Opaque Pointer workstream definition

workstreams/Opaque_Pointers_Workstream.md

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+# Workstream: Opaque Pointer Support
+
+## Motivation & Benefits
+
+Support for typed pointers in LLVM has been removed. More information about the
+motivation for this change can be found in LLVM forums. For example, the LLVM
+documentation on [opaque pointers](https://llvm.org/docs/OpaquePointers.html) as
+well as [this
+presentation](https://llvm.org/devmtg/2022-04-03/slides/keynote.Opaque.Pointers.Are.Coming.pdf)
+give a brief overview.
+
+The QIR specification currently makes use of typed pointers to represent qubit
+values and result values. While in LLVM 16, support for these types can still be
+enabled, an update to LLVM 17 requires us to change this representation.
+
+The goal of this workstream is hence to update the QIR specification to support
+newer versions of LLVM, and to document what LLVM versions can be used to
+support each version of the QIR specification.
+
+## Requirements
+
+Generally, LLVM maintains very good [backwards
+compatibility](https://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility);
+according to the LLVM 18 docs, it is possible to load even bitcode generated by
+an LLVM version as early as 3.0. However, it is not possible in general to
+generate bitcode that can be loaded with an older version of LLVM than the one
+used to emit it. This means that while *QIR consumers*, i.e. backend providers,
+can update to the latest LLVM version without sacrificing support for prior QIR
+versions, *QIR producers*, i.e. language frontends, cannot use LLVM
+versions that are newer than the one required by the versions of the QIR
+specification they support. This can pose challenges for maintaining a
+state-of-the-art front end.
+
+As part of this workstream, we hence specifically propose to update the required
+LLVM version for the next version of the QIR specification to LLVM 17.
+
+## Dependencies & Related Projects
+
+There are no related projects within the QIR Alliance at this time. However,
+there are a number of threads in the LLVM community by projects with similar
+needs. In particular, the considerations discussed in [this
+thread](https://discourse.llvm.org/t/rfc-better-support-for-typed-pointers-in-an-opaque-pointer-world/63339)
+and the follow up
+[here](https://discourse.llvm.org/t/rfc-adding-opaque-types-to-llvm-ir/65326)
+also apply to the QIR ecosystem.
+
+See also the section on [Open Questions](#open-questions) regarding possible
+alternatives to consider.
+
+A related topic to consider when discussing options for this workstream is
+whether the new representation would also allow to support qudits in the future.
+
+## Deliverable(s) & Expected Outcome
+
+The expected outcome of the workstream is a pull request to update the 0.1
+version of the QIR specification with the necessary changes to be able to emit
+the full QIR and all its profiles using LLVM 17.
+Additionally, the working group should outline a migration plan/recommendation 
+for front- and backend implementers to migrate from supporting the current 0.1 version 
+of the spec to the new version supporting LLVM 17. This migration plan could for
+example involve the creation of helper scripts and tools that would convert a 
+0.1 compliant representation to the new representation and vice versa.
+
+## Future Work (Out of Scope)
+
+We look for a recommendation by the working group whether 
+adding support for qudits and non-binary measurement results should 
+be discussed and fleshed out by this workstream, or deferred to a separate one.
+
+## Working Group & Getting Involved
+
+Members: <br/>
+Chair:
+
+If you would like to contribute to the workstream, please contact \<insert
+contact\>.
+
+## Schedule
+
+Launch date: <br/>
+Estimated end date: <br/>
+Meeting schedule and/or channel(s) of communication:
+
+## Status & Discussions
+
+The work and status is tracked in the form of a GitHub issue \<insert link\>.
+<br/>
+We encourage comments, inputs, and discussions on that issue.
+
+The GitHub issue is labeled as `Approved` after approval by the steering
+committee.
+
+## Open Questions
+
+As part of the workstream, we could re-examine the use of address spaces to
+distinguish memory on the QPU itself (i.e. qubits) from memory located on an
+adjacent classical processor. See also [this
+thread](https://discourse.llvm.org/t/casting-between-address-spaces-and-address-space-semantics/10920/16)
+that contains a discussion about the nature and use of different address spaces.
+
+There may be options to consider for representing quantum specific data types,
+and merely migrating from typed to opaque pointers may not be the best option.
+E.g. the working group should consider if it is desirable at all to work with
+pointers, especially since the QIR specification prohibits dereferencing of
+resources that are located on the QPU.