[llvm] Add 'initialized' attribute langref (PR #84803)

[Jan Voung via llvm-commits]

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@@ -1621,6 +1621,28 @@ Currently, only the following parameter attributes are defined:
     ``readonly`` or a ``memory`` attribute that does not contain
     ``argmem: write``.
 
+``initialized((Lo1,Hi1),...)``
+    This attribute is a list of const ranges in ascending order with no
+    overlapping or continuous. It indicates that the function initializes the
+    memory through the pointer argument, [%p+LoN, %p+HiN): there are no reads,
+    and no special accesses (such as volatile access or untrackable capture)
+    before the initialization in the function. LoN/HiN are 64-bit ints;
+    negative values are allowed in case a pointer to partway through the
+    allocation is passed to.
+
+    This attribute implies that the function initializes and does not read
+    before initialization through this pointer argument, even though it may
+    read the memory before initialization that the pointer points to, such
+    as through other arguments.
+
+    The ``writable`` or ``dereferenceable`` attribute does not imply
+    ``initialized`` attribute, and ``initialized`` does not imply ``writeonly``
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jvoung wrote:

nit: "does not imply [the] ``initialized`` attribute, and ... ``writeonly``, since ``initialized`` allows reading from the pointer after writing." ?

https://github.com/llvm/llvm-project/pull/84803