Issue 260211.1: General Support for Address Spaces

Tony Tye, Ben Woodard, & GPU Subcommittee

Depends on Issue [260617.1][260617.1], "Clarify Address Classes vs. Address Spaces".

## PROBLEM DESCRIPTION

GPUs need to be able to describe addresses that are in different pools
of memory that are not part of the system's normal memory pool. These
memory pools can often be quite different than the system's normal
memory address space and this requires that they be treated
differently.

## How GPU memory addresses are different

While the system's normal address space is universally accessible and
therefore context independent memory, GPU memory is often context
dependent. The definition of this context has already been accepted
into the DWARF6 standard with issue [241011.1 Expression Evaluation
Context][242011.1]. Unlike system
memory where an address like 0x1000 refers to the same location
independent of the evaluation context, GPU memory pools can be local
to a GPU's processing unit and every processing unit may have its own
memory pool. A couple of examples of this are: [Intel's Shared Local
Memory][shared-local-mem] and [AMD's LDS][lds].
Thus, within a specific address space, the address 0x1000 may
reference different bits depending on which context it is evaluated
from. This is very similar to registers where every processor has its
own set of registers and the context disambiguates which one the
consumer should refer to. More formally when a pool of memory is
context dependent each address space defines a locality scope and
locations within that address space are bound to a particular instance
of storage with that scope.

[242011.1]: https://dwarfstd.org/issues/241011.1.html
[shared-local-mem]: https://www.intel.com/content/www/us/en/docs/oneapi/optimization-guide-gpu/2025-2/shared-local-memory.html
[lds]: https://rocm.docs.amd.com/projects/HIP/en/develop/understand/hardware_implementation.html#local-data-share-lds

There is no requirement that a particular set of bits is only
accessible through only one address space. In fact, it is common for a
GPU to provide access to the same bits through different address
spaces. This allows uses of memory locations within alternative
address spaces that are quite different than on CPUs. In some GPUs
some address spaces have embedded access patterns. Compilers often use
one access pattern for serial code while another is used for
vectorized code. If this access pattern were not conceptually
abstracted into an address space, then the compiler would have to
generate DWARF expressions that would undo the work done by the
addressing mode in order to point the consumer to the correct
location. This would expand the size of the DWARF generated and it
would make the job of generating debuginfo harder for the
producer. Another common example of an embedded access pattern is
strided memory.

These GPU memories can also have address widths that are different
than the system memory addresses. It is currently common for an
address in GPU memory to be 32b long while a system's memory address
is 64b.

## An address as a location rather than as a value

DWARF 5 has the concept of an address in many expressions but does not
define how it relates to address spaces. For example,
`DW_OP_push_object_address` pushes the address of an object. Other
contexts implicitly push an address on the stack before evaluating an
expression. For example, the `DW_AT_use_location` attribute of the
`DW_TAG_ptr_to_member_type`. The expression belongs to a source
language type that may apply to objects allocated in different kinds
of storage. Therefore, it is important that an expression that uses
the location can do so without regard to what kind of storage it
specifies. As it applies to memory location this means that a simple
value is insufficient. The address space of a memory location
description is also required. For example, a pointer to member value
may want to be applied to an object that may reside in any address
space.

### Why `DW_OP_xderef` is not sufficient

The DWARF 5 `DW_OP_xderef*` operations allow a value to be converted
into an address within a specified address space which is then
read. But it provides no way to create a memory location for an
address in the non-default address space. For example, GPU variables
can be allocated in local scratch pad memory at a fixed address.

### Address spaces are not address classes

Address class is an attribute of a pointer type that distinguishes
between different representations of a pointer; e.g., NEAR and FAR
pointers from old 16-bit x86 code. The attribute affects the size and
interpretation of the bits of the pointer, but the pointer still
refers to the "default" address space of the target.

The attribute has also been used for subprograms and subroutine types,
to specify what addressing mode should be used to call the subprogram.

While it is similar to address classes in that a pointer type for one
address space may be a different size from that for another address
space, the address space is not part of the pointer value, but is
implicit in the type of the pointer. The bits of the pointer refer to
an address in a completely separate address space.

### Address spaces are not just swizzled pointers

While some target's memory controllers may implement different address
spaces by what is sometimes called "swizzling pointers" (e.g., using
otherwise unused bits of the address to encode the address spaces), it
cannot be assumed that this is a universal hardware design. Different
GPU designs could have a completely separate connection to a
particular pool of memory and have a completely different numbering
system to reference it. On the other hand, there is nothing in this
design that precludes hardware from using swizzled pointers to
implement address spaces.

If address spaces were implemented by a consumer by swizzling
pointers, then there would likely need to be reserved ranges of
addresses defined in the ABI that could interfere with other
addresses in the system memory. That would require the consumer to
have special treatment for such values. Furthermore, purely swizzled
pointers do not capture the semantic differences of address spaces
like context dependence, pointer size, or embedded access patterns.

## Proposed solution

Add a new attribute `DW_AT_address_space` to pointer and reference
types.  This allows the compiler to specify which address space is
being used to encode the value of the pointer or reference type.

Add a new `DW_OP_mem` operation defined to create a memory location
description from an address and address space. It can be used to
specify the location of a variable that is allocated in a specific
address space.

Unlike `DW_OP_addr` which takes the address as an inline operand,
`DW_OP_mem` takes both of its parameters from the stack. This allows
them both to be dynamically computed. The case for the address being
dynamically computed is fairly obvious. One unobvious use is to allow
the address to be relocated when it is supplied using `DW_OP_constx`.

`DW_OP_mem` can be a thought of as a more general form of `DW_OP_addr`
with `DW_OP_addr(X)` being a short hand form of:

    DW_OP_lit0 ; DW_ASPACE_default is by definition 0
    DW_OP_const<N>u X
    DW_OP_mem

The address space being a stack parameter may be less obvious but at
least one language, OpenCL, allows the address space to be computed
and so it was determined that it was best to make this a stack
parameter as well.

Implicit conversion back to a value is limited only to the default
address space to maintain compatibility with DWARF 5. A very common
idiom in previous versions of DWARF would be:

    DW_OP_addr 0xf00
    DW_OP_lit1
    DW_OP_plus

In previous versions DWARF, `DW_OP_addr` pushed a generic-type stack
value. In DWARF 6, `DW_OP_addr` pushes a location. `DW_OP_plus` pops
two VALUEs from the stack.  If we didn't have the backward
compatability rule, this common DWARF expression would be invalid in
DWARF 6.

A similar problem arises with `DW_OP_push_object_address`:

    DW_OP_push_object_address  # pushes generic-type value in DWARF 5
    DW_OP_lit1
    DW_OP_plus

In DWARF 6:

    DW_OP_push_object_location # pushes location in DWARF 6
    DW_OP_lit1
    DW_OP_plus

Still works, provided the object location is global memory, which was
the only case possible in DWARF 5. This approach of extending memory
location to support address spaces, allows all existing DWARF 5
expressions to have the identical semantics. However the correct DWARF
6 expression that does not rely on implicit conversion would be:

    DW_OP_push_object_location
    DW_OP_lit1
    DW_OP_offset

Unlike `DW_OP_plus` which pops two VALUEs, `DW_OP_offset` pops a
LOCATION and a VALUE. Therefore the expression still works even if the
object location is another address space rather than in global memory
or even if the object's location is not in memory at all.

Having the address space included as part of the memory location as
opposed to being separate value, fixes one of the problems with
`DW_OP_xderef` where the address space is a separate value from the
address. This requires producers and consumers to manage two values
rather than one location.

Address spaces need not uniquely reference bits. The same bits may be
accessible through multiple address spaces. This allows great
implementation freedom. For example, a compiler could choose to access
private memory when mapping a source language thread to the lane of a
wavefront in a SIMT manner. Or a compiler could choose to access the
same private memory by mapping the same language construct with a
wavefront being the thread. It also allows the compiler to mix the
address space it uses to access private memory. For example, the
compiler can still spill entire vector registers to an address space
without a strided access pattern, while accessing the same bits for
SIMT variable access using the strided address space.

Having address spaces within the location also makes it available to
consumers and therefore developers. This can also be an aid to
programmers who are seeking to optimize their code.

Since memory locations are an abstraction of storage, the same set of
operations can operate on memory locations independent of their
underlying storage. Therefore, operations like `DW_OP_deref*` can be
used on any memory locations even those referring to different address
spaces. Consequently, the `DW_OP_xderef*` operations are unnecessary,
except as a more compact way to encode a non-default address space
address followed by dereferencing it. Therefore we propose renaming
those operators to `DW_OP_aspace_deref*` to be consistent with the
other operators that refer to address spaces and making the old
`DW_OP_xderef` names aliases.

### Scope of proposal

The scope of this proposal is limited to introducing address spaces to
memory locations. Additional changes are needed to support address
spaces in CFI. However, they are not included in this proposal. Those
changes will be presented in a subsequent proposal.

## PROPOSAL

In Section 2.2 "Attribute Types", add the following row to Table 2.2
"Attribute names":

Table 2.2: Attribute names

| Attribute | Usage |
| :---- | :---- |
| `DW_AT_address_space` | Architecture specific address space |

Author:	Tony Tye, Ben Woodard, & GPU Subcommittee
Champion:	Ben Woodard
Date submitted:	2026-02-11
Date revised:	2026-08-03
Date closed:	2026-08-17
Type:	Enhancement
Status:	Accepted
DWARF version:	6