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

[Pierre van Houtryve 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 nodes:
+
+    * a *prefix* string.
+    * a *suffix* integer or string.
----------------
Pierre-vh wrote:

I just remembered ints aren't supported yet in the implementation.
I'm also doubting they'll be useful at all, and we can always store them as string which avoids questions like "which integer type to use to represent them".

Is it ok to just remove it from the spec? @ssahasra 

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