+    MDNode *MDOp = cast<MDNode>(Op.get());
+    if (!isTagMD(MDOp)) {
+      errs() << "MD Node:\n";
+      MD->print(errs());
+      errs() << "Operand:\n";
+      Op->print(errs());
+      report_fatal_error("Invalid MMRA Metadata Structure!");
+    }
+
+    HandleTagMD(MDOp);
+  }
+}
+
+bool MMRAMetadata::isCompatibleWith(const MMRAMetadata &Other) const {
+  // Two sets of tags are compatible iff, for every unique tag prefix P
+  // present in at least one set:
+  //   - the other set contains no tag that with the same prefix, or
+  //   - at least one tag with the same prefix is present in both sets.
+
+  // These sets are generally small so we don't bother uniquing
+  // the prefixes beforehand. Checking a prefix twice is likely cheaper
+  // than building a map.
+  for (const auto &[P, S] : Tags) {
+    if (!Other.hasTag(P, S) && Other.hasTagWithPrefix(P))
+      return false;
+  }
+
+  for (const auto &[P, S] : Other) {
+    if (!hasTag(P, S) && hasTagWithPrefix(P))
+      return false;
+  }
+
+  return true;
+}
+
+MMRAMetadata MMRAMetadata::combine(const MMRAMetadata &Other) const {
+  // Let A and B be two tags set, and U be the prefix-wise union of A and B.
+  // For every unique tag prefix P present in A or B:
+  // * If either A or B has no tags with prefix P, no tags with prefix
+  //   P are added to U.
+  // * If both A and B have at least one tag with prefix P, only the tags
+  //   common to A and B are added to U.
+
+  StringSet<> Prefixes;
+  for (const auto &[P, S] : Tags)
+    Prefixes.insert(P);
+  for (const auto &[P, S] : Other)
+    Prefixes.insert(P);
+
+  MMRAMetadata U;
+  for (StringRef P : Prefixes.keys()) {
+    auto A = getAllTagsWithPrefix(P);
+    auto B = Other.getAllTagsWithPrefix(P);
+
+    if (A.empty() || B.empty())
+      continue;
+
+    for (const auto &Tag : A) {
+      if (hasTag(Tag) && Other.hasTag(Tag))
+        U.addTag(Tag);
+    }
+    for (const auto &Tag : B) {
+      if (hasTag(Tag) && Other.hasTag(Tag))
+        U.addTag(Tag);
+    }
+  }
+
+  return U;
+}
+
+MMRAMetadata &MMRAMetadata::addTag(StringRef Prefix, StringRef Suffix) {
+  Tags.insert(std::make_pair(Prefix.str(), Suffix.str()));
+  return *this;
+}
+
+bool MMRAMetadata::hasTag(StringRef Prefix, StringRef Suffix) const {
+  return Tags.count(std::make_pair(Prefix.str(), Suffix.str()));
+}
+
+std::vector<MMRAMetadata::TagT>
+MMRAMetadata::getAllTagsWithPrefix(StringRef Prefix) const {
+  std::vector<TagT> Result;
+  for (const auto &T : Tags) {
+    if (T.first == Prefix)
+      Result.push_back(T);
+  }
+  return Result;
+}
+
+bool MMRAMetadata::hasTagWithPrefix(StringRef Prefix) const {
+  for (const auto &[P, S] : Tags)
+    if (P == Prefix)
+      return true;
+  return false;
+}
+
+MMRAMetadata::const_iterator MMRAMetadata::begin() const {
+  return Tags.begin();
+}
+
+MMRAMetadata::const_iterator MMRAMetadata::end() const { return Tags.end(); }
+
+bool MMRAMetadata::empty() const { return Tags.empty(); }
+
+unsigned MMRAMetadata::size() const { return Tags.size(); }
+
+static MDTuple *getMDForPair(LLVMContext &Ctx, StringRef P, StringRef S) {
+  return MDTuple::get(Ctx, {MDString::get(Ctx, P), MDString::get(Ctx, S)});
+}
+
+MDTuple *MMRAMetadata::getAsMD(LLVMContext &Ctx) const {
+  if (empty())
+    return MDTuple::get(Ctx, {});
+
+  std::vector<Metadata *> TagMDs;
+  TagMDs.reserve(Tags.size());
+
+  for (const auto &[P, S] : Tags)
+    TagMDs.push_back(getMDForPair(Ctx, P, S));
+
+  if (TagMDs.size() == 1)
+    return cast<MDTuple>(TagMDs.front());
+  return MDTuple::get(Ctx, TagMDs);
+}
+
+void MMRAMetadata::print(raw_ostream &OS) const {
+  bool IsFirst = true;
+  // TODO: use map_iter + join
+  for (const auto &[P, S] : Tags) {
+    if (IsFirst)
+      IsFirst = false;
+    else
+      OS << ", ";
+    OS << P << ":" << S;
+  }
+}
+
+LLVM_DUMP_METHOD
+void MMRAMetadata::dump() const { print(dbgs()); }
+
+static bool isReadWriteMemCall(const Instruction &I) {
+  if (const auto *C = dyn_cast<CallBase>(&I))
+    return C->mayReadOrWriteMemory() ||
+           !C->getMemoryEffects().doesNotAccessMemory();
+  return false;
+}
+
+bool llvm::canInstructionHaveMMRAs(const Instruction &I) {
+  return isa<LoadInst>(I) || isa<StoreInst>(I) || isa<AtomicCmpXchgInst>(I) ||
+         isa<AtomicRMWInst>(I) || isa<FenceInst>(I) || isReadWriteMemCall(I);
+}
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index 7ee658c71d3df..5d84975c14327 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -99,6 +99,7 @@
 #include "llvm/IR/IntrinsicsNVPTX.h"
 #include "llvm/IR/IntrinsicsWebAssembly.h"
 #include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/ModuleSlotTracker.h"
@@ -116,6 +117,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
+#include "llvm/Support/ModRef.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
@@ -502,6 +504,7 @@ class Verifier : public InstVisitor<Verifier>, VerifierSupport {
   void visitMemProfMetadata(Instruction &I, MDNode *MD);
   void visitCallsiteMetadata(Instruction &I, MDNode *MD);
   void visitDIAssignIDMetadata(Instruction &I, MDNode *MD);
+  void visitMMRAMetadata(Instruction &I, MDNode *MD);
   void visitAnnotationMetadata(MDNode *Annotation);
   void visitAliasScopeMetadata(const MDNode *MD);
   void visitAliasScopeListMetadata(const MDNode *MD);
@@ -4742,6 +4745,27 @@ void Verifier::visitDIAssignIDMetadata(Instruction &I, MDNode *MD) {
   }
 }
 
+void Verifier::visitMMRAMetadata(Instruction &I, MDNode *MD) {
+  Check(canInstructionHaveMMRAs(I),
+        "!mmra metadata attached to unexpected instruction kind", I, MD);
+
+  const auto IsLeaf = [](const Metadata *CurMD) {
+    const MDNode *Tuple = dyn_cast<MDTuple>(CurMD);
+    return Tuple && Tuple->getNumOperands() == 2 &&
+           isa<MDString>(Tuple->getOperand(0)) &&
+           isa<MDString>(Tuple->getOperand(1));
+  };
+
+  // Simple MMRA metadata like !{!"foo", "!bar"} -> ok.
+  if (IsLeaf(MD))
+    return;
+
+  Check(isa<MDTuple>(MD), "!mmra expected to be a metadata tuple", I, MD);
+  for (const MDOperand &MDOp : MD->operands())
+    Check(IsLeaf(MDOp), "!mmra metadata tuple operand is not an MMRA tag", I,
+          MDOp.get());
+}
+
 void Verifier::visitCallStackMetadata(MDNode *MD) {
   // Call stack metadata should consist of a list of at least 1 constant int
   // (representing a hash of the location).
@@ -5054,6 +5078,9 @@ void Verifier::visitInstruction(Instruction &I) {
   if (MDNode *MD = I.getMetadata(LLVMContext::MD_DIAssignID))
     visitDIAssignIDMetadata(I, MD);
 
+  if (MDNode *MMRA = I.getMetadata(LLVMContext::MD_MMRA))
+    visitMMRAMetadata(I, MMRA);
+
   if (MDNode *Annotation = I.getMetadata(LLVMContext::MD_annotation))
     visitAnnotationMetadata(Annotation);
 
diff --git a/llvm/lib/Transforms/Scalar/GVNHoist.cpp b/llvm/lib/Transforms/Scalar/GVNHoist.cpp
index b564f00eb9d16..3318b21c44a77 100644
--- a/llvm/lib/Transforms/Scalar/GVNHoist.cpp
+++ b/llvm/lib/Transforms/Scalar/GVNHoist.cpp
@@ -246,7 +246,8 @@ static void combineKnownMetadata(Instruction *ReplInst, Instruction *I) {
                                       LLVMContext::MD_fpmath,
                                       LLVMContext::MD_invariant_load,
                                       LLVMContext::MD_invariant_group,
-                                      LLVMContext::MD_access_group};
+                                      LLVMContext::MD_access_group,
+                                      LLVMContext::MD_MMRA};
   combineMetadata(ReplInst, I, KnownIDs, true);
 }
 
diff --git a/llvm/lib/Transforms/Utils/FunctionComparator.cpp b/llvm/lib/Transforms/Utils/FunctionComparator.cpp
index 09e19be0d293b..4e60fecd83d8f 100644
--- a/llvm/lib/Transforms/Utils/FunctionComparator.cpp
+++ b/llvm/lib/Transforms/Utils/FunctionComparator.cpp
@@ -31,6 +31,7 @@
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
@@ -641,6 +642,10 @@ int FunctionComparator::cmpOperations(const Instruction *L,
       return Res;
   }
 
+  // TODO: Does this even make sense? Why do we need to return +-1?
+  if (MMRAMetadata LMD(*L), RMD(*R); LMD != RMD)
+    return LMD.size() >= RMD.size() ? 1 : -1;
+
   // Check special state that is a part of some instructions.
   if (const AllocaInst *AI = dyn_cast<AllocaInst>(L)) {
     if (int Res = cmpTypes(AI->getAllocatedType(),
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index d1b42f28923f5..3215c60bf92e3 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -59,6 +59,7 @@
 #include "llvm/IR/IntrinsicsWebAssembly.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/MDBuilder.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/PatternMatch.h"
@@ -3294,6 +3295,17 @@ void llvm::combineMetadata(Instruction *K, const Instruction *J,
   if (auto *JMD = J->getMetadata(LLVMContext::MD_invariant_group))
     if (isa<LoadInst>(K) || isa<StoreInst>(K))
       K->setMetadata(LLVMContext::MD_invariant_group, JMD);
+
+  // Merge MMRAs.
+  // This is handled separately because we also want to handle cases where K
+  // doesn't have tags but J does.
+
+  auto JTags = MMRAMetadata(J->getMetadata(LLVMContext::MD_MMRA));
+  auto KTags = MMRAMetadata(K->getMetadata(LLVMContext::MD_MMRA));
+  if (JTags || KTags) {
+    K->setMetadata(LLVMContext::MD_MMRA,
+                   JTags.combine(KTags).getAsMD(K->getContext()));
+  }
 }
 
 void llvm::combineMetadataForCSE(Instruction *K, const Instruction *J,
@@ -3313,7 +3325,8 @@ void llvm::combineMetadataForCSE(Instruction *K, const Instruction *J,
                          LLVMContext::MD_preserve_access_index,
                          LLVMContext::MD_prof,
                          LLVMContext::MD_nontemporal,
-                         LLVMContext::MD_noundef};
+                         LLVMContext::MD_noundef,
+                         LLVMContext::MD_MMRA};
   combineMetadata(K, J, KnownIDs, KDominatesJ);
 }
 
diff --git a/llvm/test/CodeGen/AMDGPU/GlobalISel/mmra.ll b/llvm/test/CodeGen/AMDGPU/GlobalISel/mmra.ll
new file mode 100644
index 0000000000000..5270f3d20bc84
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/GlobalISel/mmra.ll
@@ -0,0 +1,49 @@
+; NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 2
+; RUN: llc -global-isel -march=amdgcn -mcpu=gfx900 -stop-after=finalize-isel < %s | FileCheck %s
+
+define void @readsMem(ptr %ptr) #0 {
+  ; CHECK-LABEL: name: readsMem
+  ; CHECK: bb.1 (%ir-block.0):
+  ; CHECK-NEXT:   liveins: $vgpr0, $vgpr1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   SI_RETURN
+  ret void
+}
+
+define void @writesMem(ptr %ptr) #1 {
+  ; CHECK-LABEL: name: writesMem
+  ; CHECK: bb.1 (%ir-block.0):
+  ; CHECK-NEXT:   liveins: $vgpr0, $vgpr1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   SI_RETURN
+  ret void
+}
+
+define void @fence_loads(ptr %ptr) {
+  ; CHECK-LABEL: name: fence_loads
+  ; CHECK: bb.1 (%ir-block.0):
+  ; CHECK-NEXT:   liveins: $vgpr0, $vgpr1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:vgpr_32 = COPY $vgpr1
+  ; CHECK-NEXT:   [[REG_SEQUENCE:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[COPY]], %subreg.sub0, [[COPY1]], %subreg.sub1
+  ; CHECK-NEXT:   ATOMIC_FENCE 5, 1, mmra !0
+  ; CHECK-NEXT:   [[FLAT_LOAD_UBYTE:%[0-9]+]]:vgpr_32 = FLAT_LOAD_UBYTE [[REG_SEQUENCE]], 0, 0, implicit $exec, implicit $flat_scr, mmra !1 :: (load acquire (s8) from %ir.ptr, align 4)
+  ; CHECK-NEXT:   [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 1
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:vgpr_32 = COPY [[S_MOV_B32_]]
+  ; CHECK-NEXT:   FLAT_STORE_BYTE [[REG_SEQUENCE]], [[COPY2]], 0, 0, implicit $exec, implicit $flat_scr, mmra !2 :: (store release (s8) into %ir.ptr, align 4)
+  ; CHECK-NEXT:   SI_RETURN
+  fence release,                                        !mmra !0
+  %ld = load atomic i8, ptr %ptr acquire, align 4,      !mmra !2
+  store atomic i8 1, ptr %ptr release, align 4,         !mmra !1
+  ret void
+}
+
+; TODO: test atomicrmw, cmpxchg - current lowering doesn't work and blows up on i1 PHIs.
+
+attributes #0 = { memory(read) }
+attributes #1 = { memory(write) }
+
+!0 = !{!"foo", !"bar"}
+!1 = !{!"bux", !"baz"}
+!2 = !{!0, !1}
diff --git a/llvm/test/CodeGen/AMDGPU/mmra.ll b/llvm/test/CodeGen/AMDGPU/mmra.ll
new file mode 100644
index 0000000000000..55654d600c50e
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/mmra.ll
@@ -0,0 +1,254 @@
+; NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 2
+; RUN: llc -march=amdgcn -mcpu=gfx900 -stop-after=finalize-isel < %s | FileCheck %s
+
+define void @readsMem(ptr %ptr) #0 {
+  ; CHECK-LABEL: name: readsMem
+  ; CHECK: bb.0 (%ir-block.0):
+  ; CHECK-NEXT:   SI_RETURN
+  ret void
+}
+
+define void @writesMem(ptr %ptr) #1 {
+  ; CHECK-LABEL: name: writesMem
+  ; CHECK: bb.0 (%ir-block.0):
+  ; CHECK-NEXT:   SI_RETURN
+  ret void
+}
+
+define void @fence_loads(ptr %ptr) {
+  ; CHECK-LABEL: name: fence_loads
+  ; CHECK: bb.0 (%ir-block.0):
+  ; CHECK-NEXT:   liveins: $vgpr0, $vgpr1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr1
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+  ; CHECK-NEXT:   [[DEF:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF1:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[REG_SEQUENCE:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[COPY1]], %subreg.sub0, [[COPY]], %subreg.sub1
+  ; CHECK-NEXT:   ATOMIC_FENCE 5, 1, mmra !0
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE]], mmra !1
+  ; CHECK-NEXT:   [[FLAT_LOAD_UBYTE:%[0-9]+]]:vgpr_32 = FLAT_LOAD_UBYTE [[COPY2]], 0, 0, implicit $exec, implicit $flat_scr, mmra !1 :: (load acquire (s8) from %ir.ptr, align 4)
+  ; CHECK-NEXT:   [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 1, mmra !2
+  ; CHECK-NEXT:   [[COPY3:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE]], mmra !2
+  ; CHECK-NEXT:   [[COPY4:%[0-9]+]]:vgpr_32 = COPY [[S_MOV_B32_]], mmra !2
+  ; CHECK-NEXT:   FLAT_STORE_BYTE [[COPY3]], killed [[COPY4]], 0, 0, implicit $exec, implicit $flat_scr, mmra !2 :: (store release (s8) into %ir.ptr, align 4)
+  ; CHECK-NEXT:   SI_RETURN
+  fence release,                                        !mmra !0
+  %ld = load atomic i8, ptr %ptr acquire, align 4,      !mmra !2
+  store atomic i8 1, ptr %ptr release, align 4,         !mmra !1
+  ret void
+}
+
+define void @atomicrmw_rel(ptr %ptr) {
+  ; CHECK-LABEL: name: atomicrmw_rel
+  ; CHECK: bb.0 (%ir-block.0):
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $vgpr0, $vgpr1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr1
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+  ; CHECK-NEXT:   [[DEF:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF1:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[REG_SEQUENCE:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[COPY1]], %subreg.sub0, [[COPY]], %subreg.sub1
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:vgpr_32 = COPY [[REG_SEQUENCE]].sub1
+  ; CHECK-NEXT:   [[COPY3:%[0-9]+]]:vgpr_32 = COPY [[REG_SEQUENCE]].sub0
+  ; CHECK-NEXT:   [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 -4
+  ; CHECK-NEXT:   [[V_AND_B32_e64_:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 [[COPY3]], killed [[S_MOV_B32_]], implicit $exec
+  ; CHECK-NEXT:   [[DEF2:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF3:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[REG_SEQUENCE1:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[V_AND_B32_e64_]], %subreg.sub0, [[COPY2]], %subreg.sub1
+  ; CHECK-NEXT:   [[COPY4:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE1]]
+  ; CHECK-NEXT:   [[S_MOV_B32_1:%[0-9]+]]:sreg_32 = S_MOV_B32 3
+  ; CHECK-NEXT:   [[V_AND_B32_e64_1:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 [[COPY3]], [[S_MOV_B32_1]], implicit $exec
+  ; CHECK-NEXT:   [[V_LSHLREV_B32_e64_:%[0-9]+]]:vgpr_32 = V_LSHLREV_B32_e64 [[S_MOV_B32_1]], killed [[V_AND_B32_e64_1]], implicit $exec
+  ; CHECK-NEXT:   [[S_MOV_B32_2:%[0-9]+]]:sreg_32 = S_MOV_B32 255
+  ; CHECK-NEXT:   [[V_LSHLREV_B32_e64_1:%[0-9]+]]:vgpr_32 = V_LSHLREV_B32_e64 [[V_LSHLREV_B32_e64_]], killed [[S_MOV_B32_2]], implicit $exec
+  ; CHECK-NEXT:   [[V_NOT_B32_e32_:%[0-9]+]]:vgpr_32 = V_NOT_B32_e32 [[V_LSHLREV_B32_e64_1]], implicit $exec
+  ; CHECK-NEXT:   [[COPY5:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE1]]
+  ; CHECK-NEXT:   [[FLAT_LOAD_DWORD:%[0-9]+]]:vgpr_32 = FLAT_LOAD_DWORD [[COPY5]], 0, 0, implicit $exec, implicit $flat_scr :: (load (s32) from %ir.AlignedAddr)
+  ; CHECK-NEXT:   [[S_MOV_B64_:%[0-9]+]]:sreg_64 = S_MOV_B64 0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1.atomicrmw.start:
+  ; CHECK-NEXT:   successors: %bb.2(0x04000000), %bb.1(0x7c000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[PHI:%[0-9]+]]:sreg_64 = PHI [[S_MOV_B64_]], %bb.0, %8, %bb.1
+  ; CHECK-NEXT:   [[PHI1:%[0-9]+]]:vgpr_32 = PHI [[FLAT_LOAD_DWORD]], %bb.0, %7, %bb.1
+  ; CHECK-NEXT:   [[V_OR_B32_e64_:%[0-9]+]]:vgpr_32 = V_OR_B32_e64 [[V_NOT_B32_e32_]], [[V_LSHLREV_B32_e64_1]], implicit $exec
+  ; CHECK-NEXT:   [[V_AND_B32_e64_2:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 [[PHI1]], killed [[V_OR_B32_e64_]], implicit $exec
+  ; CHECK-NEXT:   [[DEF4:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF5:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[REG_SEQUENCE2:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[V_AND_B32_e64_2]], %subreg.sub0, [[PHI1]], %subreg.sub1
+  ; CHECK-NEXT:   [[COPY6:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE2]]
+  ; CHECK-NEXT:   [[FLAT_ATOMIC_CMPSWAP_RTN:%[0-9]+]]:vgpr_32 = FLAT_ATOMIC_CMPSWAP_RTN [[COPY4]], killed [[COPY6]], 0, 1, implicit $exec, implicit $flat_scr :: (load store release monotonic (s32) on %ir.AlignedAddr)
+  ; CHECK-NEXT:   [[V_CMP_EQ_U32_e64_:%[0-9]+]]:sreg_64 = V_CMP_EQ_U32_e64 [[FLAT_ATOMIC_CMPSWAP_RTN]], [[PHI1]], implicit $exec
+  ; CHECK-NEXT:   [[SI_IF_BREAK:%[0-9]+]]:sreg_64 = SI_IF_BREAK killed [[V_CMP_EQ_U32_e64_]], [[PHI]], implicit-def dead $scc
+  ; CHECK-NEXT:   SI_LOOP [[SI_IF_BREAK]], %bb.1, implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   S_BRANCH %bb.2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2.atomicrmw.end:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[PHI2:%[0-9]+]]:sreg_64 = PHI [[SI_IF_BREAK]], %bb.1
+  ; CHECK-NEXT:   SI_END_CF [[PHI2]], implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   [[S_MOV_B32_3:%[0-9]+]]:sreg_32 = S_MOV_B32 1
+  ; CHECK-NEXT:   [[V_LSHLREV_B32_e64_2:%[0-9]+]]:vgpr_32 = V_LSHLREV_B32_e64 [[V_LSHLREV_B32_e64_]], killed [[S_MOV_B32_3]], implicit $exec
+  ; CHECK-NEXT:   [[FLAT_LOAD_DWORD1:%[0-9]+]]:vgpr_32 = FLAT_LOAD_DWORD [[COPY4]], 0, 0, implicit $exec, implicit $flat_scr :: (load (s32) from %ir.AlignedAddr)
+  ; CHECK-NEXT:   [[V_AND_B32_e64_3:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 killed [[FLAT_LOAD_DWORD1]], [[V_NOT_B32_e32_]], implicit $exec
+  ; CHECK-NEXT:   [[S_MOV_B64_1:%[0-9]+]]:sreg_64 = S_MOV_B64 0
+  ; CHECK-NEXT:   [[DEF6:%[0-9]+]]:sreg_64 = IMPLICIT_DEF
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3.partword.cmpxchg.loop:
+  ; CHECK-NEXT:   successors: %bb.4(0x40000000), %bb.5(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[PHI3:%[0-9]+]]:sreg_64 = PHI [[DEF6]], %bb.2, %19, %bb.5
+  ; CHECK-NEXT:   [[PHI4:%[0-9]+]]:sreg_64 = PHI [[S_MOV_B64_1]], %bb.2, %20, %bb.5
+  ; CHECK-NEXT:   [[PHI5:%[0-9]+]]:vgpr_32 = PHI [[V_AND_B32_e64_3]], %bb.2, %18, %bb.5
+  ; CHECK-NEXT:   [[V_OR_B32_e64_1:%[0-9]+]]:vgpr_32 = V_OR_B32_e64 [[PHI5]], [[V_LSHLREV_B32_e64_2]], implicit $exec
+  ; CHECK-NEXT:   [[DEF7:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF8:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[REG_SEQUENCE3:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[V_OR_B32_e64_1]], %subreg.sub0, [[PHI5]], %subreg.sub1
+  ; CHECK-NEXT:   [[COPY7:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE3]]
+  ; CHECK-NEXT:   [[FLAT_ATOMIC_CMPSWAP_RTN1:%[0-9]+]]:vgpr_32 = FLAT_ATOMIC_CMPSWAP_RTN [[COPY4]], killed [[COPY7]], 0, 1, implicit $exec, implicit $flat_scr :: (load store acquire acquire (s32) on %ir.AlignedAddr)
+  ; CHECK-NEXT:   [[V_CMP_NE_U32_e64_:%[0-9]+]]:sreg_64 = V_CMP_NE_U32_e64 [[FLAT_ATOMIC_CMPSWAP_RTN1]], [[PHI5]], implicit $exec
+  ; CHECK-NEXT:   [[S_MOV_B64_2:%[0-9]+]]:sreg_64 = S_MOV_B64 -1
+  ; CHECK-NEXT:   [[DEF9:%[0-9]+]]:sreg_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[COPY8:%[0-9]+]]:vgpr_32 = COPY [[DEF9]]
+  ; CHECK-NEXT:   [[S_OR_B64_:%[0-9]+]]:sreg_64 = S_OR_B64 [[PHI3]], $exec, implicit-def $scc
+  ; CHECK-NEXT:   [[SI_IF:%[0-9]+]]:sreg_64 = SI_IF killed [[V_CMP_NE_U32_e64_]], %bb.5, implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   S_BRANCH %bb.4
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.4.partword.cmpxchg.failure:
+  ; CHECK-NEXT:   successors: %bb.5(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[V_AND_B32_e64_4:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 [[FLAT_ATOMIC_CMPSWAP_RTN1]], [[V_NOT_B32_e32_]], implicit $exec
+  ; CHECK-NEXT:   [[V_CMP_EQ_U32_e64_1:%[0-9]+]]:sreg_64 = V_CMP_EQ_U32_e64 [[PHI5]], [[V_AND_B32_e64_4]], implicit $exec
+  ; CHECK-NEXT:   [[S_ANDN2_B64_:%[0-9]+]]:sreg_64 = S_ANDN2_B64 [[S_OR_B64_]], $exec, implicit-def $scc
+  ; CHECK-NEXT:   [[S_AND_B64_:%[0-9]+]]:sreg_64 = S_AND_B64 [[V_CMP_EQ_U32_e64_1]], $exec, implicit-def $scc
+  ; CHECK-NEXT:   [[S_OR_B64_1:%[0-9]+]]:sreg_64 = S_OR_B64 [[S_ANDN2_B64_]], [[S_AND_B64_]], implicit-def $scc
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.5.Flow:
+  ; CHECK-NEXT:   successors: %bb.6(0x04000000), %bb.3(0x7c000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[PHI6:%[0-9]+]]:sreg_64 = PHI [[S_OR_B64_]], %bb.3, [[S_OR_B64_1]], %bb.4
+  ; CHECK-NEXT:   [[PHI7:%[0-9]+]]:vgpr_32 = PHI [[COPY8]], %bb.3, [[V_AND_B32_e64_4]], %bb.4
+  ; CHECK-NEXT:   SI_END_CF [[SI_IF]], implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   [[COPY9:%[0-9]+]]:sreg_64 = COPY [[PHI6]]
+  ; CHECK-NEXT:   [[SI_IF_BREAK1:%[0-9]+]]:sreg_64 = SI_IF_BREAK [[COPY9]], [[PHI4]], implicit-def dead $scc
+  ; CHECK-NEXT:   SI_LOOP [[SI_IF_BREAK1]], %bb.3, implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   S_BRANCH %bb.6
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.6.partword.cmpxchg.end:
+  ; CHECK-NEXT:   [[PHI8:%[0-9]+]]:sreg_64 = PHI [[SI_IF_BREAK1]], %bb.5
+  ; CHECK-NEXT:   [[PHI9:%[0-9]+]]:vgpr_32 = PHI [[FLAT_ATOMIC_CMPSWAP_RTN1]], %bb.5
+  ; CHECK-NEXT:   SI_END_CF [[PHI8]], implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   SI_RETURN
+  %old = atomicrmw add ptr %ptr, i8 0 release,          !mmra !2
+  %pair = cmpxchg ptr %ptr, i8 0, i8 1 acquire acquire, !mmra !2
+  ret void
+}
+
+define void @atomicrmw_acq(ptr %ptr) {
+  ; CHECK-LABEL: name: atomicrmw_acq
+  ; CHECK: bb.0 (%ir-block.0):
+  ; CHECK-NEXT:   liveins: $vgpr0, $vgpr1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr1
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+  ; CHECK-NEXT:   [[DEF:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF1:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[REG_SEQUENCE:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[COPY1]], %subreg.sub0, [[COPY]], %subreg.sub1
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE]], mmra !1
+  ; CHECK-NEXT:   [[FLAT_LOAD_UBYTE:%[0-9]+]]:vgpr_32 = FLAT_LOAD_UBYTE killed [[COPY2]], 0, 0, implicit $exec, implicit $flat_scr, mmra !1 :: (load acquire (s8) from %ir.ptr)
+  ; CHECK-NEXT:   SI_RETURN
+  %old.2 = atomicrmw add ptr %ptr, i8 0 acquire,        !mmra !2
+  ret void
+}
+
+define void @cmpxchg(ptr %ptr) {
+  ; CHECK-LABEL: name: cmpxchg
+  ; CHECK: bb.0 (%ir-block.0):
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $vgpr0, $vgpr1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr1
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+  ; CHECK-NEXT:   [[DEF:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF1:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[REG_SEQUENCE:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[COPY1]], %subreg.sub0, [[COPY]], %subreg.sub1
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:vgpr_32 = COPY [[REG_SEQUENCE]].sub1
+  ; CHECK-NEXT:   [[COPY3:%[0-9]+]]:vgpr_32 = COPY [[REG_SEQUENCE]].sub0
+  ; CHECK-NEXT:   [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 -4
+  ; CHECK-NEXT:   [[V_AND_B32_e64_:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 [[COPY3]], killed [[S_MOV_B32_]], implicit $exec
+  ; CHECK-NEXT:   [[DEF2:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF3:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[REG_SEQUENCE1:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[V_AND_B32_e64_]], %subreg.sub0, [[COPY2]], %subreg.sub1
+  ; CHECK-NEXT:   [[COPY4:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE1]]
+  ; CHECK-NEXT:   [[S_MOV_B32_1:%[0-9]+]]:sreg_32 = S_MOV_B32 3
+  ; CHECK-NEXT:   [[V_AND_B32_e64_1:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 [[COPY3]], [[S_MOV_B32_1]], implicit $exec
+  ; CHECK-NEXT:   [[V_LSHLREV_B32_e64_:%[0-9]+]]:vgpr_32 = V_LSHLREV_B32_e64 [[S_MOV_B32_1]], killed [[V_AND_B32_e64_1]], implicit $exec
+  ; CHECK-NEXT:   [[S_MOV_B32_2:%[0-9]+]]:sreg_32 = S_MOV_B32 255
+  ; CHECK-NEXT:   [[V_LSHLREV_B32_e64_1:%[0-9]+]]:vgpr_32 = V_LSHLREV_B32_e64 [[V_LSHLREV_B32_e64_]], killed [[S_MOV_B32_2]], implicit $exec
+  ; CHECK-NEXT:   [[V_NOT_B32_e32_:%[0-9]+]]:vgpr_32 = V_NOT_B32_e32 killed [[V_LSHLREV_B32_e64_1]], implicit $exec
+  ; CHECK-NEXT:   [[S_MOV_B32_3:%[0-9]+]]:sreg_32 = S_MOV_B32 1
+  ; CHECK-NEXT:   [[V_LSHLREV_B32_e64_2:%[0-9]+]]:vgpr_32 = V_LSHLREV_B32_e64 [[V_LSHLREV_B32_e64_]], killed [[S_MOV_B32_3]], implicit $exec
+  ; CHECK-NEXT:   [[COPY5:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE1]]
+  ; CHECK-NEXT:   [[FLAT_LOAD_DWORD:%[0-9]+]]:vgpr_32 = FLAT_LOAD_DWORD [[COPY5]], 0, 0, implicit $exec, implicit $flat_scr :: (load (s32) from %ir.AlignedAddr)
+  ; CHECK-NEXT:   [[V_AND_B32_e64_2:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 killed [[FLAT_LOAD_DWORD]], [[V_NOT_B32_e32_]], implicit $exec
+  ; CHECK-NEXT:   [[S_MOV_B64_:%[0-9]+]]:sreg_64 = S_MOV_B64 0
+  ; CHECK-NEXT:   [[DEF4:%[0-9]+]]:sreg_64 = IMPLICIT_DEF
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1.partword.cmpxchg.loop:
+  ; CHECK-NEXT:   successors: %bb.2(0x40000000), %bb.3(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[PHI:%[0-9]+]]:sreg_64 = PHI [[DEF4]], %bb.0, %12, %bb.3
+  ; CHECK-NEXT:   [[PHI1:%[0-9]+]]:sreg_64 = PHI [[S_MOV_B64_]], %bb.0, %13, %bb.3
+  ; CHECK-NEXT:   [[PHI2:%[0-9]+]]:vgpr_32 = PHI [[V_AND_B32_e64_2]], %bb.0, %11, %bb.3
+  ; CHECK-NEXT:   [[V_OR_B32_e64_:%[0-9]+]]:vgpr_32 = V_OR_B32_e64 [[PHI2]], [[V_LSHLREV_B32_e64_2]], implicit $exec
+  ; CHECK-NEXT:   [[DEF5:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF6:%[0-9]+]]:sgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[REG_SEQUENCE2:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[V_OR_B32_e64_]], %subreg.sub0, [[PHI2]], %subreg.sub1
+  ; CHECK-NEXT:   [[COPY6:%[0-9]+]]:vreg_64 = COPY [[REG_SEQUENCE2]]
+  ; CHECK-NEXT:   [[FLAT_ATOMIC_CMPSWAP_RTN:%[0-9]+]]:vgpr_32 = FLAT_ATOMIC_CMPSWAP_RTN [[COPY4]], killed [[COPY6]], 0, 1, implicit $exec, implicit $flat_scr :: (load store acquire acquire (s32) on %ir.AlignedAddr)
+  ; CHECK-NEXT:   [[V_CMP_NE_U32_e64_:%[0-9]+]]:sreg_64 = V_CMP_NE_U32_e64 [[FLAT_ATOMIC_CMPSWAP_RTN]], [[PHI2]], implicit $exec
+  ; CHECK-NEXT:   [[S_MOV_B64_1:%[0-9]+]]:sreg_64 = S_MOV_B64 -1
+  ; CHECK-NEXT:   [[DEF7:%[0-9]+]]:sreg_32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[COPY7:%[0-9]+]]:vgpr_32 = COPY [[DEF7]]
+  ; CHECK-NEXT:   [[S_OR_B64_:%[0-9]+]]:sreg_64 = S_OR_B64 [[PHI]], $exec, implicit-def $scc
+  ; CHECK-NEXT:   [[SI_IF:%[0-9]+]]:sreg_64 = SI_IF killed [[V_CMP_NE_U32_e64_]], %bb.3, implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   S_BRANCH %bb.2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2.partword.cmpxchg.failure:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[V_AND_B32_e64_3:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 [[FLAT_ATOMIC_CMPSWAP_RTN]], [[V_NOT_B32_e32_]], implicit $exec
+  ; CHECK-NEXT:   [[V_CMP_EQ_U32_e64_:%[0-9]+]]:sreg_64 = V_CMP_EQ_U32_e64 [[PHI2]], [[V_AND_B32_e64_3]], implicit $exec
+  ; CHECK-NEXT:   [[S_ANDN2_B64_:%[0-9]+]]:sreg_64 = S_ANDN2_B64 [[S_OR_B64_]], $exec, implicit-def $scc
+  ; CHECK-NEXT:   [[S_AND_B64_:%[0-9]+]]:sreg_64 = S_AND_B64 [[V_CMP_EQ_U32_e64_]], $exec, implicit-def $scc
+  ; CHECK-NEXT:   [[S_OR_B64_1:%[0-9]+]]:sreg_64 = S_OR_B64 [[S_ANDN2_B64_]], [[S_AND_B64_]], implicit-def $scc
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3.Flow:
+  ; CHECK-NEXT:   successors: %bb.4(0x04000000), %bb.1(0x7c000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[PHI3:%[0-9]+]]:sreg_64 = PHI [[S_OR_B64_]], %bb.1, [[S_OR_B64_1]], %bb.2
+  ; CHECK-NEXT:   [[PHI4:%[0-9]+]]:vgpr_32 = PHI [[COPY7]], %bb.1, [[V_AND_B32_e64_3]], %bb.2
+  ; CHECK-NEXT:   SI_END_CF [[SI_IF]], implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   [[COPY8:%[0-9]+]]:sreg_64 = COPY [[PHI3]]
+  ; CHECK-NEXT:   [[SI_IF_BREAK:%[0-9]+]]:sreg_64 = SI_IF_BREAK [[COPY8]], [[PHI1]], implicit-def dead $scc
+  ; CHECK-NEXT:   SI_LOOP [[SI_IF_BREAK]], %bb.1, implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   S_BRANCH %bb.4
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.4.partword.cmpxchg.end:
+  ; CHECK-NEXT:   [[PHI5:%[0-9]+]]:sreg_64 = PHI [[SI_IF_BREAK]], %bb.3
+  ; CHECK-NEXT:   [[PHI6:%[0-9]+]]:vgpr_32 = PHI [[FLAT_ATOMIC_CMPSWAP_RTN]], %bb.3
+  ; CHECK-NEXT:   SI_END_CF [[PHI5]], implicit-def dead $exec, implicit-def dead $scc, implicit $exec
+  ; CHECK-NEXT:   SI_RETURN
+  %pair = cmpxchg ptr %ptr, i8 0, i8 1 acquire acquire, !mmra !2
+  ret void
+}
+
+attributes #0 = { memory(read) }
+attributes #1 = { memory(write) }
+
+!0 = !{!"foo", !"bar"}
+!1 = !{!"bux", !"baz"}
+!2 = !{!0, !1}
diff --git a/llvm/test/Verifier/mmra-allowed.ll b/llvm/test/Verifier/mmra-allowed.ll
new file mode 100644
index 0000000000000..b7584c6d7ecbc
--- /dev/null
+++ b/llvm/test/Verifier/mmra-allowed.ll
@@ -0,0 +1,33 @@
+; RUN: opt -S < %s
+
+; This file contains MMRA metadata that is okay and should pass the verifier.
+
+define void @readsMem(ptr %ptr) #0 {
+  ret void
+}
+
+define void @writesMem(ptr %ptr) #1 {
+  ret void
+}
+
+define void @test(ptr %ptr) {
+  %ld = load i8, ptr %ptr,                                !mmra !0
+  store i8 1, ptr %ptr,                                   !mmra !1
+  call void @writesMem(),                                 !mmra !2
+  call void @readsMem(),                                  !mmra !2
+  fence release,                                          !mmra !0
+  %rmw.1 = atomicrmw add ptr %ptr, i8 0 release,          !mmra !0
+  %rmw.2 = atomicrmw add ptr %ptr, i8 0 acquire,          !mmra !0
+  %pair = cmpxchg ptr %ptr, i8 0, i8 1 acquire acquire,   !mmra !1
+  %ld.atomic = load atomic i8, ptr %ptr acquire, align 4, !mmra !1
+  store atomic i8 1, ptr %ptr release, align 4,           !mmra !2
+  ; TODO: barrier
+  ret void
+}
+
+attributes #0 = { memory(read) }
+attributes #1 = { memory(write) }
+
+!0 = !{!"scope", !"workgroup"}
+!1 = !{!"as", !"private"}
+!2 = !{!0, !1}
diff --git a/llvm/test/Verifier/mmra.ll b/llvm/test/Verifier/mmra.ll
new file mode 100644
index 0000000000000..8b4213305790a
--- /dev/null
+++ b/llvm/test/Verifier/mmra.ll
@@ -0,0 +1,37 @@
+; RUN: not llvm-as < %s 2>&1 | FileCheck %s
+
+define void @foo(ptr %ptr, i32 %x) {
+
+  ; CHECK: !mmra metadata attached to unexpected instruction kind
+  ; CHECK-NEXT: %bad.add
+  %bad.add = add i32 %x, 42, !mmra !{}
+
+  ; CHECK: !mmra metadata attached to unexpected instruction kind
+  ; CHECK-NEXT: %bad.sub
+  %bad.sub = sub i32 %x, 42, !mmra !{}
+
+  ; CHECK: !mmra expected to be a metadata tuple
+  ; CHECK-NEXT: %bad.md0
+  ; CHECK-NEXT: !DIFile
+  %bad.md0 = load atomic i32, ptr %ptr acquire, align 4, !mmra !0
+
+  ; CHECK: !mmra expected to be a metadata tuple
+  ; CHECK-NEXT: %bad.md1
+  ; CHECK-NEXT: !DIFile
+  %bad.md1 = load atomic i32, ptr %ptr acquire, align 4, !mmra !0
+
+  ; CHECK: !mmra metadata tuple operand is not an MMRA tag
+  ; CHECK-NEXT: %bad.md2
+  ; CHECK-NEXT: !"foo"
+  %bad.md2 = load atomic i32, ptr %ptr acquire, align 4, !mmra !1
+
+  ; CHECK: !mmra metadata tuple operand is not an MMRA tag
+  ; CHECK-NEXT: %bad.md3
+  ; CHECK-NEXT: !"baz"
+  %bad.md3 = load atomic i32, ptr %ptr acquire, align 4, !mmra !2
+  ret void
+}
+
+!0 = !DIFile(filename: "test.c", directory: "")
+!1 = !{!"foo", !"bar", !"bux"}
+!2 = !{!"baz", !0}
diff --git a/llvm/unittests/CodeGen/MachineInstrTest.cpp b/llvm/unittests/CodeGen/MachineInstrTest.cpp
index 7c45d9784bb8e..0734a288ffc52 100644
--- a/llvm/unittests/CodeGen/MachineInstrTest.cpp
+++ b/llvm/unittests/CodeGen/MachineInstrTest.cpp
@@ -277,12 +277,14 @@ TEST(MachineInstrExtraInfo, AddExtraInfo) {
   MCSymbol *Sym2 = MC->createTempSymbol("post_label", false);
   MDNode *HAM = MDNode::getDistinct(Ctx, std::nullopt);
   MDNode *PCS = MDNode::getDistinct(Ctx, std::nullopt);
+  MDNode *MMRA = MMRAMetadata().addTag("foo", "bar").getAsMD(Ctx);
 
   ASSERT_TRUE(MI->memoperands_empty());
   ASSERT_FALSE(MI->getPreInstrSymbol());
   ASSERT_FALSE(MI->getPostInstrSymbol());
   ASSERT_FALSE(MI->getHeapAllocMarker());
   ASSERT_FALSE(MI->getPCSections());
+  ASSERT_FALSE(MI->getMMRAMetadata());
 
   MI->setMemRefs(*MF, MMOs);
   ASSERT_TRUE(MI->memoperands().size() == 1);
@@ -290,6 +292,7 @@ TEST(MachineInstrExtraInfo, AddExtraInfo) {
   ASSERT_FALSE(MI->getPostInstrSymbol());
   ASSERT_FALSE(MI->getHeapAllocMarker());
   ASSERT_FALSE(MI->getPCSections());
+  ASSERT_FALSE(MI->getMMRAMetadata());
 
   MI->setPreInstrSymbol(*MF, Sym1);
   ASSERT_TRUE(MI->memoperands().size() == 1);
@@ -297,6 +300,7 @@ TEST(MachineInstrExtraInfo, AddExtraInfo) {
   ASSERT_FALSE(MI->getPostInstrSymbol());
   ASSERT_FALSE(MI->getHeapAllocMarker());
   ASSERT_FALSE(MI->getPCSections());
+  ASSERT_FALSE(MI->getMMRAMetadata());
 
   MI->setPostInstrSymbol(*MF, Sym2);
   ASSERT_TRUE(MI->memoperands().size() == 1);
@@ -304,6 +308,7 @@ TEST(MachineInstrExtraInfo, AddExtraInfo) {
   ASSERT_TRUE(MI->getPostInstrSymbol() == Sym2);
   ASSERT_FALSE(MI->getHeapAllocMarker());
   ASSERT_FALSE(MI->getPCSections());
+  ASSERT_FALSE(MI->getMMRAMetadata());
 
   MI->setHeapAllocMarker(*MF, HAM);
   ASSERT_TRUE(MI->memoperands().size() == 1);
@@ -311,6 +316,7 @@ TEST(MachineInstrExtraInfo, AddExtraInfo) {
   ASSERT_TRUE(MI->getPostInstrSymbol() == Sym2);
   ASSERT_TRUE(MI->getHeapAllocMarker() == HAM);
   ASSERT_FALSE(MI->getPCSections());
+  ASSERT_FALSE(MI->getMMRAMetadata());
 
   MI->setPCSections(*MF, PCS);
   ASSERT_TRUE(MI->memoperands().size() == 1);
@@ -318,6 +324,21 @@ TEST(MachineInstrExtraInfo, AddExtraInfo) {
   ASSERT_TRUE(MI->getPostInstrSymbol() == Sym2);
   ASSERT_TRUE(MI->getHeapAllocMarker() == HAM);
   ASSERT_TRUE(MI->getPCSections() == PCS);
+  ASSERT_FALSE(MI->getMMRAMetadata());
+
+  MI->setMMRAMetadata(*MF, MMRA);
+  ASSERT_TRUE(MI->memoperands().size() == 1);
+  ASSERT_TRUE(MI->getPreInstrSymbol() == Sym1);
+  ASSERT_TRUE(MI->getPostInstrSymbol() == Sym2);
+  ASSERT_TRUE(MI->getHeapAllocMarker() == HAM);
+  ASSERT_TRUE(MI->getPCSections() == PCS);
+  ASSERT_TRUE(MI->getMMRAMetadata() == MMRA);
+
+  // Check with nothing but MMRAs.
+  MachineInstr *MMRAMI = MF->CreateMachineInstr(MCID, DebugLoc());
+  ASSERT_FALSE(MMRAMI->getMMRAMetadata());
+  MMRAMI->setMMRAMetadata(*MF, MMRA);
+  ASSERT_TRUE(MMRAMI->getMMRAMetadata() == MMRA);
 }
 
 TEST(MachineInstrExtraInfo, ChangeExtraInfo) {
@@ -338,11 +359,15 @@ TEST(MachineInstrExtraInfo, ChangeExtraInfo) {
   MDNode *HAM = MDNode::getDistinct(Ctx, std::nullopt);
   MDNode *PCS = MDNode::getDistinct(Ctx, std::nullopt);
 
+  MDNode *MMRA1 = MMRAMetadata().addTag("foo", "bar").getAsMD(Ctx);
+  MDNode *MMRA2 = MMRAMetadata().addTag("bar", "bux").getAsMD(Ctx);
+
   MI->setMemRefs(*MF, MMOs);
   MI->setPreInstrSymbol(*MF, Sym1);
   MI->setPostInstrSymbol(*MF, Sym2);
   MI->setHeapAllocMarker(*MF, HAM);
   MI->setPCSections(*MF, PCS);
+  MI->setMMRAMetadata(*MF, MMRA1);
 
   MMOs.push_back(MMO);
 
@@ -352,6 +377,7 @@ TEST(MachineInstrExtraInfo, ChangeExtraInfo) {
   ASSERT_TRUE(MI->getPostInstrSymbol() == Sym2);
   ASSERT_TRUE(MI->getHeapAllocMarker() == HAM);
   ASSERT_TRUE(MI->getPCSections() == PCS);
+  ASSERT_TRUE(MI->getMMRAMetadata() == MMRA1);
 
   MI->setPostInstrSymbol(*MF, Sym1);
   ASSERT_TRUE(MI->memoperands().size() == 2);
@@ -359,6 +385,15 @@ TEST(MachineInstrExtraInfo, ChangeExtraInfo) {
   ASSERT_TRUE(MI->getPostInstrSymbol() == Sym1);
   ASSERT_TRUE(MI->getHeapAllocMarker() == HAM);
   ASSERT_TRUE(MI->getPCSections() == PCS);
+  ASSERT_TRUE(MI->getMMRAMetadata() == MMRA1);
+
+  MI->setMMRAMetadata(*MF, MMRA2);
+  ASSERT_TRUE(MI->memoperands().size() == 2);
+  ASSERT_TRUE(MI->getPreInstrSymbol() == Sym1);
+  ASSERT_TRUE(MI->getPostInstrSymbol() == Sym1);
+  ASSERT_TRUE(MI->getHeapAllocMarker() == HAM);
+  ASSERT_TRUE(MI->getPCSections() == PCS);
+  ASSERT_TRUE(MI->getMMRAMetadata() == MMRA2);
 }
 
 TEST(MachineInstrExtraInfo, RemoveExtraInfo) {
@@ -380,11 +415,14 @@ TEST(MachineInstrExtraInfo, RemoveExtraInfo) {
   MDNode *HAM = MDNode::getDistinct(Ctx, std::nullopt);
   MDNode *PCS = MDNode::getDistinct(Ctx, std::nullopt);
 
+  MDNode *MMRA = MMRAMetadata().getAsMD(Ctx);
+
   MI->setMemRefs(*MF, MMOs);
   MI->setPreInstrSymbol(*MF, Sym1);
   MI->setPostInstrSymbol(*MF, Sym2);
   MI->setHeapAllocMarker(*MF, HAM);
   MI->setPCSections(*MF, PCS);
+  MI->setMMRAMetadata(*MF, MMRA);
 
   MI->setPostInstrSymbol(*MF, nullptr);
   ASSERT_TRUE(MI->memoperands().size() == 2);
@@ -392,6 +430,7 @@ TEST(MachineInstrExtraInfo, RemoveExtraInfo) {
   ASSERT_FALSE(MI->getPostInstrSymbol());
   ASSERT_TRUE(MI->getHeapAllocMarker() == HAM);
   ASSERT_TRUE(MI->getPCSections() == PCS);
+  ASSERT_TRUE(MI->getMMRAMetadata() == MMRA);
 
   MI->setHeapAllocMarker(*MF, nullptr);
   ASSERT_TRUE(MI->memoperands().size() == 2);
@@ -399,6 +438,7 @@ TEST(MachineInstrExtraInfo, RemoveExtraInfo) {
   ASSERT_FALSE(MI->getPostInstrSymbol());
   ASSERT_FALSE(MI->getHeapAllocMarker());
   ASSERT_TRUE(MI->getPCSections() == PCS);
+  ASSERT_TRUE(MI->getMMRAMetadata() == MMRA);
 
   MI->setPCSections(*MF, nullptr);
   ASSERT_TRUE(MI->memoperands().size() == 2);
@@ -406,6 +446,7 @@ TEST(MachineInstrExtraInfo, RemoveExtraInfo) {
   ASSERT_FALSE(MI->getPostInstrSymbol());
   ASSERT_FALSE(MI->getHeapAllocMarker());
   ASSERT_FALSE(MI->getPCSections());
+  ASSERT_TRUE(MI->getMMRAMetadata() == MMRA);
 
   MI->setPreInstrSymbol(*MF, nullptr);
   ASSERT_TRUE(MI->memoperands().size() == 2);
@@ -413,6 +454,7 @@ TEST(MachineInstrExtraInfo, RemoveExtraInfo) {
   ASSERT_FALSE(MI->getPostInstrSymbol());
   ASSERT_FALSE(MI->getHeapAllocMarker());
   ASSERT_FALSE(MI->getPCSections());
+  ASSERT_TRUE(MI->getMMRAMetadata() == MMRA);
 
   MI->setMemRefs(*MF, {});
   ASSERT_TRUE(MI->memoperands_empty());
@@ -420,6 +462,15 @@ TEST(MachineInstrExtraInfo, RemoveExtraInfo) {
   ASSERT_FALSE(MI->getPostInstrSymbol());
   ASSERT_FALSE(MI->getHeapAllocMarker());
   ASSERT_FALSE(MI->getPCSections());
+  ASSERT_TRUE(MI->getMMRAMetadata() == MMRA);
+
+  MI->setMMRAMetadata(*MF, nullptr);
+  ASSERT_TRUE(MI->memoperands_empty());
+  ASSERT_FALSE(MI->getPreInstrSymbol());
+  ASSERT_FALSE(MI->getPostInstrSymbol());
+  ASSERT_FALSE(MI->getHeapAllocMarker());
+  ASSERT_FALSE(MI->getPCSections());
+  ASSERT_FALSE(MI->getMMRAMetadata());
 }
 
 TEST(MachineInstrDebugValue, AddDebugValueOperand) {
diff --git a/llvm/unittests/IR/CMakeLists.txt b/llvm/unittests/IR/CMakeLists.txt
index 803164b8f1eac..a03b0711ba33f 100644
--- a/llvm/unittests/IR/CMakeLists.txt
+++ b/llvm/unittests/IR/CMakeLists.txt
@@ -31,6 +31,7 @@ add_llvm_unittest(IRTests
   IntrinsicsTest.cpp
   LegacyPassManagerTest.cpp
   MDBuilderTest.cpp
+  MemoryModelRelaxationAnnotationsTest.cpp
   ManglerTest.cpp
   MetadataTest.cpp
   ModuleTest.cpp
diff --git a/llvm/unittests/IR/MemoryModelRelaxationAnnotationsTest.cpp b/llvm/unittests/IR/MemoryModelRelaxationAnnotationsTest.cpp
new file mode 100644
index 0000000000000..678973b0a1652
--- /dev/null
+++ b/llvm/unittests/IR/MemoryModelRelaxationAnnotationsTest.cpp
@@ -0,0 +1,34 @@
+//===- llvm/unittests/IR/MemoryModelRelaxationAnnotationsTest.cpp ---------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+
+TEST(MMRATest, Equality_Ordering) {
+  MMRAMetadata A, B;
+
+  std::array<MMRAMetadata::TagT, 5> Tags{{{"opencl-fence-mem", "local"},
+                                          {"opencl-fence-mem", "global"},
+                                          {"foo", "0"},
+                                          {"foo", "2"},
+                                          {"foo", "4"}}};
+
+  // Test that ordering does not matter.
+  for (unsigned K : {0, 2, 3, 1, 4})
+    A.addTag(Tags[K]);
+  for (unsigned K : {2, 3, 0, 4, 1})
+    B.addTag(Tags[K]);
+
+  EXPECT_EQ(A, B);
+}
+
+} // namespace

>From 23a882c3ec91fed9a5a21f4d54c79e9503fe0f98 Mon Sep 17 00:00:00 2001
From: pvanhout <pierre.vanhoutryve at amd.com>
Date: Fri, 19 Jan 2024 11:11:41 +0100
Subject: [PATCH 2/3] Comments

---
 llvm/docs/MemoryModelRelaxationAnnotations.rst          | 4 ++--
 llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h | 4 ++++
 llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp        | 4 ++--
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/llvm/docs/MemoryModelRelaxationAnnotations.rst b/llvm/docs/MemoryModelRelaxationAnnotations.rst
index e83e365a3a57d..a141bab548097 100644
--- a/llvm/docs/MemoryModelRelaxationAnnotations.rst
+++ b/llvm/docs/MemoryModelRelaxationAnnotations.rst
@@ -145,8 +145,8 @@ Ordering
 
 Synchronization
     If an synchronizing operation has one or more tags, then whether it
-    participate in the  ``seq_cst`` order with other operations is target
-    dependent.
+    synchronizes-with and participates in the  ``seq_cst`` order with
+    other operations is target dependent.
 
     .. code-block::
 
diff --git a/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h b/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h
index 8daf491cc89ce..2531df2b53340 100644
--- a/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h
+++ b/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h
@@ -52,6 +52,10 @@ class MMRAMetadata {
   MMRAMetadata(const Instruction &I);
   MMRAMetadata(MDNode *MD);
 
+  static bool isCompatible(const Instruction &A, const Instruction &B) {
+    return MMRAMetadata(A).isCompatibleWith(B);
+  }
+
   /// Checks another set of tag for compatibility with this set of tags.
   // TODO: Unit test this
   bool isCompatibleWith(const MMRAMetadata &Other) const;
diff --git a/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp b/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp
index 40ec80a1b70ad..71c7d0a7cee5e 100644
--- a/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp
+++ b/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp
@@ -64,8 +64,8 @@ MMRAMetadata::MMRAMetadata(MDNode *MD) {
 bool MMRAMetadata::isCompatibleWith(const MMRAMetadata &Other) const {
   // Two sets of tags are compatible iff, for every unique tag prefix P
   // present in at least one set:
-  //   - the other set contains no tag that with the same prefix, or
-  //   - at least one tag with the same prefix is present in both sets.
+  //   - the other set contains no tag with prefix P, or
+  //   - at least one tag with prefix P is common to both sets.
 
   // These sets are generally small so we don't bother uniquing
   // the prefixes beforehand. Checking a prefix twice is likely cheaper

>From 914491192e11ea4f4622169d03de4a7496b028ca Mon Sep 17 00:00:00 2001
From: pvanhout <pierre.vanhoutryve at amd.com>
Date: Tue, 6 Feb 2024 09:58:59 +0100
Subject: [PATCH 3/3] Clean-ups, add more tests

---
 .../CodeGen/GlobalISel/MachineIRBuilder.h     |   6 +-
 llvm/include/llvm/CodeGen/MachineInstr.h      |   4 -
 .../llvm/CodeGen/MachineInstrBuilder.h        |   1 -
 llvm/include/llvm/CodeGen/SelectionDAG.h      |   1 -
 .../IR/MemoryModelRelaxationAnnotations.h     |  19 +-
 .../IR/MemoryModelRelaxationAnnotations.cpp   |  16 +-
 llvm/unittests/CodeGen/MachineInstrTest.cpp   |   1 +
 .../MemoryModelRelaxationAnnotationsTest.cpp  | 245 +++++++++++++++++-
 8 files changed, 265 insertions(+), 28 deletions(-)

diff --git a/llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h b/llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
index 2dc23e8643765..55bca9d08f712 100644
--- a/llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
+++ b/llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
@@ -386,12 +386,12 @@ class MachineIRBuilder {
   /// Set the PC sections metadata to \p MD for all the next build instructions.
   void setPCSections(MDNode *MD) { State.PCSections = MD; }
 
-  /// Set the PC sections metadata to \p MD for all the next build instructions.
-  void setMMRAMetadata(MDNode *MMRA) { State.MMRA = MMRA; }
-
   /// Get the current instruction's PC sections metadata.
   MDNode *getPCSections() { return State.PCSections; }
 
+  /// Set the PC sections metadata to \p MD for all the next build instructions.
+  void setMMRAMetadata(MDNode *MMRA) { State.MMRA = MMRA; }
+
   /// Get the current instruction's MMRA metadata.
   MDNode *getMMRAMetadata() { return State.MMRA; }
 
diff --git a/llvm/include/llvm/CodeGen/MachineInstr.h b/llvm/include/llvm/CodeGen/MachineInstr.h
index e7285a150b58a..e9d3eca5a692f 100644
--- a/llvm/include/llvm/CodeGen/MachineInstr.h
+++ b/llvm/include/llvm/CodeGen/MachineInstr.h
@@ -25,7 +25,6 @@
 #include "llvm/CodeGen/TargetOpcodes.h"
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/IR/InlineAsm.h"
-#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/MC/MCInstrDesc.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/ArrayRecycler.h"
@@ -260,9 +259,6 @@ class MachineInstr
     size_t numTrailingObjects(OverloadToken<uint32_t>) const {
       return HasCFIType;
     }
-    size_t numTrailingObjects(OverloadToken<MMRAMetadata>) const {
-      return HasMMRAs;
-    }
 
     // Just a boring constructor to allow us to initialize the sizes. Always use
     // the `create` routine above.
diff --git a/llvm/include/llvm/CodeGen/MachineInstrBuilder.h b/llvm/include/llvm/CodeGen/MachineInstrBuilder.h
index 4fcb99aa410a9..a5b8d3af3cc9b 100644
--- a/llvm/include/llvm/CodeGen/MachineInstrBuilder.h
+++ b/llvm/include/llvm/CodeGen/MachineInstrBuilder.h
@@ -29,7 +29,6 @@
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Intrinsics.h"
-#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cassert>
 #include <cstdint>
diff --git a/llvm/include/llvm/CodeGen/SelectionDAG.h b/llvm/include/llvm/CodeGen/SelectionDAG.h
index 8d7b02cc613b8..e73852a95f744 100644
--- a/llvm/include/llvm/CodeGen/SelectionDAG.h
+++ b/llvm/include/llvm/CodeGen/SelectionDAG.h
@@ -33,7 +33,6 @@
 #include "llvm/CodeGen/SelectionDAGNodes.h"
 #include "llvm/CodeGen/ValueTypes.h"
 #include "llvm/IR/DebugLoc.h"
-#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/ArrayRecycler.h"
diff --git a/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h b/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h
index 2531df2b53340..96a5621b24c21 100644
--- a/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h
+++ b/llvm/include/llvm/IR/MemoryModelRelaxationAnnotations.h
@@ -17,10 +17,10 @@
 #ifndef LLVM_IR_MEMORYMODELRELAXATIONANNOTATIONS_H
 #define LLVM_IR_MEMORYMODELRELAXATIONANNOTATIONS_H
 
-#include "llvm/ADT/STLExtras.h"
+#include <vector>
 #include <string>
 #include <tuple>
-#include <unordered_set>
+#include <set>
 
 namespace llvm {
 
@@ -45,22 +45,23 @@ class Instruction;
 class MMRAMetadata {
 public:
   using TagT = std::pair<std::string, std::string>;
-  using SetT = std::unordered_set<TagT, pair_hash<std::string, std::string>>;
+  using SetT = std::set<TagT>;
   using const_iterator = SetT::const_iterator;
 
   MMRAMetadata() = default;
   MMRAMetadata(const Instruction &I);
   MMRAMetadata(MDNode *MD);
 
-  static bool isCompatible(const Instruction &A, const Instruction &B) {
+  /// \returns whether the MMRAs on \p A and \p B are compatible.
+  static bool checkCompatibility(const Instruction &A, const Instruction &B) {
     return MMRAMetadata(A).isCompatibleWith(B);
   }
 
-  /// Checks another set of tag for compatibility with this set of tags.
-  // TODO: Unit test this
+  /// \returns whether this set of tags is compatible with \p Other.
   bool isCompatibleWith(const MMRAMetadata &Other) const;
 
-  // TODO: Unit test this
+  /// Combines this set of tags with \p Other.
+  /// \returns a new set of tags containing the result.
   MMRAMetadata combine(const MMRAMetadata &Other) const;
 
   MMRAMetadata &addTag(StringRef Prefix, StringRef Suffix);
@@ -72,10 +73,10 @@ class MMRAMetadata {
   bool hasTag(StringRef Prefix, StringRef Suffix) const;
   bool hasTag(const TagT &Tag) const { return Tags.count(Tag); }
 
-  std::vector<TagT> getAllTagsWithPrefix(StringRef Prefix) const;
-
   bool hasTagWithPrefix(StringRef Prefix) const;
 
+  std::vector<TagT> getAllTagsWithPrefix(StringRef Prefix) const;
+
   MDTuple *getAsMD(LLVMContext &Ctx) const;
 
   const_iterator begin() const;
diff --git a/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp b/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp
index 71c7d0a7cee5e..b7f76e2db67d6 100644
--- a/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp
+++ b/llvm/lib/IR/MemoryModelRelaxationAnnotations.cpp
@@ -67,16 +67,14 @@ bool MMRAMetadata::isCompatibleWith(const MMRAMetadata &Other) const {
   //   - the other set contains no tag with prefix P, or
   //   - at least one tag with prefix P is common to both sets.
 
-  // These sets are generally small so we don't bother uniquing
-  // the prefixes beforehand. Checking a prefix twice is likely cheaper
-  // than building a map.
-  for (const auto &[P, S] : Tags) {
-    if (!Other.hasTag(P, S) && Other.hasTagWithPrefix(P))
-      return false;
-  }
+  StringMap<bool> PrefixStatuses;
+  for (const auto &[P, S] : Tags)
+    PrefixStatuses[P] |= (Other.hasTag(P, S) || !Other.hasTagWithPrefix(P));
+  for (const auto &[P, S] : Other)
+    PrefixStatuses[P] |= (hasTag(P, S) || !hasTagWithPrefix(P));
 
-  for (const auto &[P, S] : Other) {
-    if (!hasTag(P, S) && hasTagWithPrefix(P))
+  for(auto &[Prefix, Status]: PrefixStatuses) {
+    if(!Status)
       return false;
   }
 
diff --git a/llvm/unittests/CodeGen/MachineInstrTest.cpp b/llvm/unittests/CodeGen/MachineInstrTest.cpp
index 0734a288ffc52..17bbc43f6b052 100644
--- a/llvm/unittests/CodeGen/MachineInstrTest.cpp
+++ b/llvm/unittests/CodeGen/MachineInstrTest.cpp
@@ -16,6 +16,7 @@
 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetLowering.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
+#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/ModuleSlotTracker.h"
diff --git a/llvm/unittests/IR/MemoryModelRelaxationAnnotationsTest.cpp b/llvm/unittests/IR/MemoryModelRelaxationAnnotationsTest.cpp
index 678973b0a1652..62bda7306a353 100644
--- a/llvm/unittests/IR/MemoryModelRelaxationAnnotationsTest.cpp
+++ b/llvm/unittests/IR/MemoryModelRelaxationAnnotationsTest.cpp
@@ -7,13 +7,16 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/IR/MemoryModelRelaxationAnnotations.h"
+#include "llvm/IR/Metadata.h"
+#include "llvm/IR/Module.h"
+#include "llvm/ADT/STLExtras.h"
 #include "gtest/gtest.h"
 
 using namespace llvm;
 
 namespace {
 
-TEST(MMRATest, Equality_Ordering) {
+TEST(MMRATest, Order) {
   MMRAMetadata A, B;
 
   std::array<MMRAMetadata::TagT, 5> Tags{{{"opencl-fence-mem", "local"},
@@ -31,4 +34,244 @@ TEST(MMRATest, Equality_Ordering) {
   EXPECT_EQ(A, B);
 }
 
+TEST(MMRATest, MDParse) {
+  LLVMContext Ctx;
+
+  // No nesting:
+  // !{!"foo", "!bar"}
+  MDNode *FooBar = MDTuple::get(Ctx, { MDString::get(Ctx, "foo"), MDString::get(Ctx, "bar")});
+  MMRAMetadata FooBarMMRA(FooBar);
+
+  EXPECT_EQ(FooBarMMRA.size(), 1u);
+  EXPECT_EQ(FooBarMMRA, MMRAMetadata().addTag("foo", "bar"));
+
+  // Nested:
+  // !{!{!"foo", "!bar"}, !{!"bux", !"qux"}}
+  MDNode *BuxQux = MDTuple::get(Ctx, { MDString::get(Ctx, "bux"), MDString::get(Ctx, "qux")});
+  MDNode *Nested = MDTuple::get(Ctx, {FooBar, BuxQux});
+  MMRAMetadata NestedMMRA(Nested);
+
+  EXPECT_EQ(NestedMMRA.size(), 2u);
+  EXPECT_EQ(NestedMMRA, MMRAMetadata().addTag("foo", "bar").addTag("bux", "qux"));
+}
+
+TEST(MMRATest, MDEmit) {
+  LLVMContext Ctx;
+
+  // Simple MD.
+  // !{!"foo", "!bar"}
+  {
+    MMRAMetadata FooBarMMRA = MMRAMetadata().addTag("foo", "bar");
+    MDTuple* FooBar = FooBarMMRA.getAsMD(Ctx);
+
+    ASSERT_NE(FooBar, nullptr);
+    ASSERT_EQ(FooBar->getNumOperands(), 2u);
+    MDString *Foo = dyn_cast<MDString>(FooBar->getOperand(0));
+    MDString *Bar = dyn_cast<MDString>(FooBar->getOperand(1));
+    ASSERT_NE(Foo, nullptr);
+    ASSERT_NE(Bar, nullptr);
+    EXPECT_EQ(Foo->getString(), std::string("foo"));
+    EXPECT_EQ(Bar->getString(), std::string("bar"));
+  }
+
+
+  // Nested MD
+  // !{!{!"foo", "!bar"}, !{!"bux", !"qux"}}
+  {
+    MMRAMetadata NestedMMRA = MMRAMetadata().addTag("foo", "bar").addTag("bux", "qux");
+    MDTuple* Nested = NestedMMRA.getAsMD(Ctx);
+
+    ASSERT_NE(Nested, nullptr);
+    ASSERT_EQ(Nested->getNumOperands(), 2u);
+    MDTuple *BuxQux = dyn_cast<MDTuple>(Nested->getOperand(0));
+    MDTuple *FooBar = dyn_cast<MDTuple>(Nested->getOperand(1));
+    ASSERT_NE(FooBar, nullptr);
+    ASSERT_NE(BuxQux, nullptr);
+    ASSERT_EQ(FooBar->getNumOperands(), 2u);
+    ASSERT_EQ(BuxQux->getNumOperands(), 2u);
+
+    MDString *Foo = dyn_cast<MDString>(FooBar->getOperand(0));
+    MDString *Bar = dyn_cast<MDString>(FooBar->getOperand(1));
+    MDString *Bux = dyn_cast<MDString>(BuxQux->getOperand(0));
+    MDString *Qux = dyn_cast<MDString>(BuxQux->getOperand(1));
+
+    EXPECT_EQ(Foo->getString(), std::string("foo"));
+    EXPECT_EQ(Bar->getString(), std::string("bar"));
+    EXPECT_EQ(Bux->getString(), std::string("bux"));
+    EXPECT_EQ(Qux->getString(), std::string("qux"));
+  }
+}
+
+TEST(MMRATest, Utility) {
+  using TagT = MMRAMetadata::TagT;
+
+  MMRAMetadata MMRA;
+  MMRA.addTag("foo", "0");
+  MMRA.addTag("foo", "1");
+  MMRA.addTag("bar", "x");
+
+  EXPECT_TRUE(MMRA.hasTagWithPrefix("foo"));
+  EXPECT_TRUE(MMRA.hasTagWithPrefix("bar"));
+  EXPECT_FALSE(MMRA.hasTagWithPrefix("x"));
+
+  EXPECT_TRUE(MMRA.hasTag("foo", "0"));
+  EXPECT_TRUE(MMRA.hasTag(TagT("foo", "0")));
+  EXPECT_TRUE(MMRA.hasTag("foo", "1"));
+  EXPECT_TRUE(MMRA.hasTag(TagT("foo", "1")));
+  EXPECT_TRUE(MMRA.hasTag("bar", "x"));
+  EXPECT_TRUE(MMRA.hasTag(TagT("bar", "x")));
+
+  auto AllFoo = MMRA.getAllTagsWithPrefix("foo");
+  auto AllBar = MMRA.getAllTagsWithPrefix("bar");
+  auto AllX = MMRA.getAllTagsWithPrefix("x");
+
+  EXPECT_EQ(AllFoo.size(), 2u);
+  EXPECT_EQ(AllBar.size(), 1u);
+  EXPECT_EQ(AllX.size(), 0u);
+
+  EXPECT_TRUE(is_contained(AllFoo, TagT("foo", "0")));
+  EXPECT_TRUE(is_contained(AllFoo, TagT("foo", "1")));
+  EXPECT_TRUE(is_contained(AllBar, TagT("bar", "x")));
+}
+
+TEST(MMRATest, Operators) {
+  MMRAMetadata A;
+  A.addTag("foo", "0");
+  A.addTag("bar", "x");
+
+  MMRAMetadata B;
+  B.addTag("foo", "0");
+  B.addTag("bar", "y");
+
+
+  // ensure we have different objects by creating copies.
+  EXPECT_EQ(MMRAMetadata(A), MMRAMetadata(A));
+  EXPECT_TRUE((bool)A);
+
+  EXPECT_EQ(MMRAMetadata(B), MMRAMetadata(B));
+  EXPECT_TRUE((bool)B);
+
+  EXPECT_NE(A, B);
+
+  EXPECT_EQ(MMRAMetadata(), MMRAMetadata());
+  EXPECT_NE(A, MMRAMetadata());
+  EXPECT_NE(B, MMRAMetadata());
+
+  MMRAMetadata Empty;
+  EXPECT_FALSE((bool)Empty);
+}
+
+TEST(MMRATest, Compatibility) {
+  MMRAMetadata Foo0;
+  Foo0.addTag("foo", "0");
+
+  MMRAMetadata Foo1;
+  Foo1.addTag("foo", "1");
+
+  MMRAMetadata Foo10;
+  Foo10.addTag("foo", "0");
+  Foo10.addTag("foo", "1");
+
+  MMRAMetadata Bar;
+  Bar.addTag("bar", "y");
+
+  MMRAMetadata Empty;
+
+  // Other set has no tag with same prefix
+  EXPECT_TRUE(Foo0.isCompatibleWith(Bar));
+  EXPECT_TRUE(Bar.isCompatibleWith(Foo0));
+
+  EXPECT_TRUE(Foo0.isCompatibleWith(Empty));
+  EXPECT_TRUE(Empty.isCompatibleWith(Foo0));
+
+  EXPECT_TRUE(Empty.isCompatibleWith(MMRAMetadata()));
+  EXPECT_TRUE(MMRAMetadata().isCompatibleWith(Empty));
+
+  // Other set has conflicting tags.
+  EXPECT_FALSE(Foo1.isCompatibleWith(Foo0));
+  EXPECT_FALSE(Foo0.isCompatibleWith(Foo1));
+
+  // Both have common tags.
+  EXPECT_TRUE(Foo0.isCompatibleWith(Foo0));
+  EXPECT_TRUE(Foo0.isCompatibleWith(Foo10));
+  EXPECT_TRUE(Foo10.isCompatibleWith(Foo0));
+
+  EXPECT_TRUE(Foo1.isCompatibleWith(Foo1));
+  EXPECT_TRUE(Foo1.isCompatibleWith(Foo10));
+  EXPECT_TRUE(Foo10.isCompatibleWith(Foo1));
+
+  // Try with more prefixes now:
+  MMRAMetadata Multiple0;
+  Multiple0.addTag("foo", "y");
+  Multiple0.addTag("foo", "x");
+  Multiple0.addTag("bar", "z");
+
+  MMRAMetadata Multiple1;
+  Multiple1.addTag("foo", "z");
+  Multiple1.addTag("foo", "x");
+  Multiple1.addTag("bar", "y");
+
+  MMRAMetadata Multiple2;
+  Multiple2.addTag("foo", "z");
+  Multiple2.addTag("foo", "x");
+  Multiple2.addTag("bux", "y");
+
+  // Multiple0 and Multiple1 are not compatible because "bar" is getting in the way.
+  EXPECT_FALSE(Multiple0.isCompatibleWith(Multiple1));
+  EXPECT_FALSE(Multiple1.isCompatibleWith(Multiple0));
+
+  EXPECT_TRUE(Multiple0.isCompatibleWith(Empty));
+  EXPECT_TRUE(Empty.isCompatibleWith(Multiple0));
+  EXPECT_TRUE(Multiple1.isCompatibleWith(Empty));
+  EXPECT_TRUE(Empty.isCompatibleWith(Multiple1));
+
+  // Multiple2 is compatible with both 1/0 because there is always "foo:x" in common, and the other prefixes are unique to each set.
+  EXPECT_TRUE(Multiple2.isCompatibleWith(Multiple0));
+  EXPECT_TRUE(Multiple0.isCompatibleWith(Multiple2));
+  EXPECT_TRUE(Multiple2.isCompatibleWith(Multiple1));
+  EXPECT_TRUE(Multiple1.isCompatibleWith(Multiple2));
+}
+
+TEST(MMRATest, Combine) {
+  MMRAMetadata Foo0;
+  Foo0.addTag("foo", "0");
+
+  MMRAMetadata Foo10;
+  Foo10.addTag("foo", "0");
+  Foo10.addTag("foo", "1");
+
+  MMRAMetadata Bar0;
+  Bar0.addTag("bar", "0");
+
+  MMRAMetadata BarFoo0;
+  BarFoo0.addTag("bar", "0");
+  BarFoo0.addTag("foo", "0");
+
+  {
+    // only foo:0 is common
+    MMRAMetadata Combined = Foo0.combine(Foo10);
+    EXPECT_EQ(Combined, Foo0);
+  }
+
+
+  {
+    // nothing is common
+    MMRAMetadata Combined = Foo0.combine(Bar0);
+    EXPECT_TRUE(Combined.empty());
+  }
+
+
+  {
+    // only foo:0 is common
+    MMRAMetadata Combined = BarFoo0.combine(Foo0);
+    EXPECT_EQ(Combined, Foo0);
+  }
+
+  {
+    // only bar:0 is common
+    MMRAMetadata Combined = BarFoo0.combine(Bar0);
+    EXPECT_EQ(Combined, Bar0);
+  }
+}
+
 } // namespace



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