================ @@ -650,48 +650,127 @@ 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>`. + +The 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 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 +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Pointers are not represented as just an address, but may instead include +additional metadata such as bounds information or a temporal identifier. +Examples include AMDGPU buffer descriptors with a 128-bit fat pointer and a +32-bit offset, or CHERI capabilities that contain bounds, permissions and a +type field (as well as an out-of-band validity bit, see next section). +In general, valid non-integral pointers cannot becreated from just an integer +value: while ``inttoptr`` yields a deterministic bitwise pattern, the resulting +value is not guaranteed to be a valid dereferenceable pointer. + +In most cases pointers with a non-integral representation 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. + +"Non-integral" pointers also impose restrictions on transformation passes, but +in general these are less restrictive than for "unstable" pointers. The main +difference compared to integral pointers is that ``inttoptr`` instructions +should not be inserted by passes as they may not be able to create a valid +pointer. This property also means that ``inttoptr(ptrtoint(x))`` cannot be +folded to ``x`` as the ``ptrtoint`` operation may destroy the necessary metadata +to reconstruct the pointer. + +Unlike "unstable" pointers, the bit-wise representation is stable and +``ptrtoint(x)`` always yields a deterministic value. +This means transformation passes are still permitted to insert new ``ptrtoint`` +instructions. +However, it is important to note that ``ptrtoint`` may not yield the same value +as storing the pointer via memory and reading it back as an integer, even if the +bitwidth of the two types matches (since ptrtoint could involve some form of +arithmetic or strip parts of the non-integral pointer representation). + +Non-integral pointers with external state +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +A special case of non-integral pointers is ones that include external state +(such as implicit bounds information or a type tag) with a target-defined size. +An example of such a type is a CHERI capability, where there is an additional +validity bit that is part of all pointer-typed registers, but is located in +memory at an implementation-defined address separate from the pointer itself. +Another example would be a fat-pointer scheme where pointers remain plain +integers, but the associated bounds are stored in an out-of-band table. + +When a ``store ptr addrspace(N) %p, ptr @dst`` of such a non-integral pointers +is performed, the external metadata is also stored to another location. +Similarly, a ``%val = load ptr addrspace(N), ptr @dst`` will fetch the +external metadata and make it available for all uses of ``%val``. +Notionally, these additional bits are part of the pointer, but since loads and +stores only operate on the "inline" bits of the pointer and the additional +bits are not explicitly exposed, they are not included in the size specified in +the :ref:`datalayout string<langref_datalayout>`. + +When a pointer type has external state, all roundtrips via memory must +be performed as loads and stores of the correct type since stores of other +types may not propagate the external data. +Therefore it is not legal to convert a load/store of a non-integral pointer +type with external state to a load/store of an integer type with same +bitwidth, as that drops the additional state. +However, it can be assumed that appropriately-aligned ``llvm.memcpy`` and +``llvm.memmove`` intrinsics will preserve the external metadata. +This is essential to allow frontends to efficiently emit of copies of +structures containing such pointers, since expanding all these copies as +individual loads and stores would significantly inhibit optimizations. +To ensure that this results in valid code, affected backends must lower +these intrinsics in a way that propagates external state. + ---------------- arichardson wrote:
I should probably drop "appropriately-aligned" - this relates to the fact that CHERI capabilities can only be loaded from/stored to naturally aligned memory locations since there is only one tag bit per capability-size location. https://github.com/llvm/llvm-project/pull/105735 _______________________________________________ llvm-branch-commits mailing list llvm-branch-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-branch-commits
