[llvm] [IR] Introduce !captures metadata (PR #160913)

[Nikita Popov via llvm-commits]

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@@ -7543,6 +7545,33 @@ The number of bytes known to be dereferenceable is specified by the integer
 value in the metadata node. This is analogous to the ''dereferenceable_or_null''
 attribute on parameters and return values.
 
+'``captures``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``!captures`` metadata can only be applied to ``store`` instructions with
+a pointer-typed value operand. It restricts the capturing behavior of the store
+value operand in the same way the ``captures(...)`` attribute would do on a
+call. See the :ref:`pointer capture section <pointercapture>` for a detailed
+discussion of capture semantics.
+
+The ``!captures`` metadata accepts a non-empty list of strings from the same
+set as the :ref:`captures attribute <captures_attr>`:
+``!"address"``, ``!"address_is_null"``, ``!"provenance"`` and
+``!"read_provenance"``. ``!"none"`` is not supported.
+
+For example ``store ptr %x, ptr %y, !captures !{!"address"}`` indicates that
+the copy of pointer ``%x`` stored to location ``%y`` will only be used to
+inspect its integral address value, and not dereferenced. Dereferencing the
----------------
nikic wrote:

Depends a bit on what kind of non-integral pointer we'd talking about here, but if it's about GC pointers, then you'd probably want the opposite, that is `!captures !{!"provenance"}` to indicate that accesses through the captured pointer are possible, but the address will not be inspected? Assuming the language does in fact not expose pointer addresses. I haven't thought too carefully on whether this would be safe. (Though I don't think we currently have any transforms that are able to take advantage of this.)

https://github.com/llvm/llvm-project/pull/160913