[llvm-branch-commits] [llvm] [DataLayout][LangRef] Split non-integral and unstable pointer properties (PR #105735)

[Nikita Popov via llvm-branch-commits]

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@@ -660,42 +660,136 @@ Non-Integral Pointer Type
 Note: non-integral pointer types are a work in progress, and they should be
 considered experimental at this time.
 
-LLVM IR optionally allows the frontend to denote pointers in certain address
-spaces as "non-integral" via the :ref:`datalayout string<langref_datalayout>`.
-Non-integral pointer types represent pointers that have an *unspecified* bitwise
-representation; that is, the integral representation may be target dependent or
-unstable (not backed by a fixed integer).
+For most targets, the pointer representation is a direct mapping from the
+bitwise representation to the address of the underlying memory allocation.
+Such pointers are considered "integral", and any pointers where the
+representation is not just an integer address are called "non-integral".
+
+Non-integral pointers have at least one of the following three properties:
+
+* the pointer representation contains non-address bits
+* the pointer representation is unstable (may changed at any time in a
+  target-specific way)
+* the pointer representation has external state
+
+These properties (or combinations thereof) can be applied to pointers via the
+:ref:`datalayout string<langref_datalayout>`.
+
+The exact implications of these properties are target-specific. The following
+subsections describe the IR semantics and restrictions to optimization passes
+for each of these properties.
+
+Pointers with non-address bits
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Pointers in this address space have a bitwise representation that not only
+has address bits, but also some other target-specific metadata.
+In most cases pointers with a non-address bits behave exactly the same as an
+integral pointer, the only difference is that it is not possible to create a
+pointer just from an address unless all the non-address bits are also recreated
+correctly in a target-specific way.
+Since the address width such a pointer is not equal to the bitwise
+representation, extracting the address will need to truncate to the index width
+of the pointer.
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nikic wrote:

This sentence is only true for ptrtoint, but not ptrtoaddr. Might make sense to just drop it.

https://github.com/llvm/llvm-project/pull/105735