[llvm] [LangRef] Allow monotonic & seq_cst accesses to inter-operate with other accesses (PR #189014)

[Sameer Sahasrabuddhe via llvm-commits]

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@@ -3983,12 +3983,13 @@ For a simpler introduction to the ordering constraints, see the
     address. All modification orders must be compatible with the
     happens-before order. There is no guarantee that the modification
     orders can be combined to a global total order for the whole program
-    (and this often will not be possible). The read in an atomic
-    read-modify-write operation (:ref:`cmpxchg <i_cmpxchg>` and
-    :ref:`atomicrmw <i_atomicrmw>`) reads the value in the modification
-    order immediately before the value it writes. If one atomic read
-    happens before another atomic read of the same address, the later
-    read must see the same value or a later value in the address's
+    (and this often will not be possible). If the read in an atomic
+    read-modify-write operation M (:ref:`cmpxchg <i_cmpxchg>` and
+    :ref:`atomicrmw <i_atomicrmw>`) reads from a ``monotonic`` (or
+    stronger) write W, W must be immediately before M in the address's
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ssahasra wrote:

No, that's not what the original sentence was about. Modification order already covers the write W' that comes before the write W in M. But M itself consists of a read R and a dependent write W. The original sentence specifies that R reads from W' (_reads the value in the modification order immediately before the value it writes_). That `rf` is missing in the new sentence. 

https://github.com/llvm/llvm-project/pull/189014