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

[Jessica Clarke via llvm-branch-commits]

================
@@ -649,48 +649,95 @@ literal types are uniqued in recent versions of LLVM.
 
 .. _nointptrtype:
 
-Non-Integral Pointer Type
--------------------------
+Non-Integral and Unstable Pointer Types
+---------------------------------------
 
-Note: non-integral pointer types are a work in progress, and they should be
-considered experimental at this time.
+Note: non-integral/unstable 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).
+spaces as "non-integral" or "unstable" (or both "non-integral" and "unstable")
+via the :ref:`datalayout string<langref_datalayout>`.
+
+These exact implications of these properties are target-specific, but the
+following IR semantics and restrictions to optimization passes apply:
+
+Unstable pointer representation
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Pointers in this address space have an *unspecified* bitwise representation
+(i.e. not backed by a fixed integer). The bitwise pattern of such pointers is
+allowed to change in a target-specific way. For example, this could be a pointer
+type used for with copying garbage collection where the garbage collector could
+update the pointer at any time in the collection sweep.
 
 ``inttoptr`` and ``ptrtoint`` instructions have the same semantics as for
 integral (i.e. normal) pointers in that they convert integers to and from
-corresponding pointer types, but there are additional implications to be
-aware of.  Because the bit-representation of a non-integral pointer may
-not be stable, two identical casts of the same operand may or may not
+corresponding pointer types, but there are additional implications to be aware
+of.
+
+For "unstable" pointer representations, the bit-representation of the pointer
+may not be stable, so two identical casts of the same operand may or may not
 return the same value.  Said differently, the conversion to or from the
-non-integral type depends on environmental state in an implementation
+"unstable" pointer type depends on environmental state in an implementation
 defined manner.
-
 If the frontend wishes to observe a *particular* value following a cast, the
 generated IR must fence with the underlying environment in an implementation
 defined manner. (In practice, this tends to require ``noinline`` routines for
 such operations.)
 
 From the perspective of the optimizer, ``inttoptr`` and ``ptrtoint`` for
-non-integral types are analogous to ones on integral types with one
+"unstable" pointer types are analogous to ones on integral types with one
 key exception: the optimizer may not, in general, insert new dynamic
 occurrences of such casts.  If a new cast is inserted, the optimizer would
 need to either ensure that a) all possible values are valid, or b)
 appropriate fencing is inserted.  Since the appropriate fencing is
 implementation defined, the optimizer can't do the latter.  The former is
 challenging as many commonly expected properties, such as
-``ptrtoint(v)-ptrtoint(v) == 0``, don't hold for non-integral types.
+``ptrtoint(v)-ptrtoint(v) == 0``, don't hold for "unstable" pointer types.
 Similar restrictions apply to intrinsics that might examine the pointer bits,
 such as :ref:`llvm.ptrmask<int_ptrmask>`.
 
-The alignment information provided by the frontend for a non-integral pointer
+The alignment information provided by the frontend for an "unstable" pointer
 (typically using attributes or metadata) must be valid for every possible
 representation of the pointer.
 
+Non-integral pointer representation
----------------
jrtc27 wrote:

Is non-integral the right term for something that is _more than_ just an integer?

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