Upstreaming ClangIR

[andykaylor]

Overview

This is a tracking issue for the project of upstreaming ClangIR support from the incubator repository into the top-of-trunk LLVM repository. This issue will be periodically updated with status, milestones, and any blocking issues.

Current Status

Upstreaming of ClangIR is currently still a work in progress, but we are making good progress. Most simple C programs can be compiled (with the embarrassing exception of any code that passes multiple arguments to printf or other variadic functions). We are beginning to implement support for C++ constructs. You can define classes, including those using simple inheritance, access member variables, and call non-virtual member functions.

We have recently reached the state where you can compile code that includes any of the standard C++ header files on Linux. You won't be able to use many of the features in the standard library yet, because fundamental things like virtual function support are still missing, but we can process the header files. (If you find that not to be the case for some version of these headers, feel free to file a bug to let us know.)

Support for complex types is nearly complete. Normal cleanup of automatic variables (calling destructors when objects go out of scope) should be working. Bitfield support is now complete. Support has been added for a number of builtin functions.

As of August 13, 2025, approximately 40% of the code from the incubator project has been upstreamed. In addition, 62% (1140 of 1833) of the single source tests from the llvm-test-suite now pass when compiled with ClangIR enabled.

Coming soon

Work is underway in the following areas:

• Calling virtual functions and using virtual inheritance
• Exception handling
• Goto
• Inline assembly
• Atomics

Getting Involved

If you would like to help with the upstreaming effort, please contact @andykaylor

Check the list of open issues for tasks that could be started. Feel free to ask about any unassigned issue.

See also: https://discourse.llvm.org/t/rfc-upstreaming-clangir/76587

[bcardosolopes]

One related idea that might help us as part of GSoC: https://discourse.llvm.org/t/clangir-gsoc2025-validate-existing-clang-codegen-test-coverage-with-clangir/84481

[andykaylor]

I've just updated this issue with the latest status and short term plans. Does GitHub send notifications to people watching the issue when I edit the main description, or should I post comments when I update it?

[h-vetinari]

Does GitHub send notifications to people watching the issue when I edit the main description, or should I post comments when I update it?

Edits don't cause updates to be sent. Very likely not every edit needs to be pointed out, but something like a biweekly or monthly update would be great IMO!

Also, if you could keep the PR references for things that have been done already in the overview, that would make it much easier to use this as a historical reference later on (and make it easier to follow the kind of work that's been done already).

If you use

* [x] a completed thing
  * first part: #12321
  * finished: #12345
* [ ] a not-yet-finished thing
  * plan: #23432
  * WIP PR: #23456

github will turn [ ] / [x] it into a checkbox and render the PR/issue title as part of the enumeration. Not everything needs sub-bullets, and of course you should partition your sections/indentations/checkboxes as you see fit, this is just an illustration.

[bcardosolopes]

should I post comments when I update it?

comments are better for visibility!

[andykaylor]

Also, if you could keep the PR references for things that have been done already in the overview, that would make it much easier to use this as a historical reference later on (and make it easier to follow the kind of work that's been done already).

I'd prefer not to sign up for that much bookkeeping. PRs are being automatically tagged with the "ClangIR" label, so you can see what's been done with this query: https://github.com/llvm/llvm-project/pulls?q=is%3Apr+label%3AClangIR+is%3Aclosed

Is that sufficient?

[andykaylor]

I've just updated the status. The highlights are that arrays, loops, binary operations, and if statements are now supported.

[andykaylor]

The status has been updated. A lot of new features have been added since the last update. Most basic C code now works (though calls to functions that are identified as builtins still cause hiccups). We're beginning to add C++ features. You can now declare classes, including non-virtual inheritance, access class members, and call member functions. Constructors are destructors aren't yet implemented, but that's underway.

[andykaylor]

The status has been updated. Many things are starting to work. See above for details. Thanks to everyone who has been contributing!