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

[Sameer Sahasrabuddhe via llvm-commits]

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+===================================
+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 string: a *prefix and a *suffix*.
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ssahasra wrote:

Missing markup (*)

https://github.com/llvm/llvm-project/pull/78569