[llvm] eb1b57c - [LangRef] Specify that syncscopes can affect the monotonic modification order (#189017)

[via llvm-commits]

Author: ritter-x2a
Date: 2026-05-28T10:14:21Z
New Revision: eb1b57c639947ad381d9dff6093c2b8085b99d0f

URL: https://github.com/llvm/llvm-project/commit/eb1b57c639947ad381d9dff6093c2b8085b99d0f
DIFF: https://github.com/llvm/llvm-project/commit/eb1b57c639947ad381d9dff6093c2b8085b99d0f.diff

LOG: [LangRef] Specify that syncscopes can affect the monotonic modification order (#189017)

If a target specifies that atomics with mismatching syncscopes appear
non-atomic to each other, there is no point in requiring them to be ordered in
the monotonic modification order. Notably, the [AMDGPU target user
guide](https://llvm.org/docs/AMDGPUUsage.html#memory-scopes) has specified
syncscopes to relax the modification order for years.

So far, I haven't found an example where this less constrained ordering would
be observable (at least with the AMDGPU inclusive scope rules). Whenever a load
would be able to see two monotonic stores with non-inclusive scope, that's
considered a data race (i.e., the load would return `undef`), so it cannot be
used to observe the order of the stores.

Related RFC: https://discourse.llvm.org/t/rfc-clarifying-llvm-irs-concurrent-memory-model/90480

Added: 
    

Modified: 
    llvm/docs/LangRef.rst

Removed: 
    


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diff  --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 44b0b626d7c30..74c90d1f06532 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -4055,17 +4055,21 @@ For a simpler introduction to the ordering constraints, see the
 .. _syncscope:
 
 If an atomic operation is marked ``syncscope("singlethread")``, it only
-*synchronizes with* and only participates in the seq\_cst total orderings of
-other operations running in the same thread (for example, in signal handlers).
+*synchronizes with* other operations running in the same thread (for
+example, in signal handlers) and it is related in the seq\_cst order and
+the monotonic modification order with other operations in the same
+thread.
 
 If an atomic operation is marked ``syncscope("<target-scope>")``, where
-``<target-scope>`` is a target-specific synchronization scope, then it is target
-dependent if it *synchronizes with* and participates in the seq\_cst total
-orderings of other operations.
-
-Otherwise, an atomic operation that is not marked ``syncscope("singlethread")``
-or ``syncscope("<target-scope>")`` *synchronizes with* and participates in the
-seq\_cst total orderings of other operations that are not marked
+``<target-scope>`` is a target-specific synchronization scope, then it
+is target-dependent if it *synchronizes with* other operations and
+if it is related with other operations in the seq\_cst order and the
+monotonic modification order.
+
+Otherwise, an atomic operation that is not marked
+``syncscope("singlethread")`` or ``syncscope("<target-scope>")``
+*synchronizes with* and is related in the seq\_cst order and the
+monotonic modification order with other operations that are not marked
 ``syncscope("singlethread")`` or ``syncscope("<target-scope>")``.
 
 .. _floatenv: