LLVM Discusses Making ClangIR a Default Build

A discussion has begun in the LLVM community over whether to include ClangIR, the new intermediate representation for the Clang compiler, in the default build. According to reporting by Michael Larabel of Phoronix, a thread soliciting opinions has been started on LLVM Discourse, and a corresponding RFC pull request has been submitted. The current proposal is not to enable ClangIR, but to make building ClangIR together when building Clang from source the standard configuration.

This proposal reflects the history that ClangIR, although already integrated upstream, has been excluded from the standard build configuration. Including it in the build targets aims to broaden development and testing. At the same time, operational burdens such as a significant increase in build time and expanded dependencies have been pointed out.

ClangIR makes use of MLIR and is higher-level than LLVM IR. With ClangIR able to preserve more C/C++ source semantics, its usage can lead to better diagnostics, code/static analysis, and more performance optimizations

ClangIR’s Role and Objectives

ClangIR is a new intermediate representation that has been developed for Clang over several years. It is based on MLIR and is characterized by a higher level of abstraction than LLVM IR. Because it can preserve more semantic information from C and C++ sources, it reduces information loss compared to the conventional approach of lowering directly from the abstract syntax tree to LLVM IR.

This characteristic directly leads to better-quality diagnostic messages and more accurate static analysis. If the compiler can accurately grasp the intent of the source, more precise warnings and fewer false positives can be expected. In addition, high-level information can be utilized during optimization, expanding opportunities for performance improvements.

ClangIR is designed not so much as a simple replacement but as a complement to the existing path to LLVM IR. It is structured to increase optimization options by performing analysis and transformations in a high-level representation before progressively lowering to lower-level representations. By adopting the MLIR framework, flexibility to bridge multiple levels of abstraction is ensured.

As with similar efforts, the advantages of MLIR are also attracting attention for offloading to GPUs and targeting heterogeneous devices. ClangIR is positioned as a foundation designed with support for such diverse backends in mind.

Proposal to Build by Default and Current Status

What is being discussed is not enabling the use of ClangIR by default. It is a proposal to make building ClangIR together when building Clang the standard. According to reporting by Michael Larabel of Phoronix, advantages and disadvantages are being organized in the LLVM Discourse thread and the RFC pull request.

Currently, ClangIR is integrated upstream but has been excluded from the standard build configuration. If the proposal is adopted, simply building Clang will also produce ClangIR artifacts. However, the compiler’s behavior itself will not change. To actually use processing via ClangIR, the -fclangir flag must be explicitly specified in a corresponding build.

This distinction is important. Including it at build time is different from using it by default at run time. The proposal is limited to the former and does not affect the default code-generation behavior. The intent is to increase testing opportunities while limiting the impact on compatibility through gradual introduction.

In the discussion, the impact of changing the build configuration on developers and continuous integration is also an issue. Building it by default has the advantage of making it easier to detect ClangIR-related bugs early. On the other hand, consideration is needed for the fact that not all developers need ClangIR.

Benefits and Future Potential of ClangIR

The benefits of ClangIR can be summarized in three areas: diagnostics, analysis, and optimization. By preserving high-level semantic information, the compiler can present warnings and suggestions that more accurately reflect the intent of the source code. In static analysis, fewer false positives and broader detection coverage are also expected.

From an optimization perspective, information available only at the high-level representation stage can be utilized. By performing transformations while preserving control structures, type information, and language-specific semantics, optimization clues that tend to be lost in low-level representations can be retained. As a result, the quality of generated code may improve.

Offloading to GPUs and various devices is cited as a future extension. ClangIR, based on MLIR, may handle lowering for SPIR-V and Vulkan more appropriately. Opening a path to efficiently deploy from a single source to multiple execution environments is an advantage for large-scale parallel processing and heterogeneous environments.

This direction aligns with the growing demand for compiler infrastructure to handle diverse targets in recent years. As seen in new hardware support efforts such as those reported in Nova Lake-S Gets 7 GPU IDs on Linux 7.3, the importance of having a flexible intermediate representation on the compiler side is increasing. If device-specific differences can be absorbed within the MLIR framework, both maintainability and extensibility can be achieved.

Doubled Build Times and Windows Support Challenges

Despite its advantages, operational challenges are also clear. The biggest concern is a significant increase in the build time for Clang itself. Some estimates suggest that including ClangIR and MLIR by default could more than double build times. The impact on developers’ iterative work and verification cycles is not small.

In terms of dependencies, building ClangIR by default would make MLIR a standard dependency of Clang. As a previously optional dependency becomes nearly mandatory, it affects fetch and build effort, disk usage, and cache efficiency. For small environments operating with limited resources, the burden becomes relatively larger.

Support for Windows is also an issue. ClangIR is said to lack support for so-called Microsoft targets. For users who primarily target Windows, including it in the standard build may have little significance. The loss of uniformity across platforms needs to be considered in terms of distribution and documentation.

Increased load on continuous integration has also been pointed out. If additional ClangIR tests are run, CI execution time will lengthen and computing resource consumption will increase. Without measures such as parallelization or selective execution, overall development speed could be affected.

Impact of MLIR Dependency on Builds

MLIR has expanded its role within the LLVM project as a foundation for handling multiple levels of abstraction in a unified way. With ClangIR adopting MLIR, the coupling between Clang and MLIR has increased. This proposal moves to make that coupling the default in terms of build configuration as well.

Making MLIR a standard dependency means more than a simple addition; it changes the assumptions for build procedures and deliverables. From the perspective of package management and binary distribution, increased dependencies affect users. For prebuilt artifacts, size and compatibility verification items increase.

On the other hand, assuming MLIR also lays the groundwork for future feature additions. If new transformations and optimizations can be implemented on a common foundation, development efficiency can be improved while avoiding duplication. Extensions in areas where MLIR excels, such as lowering for GPUs and accelerators, become easier.

For the developer community, including MLIR by default is also expected to lower barriers to ClangIR-related contributions and testing. If ClangIR is built in more environments, opportunities to discover bugs and evaluate performance will increase. As a result, quality and maturity may improve.

Future Discussions and Path to Default Enablement

The proposal is only to standardize the build targets and does not reach the stage of using ClangIR by default. Changing the default code-generation behavior will require further verification and consensus. At this stage, the aim is clear: to broaden testing coverage while limiting risk by separating building from use.

In the LLVM Discourse and RFC pull request, quantitative evaluation of advantages and disadvantages is being sought. Measured build times, estimates of CI resources, and plans for Windows support will be the focus of discussion based on concrete data. If numbers and reproducible measurements are shared, consensus will be easier to reach.

Whether to adopt it is also a choice of whether to prioritize development efficiency or future potential. Whether to accept a short-term increase in burden to broaden the testing base, or to maintain the current lightweight configuration. Adjustments that take into account the community’s diverse use cases are needed.

As a future outlook, gradual introduction or providing options could be a realistic compromise. For example, operational measures such as switching according to distribution form or build type, and selective execution in CI. In any case, the maturity of ClangIR and progress in surrounding support will determine the final decision.

Editorial Opinion

In the short term, if building ClangIR by default is adopted, we expect an immediate impact on the developer experience. Increased build times and longer CI times will burden daily development cycles. On the other hand, broader test coverage will advance early bug detection, accelerating stabilization of ClangIR. In distribution and documentation, procedures based on MLIR dependency will also need to be established.

From a long-term perspective, we see MLIR-based high-level intermediate representations becoming central to compiler evolution. More advanced diagnostics and static analysis, and flexibility for cross-device optimization, will grow in value as heterogeneous environments expand. Lagging Windows support remains a challenge, but if incremental support progresses, ClangIR could enhance the extensibility of LLVM as a whole. Given hardware diversification, we judge it a worthwhile investment.

As a question from the editorial team, the issue remains of how far the scope of the standard build should be extended. With not all users needing ClangIR, can a uniform increase in burden be justified? A design that leaves options according to build type and distribution form would be desirable.

References

• “LLVM Developers Discuss Enabling ClangIR Build By Default”, by Michael Larabel — Phoronix, 2026-09-06T10:43:10.000Z (ARR)
• Source URL: https://www.phoronix.com/news/ClangIR-Build-Default-Maybe