Optimizing CMake Dependency Checks for Submodules

Oct 13, 2025 by wbrum.org 53 views

Optimizing CMake Dependency Checks for Submodules

When working with submodules in CMake, it's not uncommon to encounter issues with dependency checks. In this article, we'll explore a common problem and two potential solutions for customizing CMake dependency checks for submodules.

The Problem: Checking for Existing Submodule Dependencies

When using submodules in CMake, it's essential to ensure that the dependencies are correctly checked and linked. However, in some cases, the default CMake behavior may lead to unnecessary checks or even errors. Let's take the example of the robin_map submodule, which is used in a project as a submodule.

Using CMakeFindDependencyMacro

In the project, the robin_map submodule is included as a submodule, and the variable NB_USE_SUBMODULE_DEPS is set to false. However, the check for this submodule uses CMakeFindDependencyMacro, which successfully finds the submodule. This might seem like a good thing, but it can lead to unnecessary checks and potential issues.

Proposed Solution 1: Omitting the Check Completely

One possible solution is to omit the check for the submodule completely and simply link to tsl::robin_map when NB_USE_SUBMODULE_DEPS is true. This approach allows the user of nanobind to decide whether to use the submodule or not.

Benefits of Omitting the Check

By omitting the check, we can avoid unnecessary dependencies and potential issues. This approach also gives the user more control over the submodule usage, which can be beneficial in certain scenarios.

Proposed Solution 2: Checking for Existing Targets

Another approach is to check if the target is already defined and do not include any subdirectory when this is the case. This solution can be particularly useful when working with multiple submodules or complex dependency graphs.

Benefits of Checking for Existing Targets

By checking for existing targets, we can avoid unnecessary checks and potential issues. This approach also helps to improve the overall build performance and reduces the risk of errors.

Implementing the Solutions in CMake

To implement the proposed solutions in CMake, we can use the following code snippets:

Omitting the Check Completely

if(NB_USE_SUBMODULE_DEPS)
  target_link_libraries(${PROJECT_NAME} tsl::robin_map)
else()
  # Do nothing
endif()

Checking for Existing Targets

if(NOT TARGET robin_map)
  add_subdirectory(submodule)
endif()

Conclusion

In conclusion, customizing CMake dependency checks for submodules can be a complex task. By understanding the problem and exploring different solutions, we can improve the overall build performance and reduce the risk of errors. The proposed solutions in this article provide a good starting point for customizing CMake dependency checks for submodules.

Best Practices for Customizing CMake Dependency Checks

When customizing CMake dependency checks for submodules, it's essential to follow best practices to ensure that the changes are safe and effective. Here are some best practices to keep in mind:

Test thoroughly: Before making any changes, test the build process thoroughly to ensure that the changes do not introduce any errors or issues.
Use version control: Use version control systems like Git to track changes and collaborate with other developers.
Document changes: Document the changes made to the CMake files to ensure that other developers understand the changes and can maintain the codebase.
Follow CMake guidelines: Follow CMake guidelines and best practices to ensure that the changes are safe and effective.

Frequently Asked Questions about Customizing CMake Dependency Checks for Submodules

In our previous article, we explored the problem of customizing CMake dependency checks for submodules and proposed two potential solutions. In this article, we'll answer some frequently asked questions about customizing CMake dependency checks for submodules.

Q: What is the main problem with the default CMake behavior when working with submodules?

A: The main problem with the default CMake behavior when working with submodules is that it can lead to unnecessary checks and potential issues. When using submodules, CMake may try to find the dependencies, which can cause errors or slow down the build process.

Q: How can I omit the check for a submodule completely and simply link to the submodule when NB_USE_SUBMODULE_DEPS is true?

A: To omit the check for a submodule completely and simply link to the submodule when NB_USE_SUBMODULE_DEPS is true, you can use the following code snippet:

if(NB_USE_SUBMODULE_DEPS)
  target_link_libraries(${PROJECT_NAME} tsl::robin_map)
else()
  # Do nothing
endif()

Q: What are the benefits of omitting the check for a submodule completely?

A: The benefits of omitting the check for a submodule completely include avoiding unnecessary dependencies and potential issues. This approach also gives the user more control over the submodule usage, which can be beneficial in certain scenarios.

Q: How can I check if a target is already defined and do not include any subdirectory when this is the case?

A: To check if a target is already defined and do not include any subdirectory when this is the case, you can use the following code snippet:

if(NOT TARGET robin_map)
  add_subdirectory(submodule)
endif()

Q: What are the benefits of checking for existing targets?

A: The benefits of checking for existing targets include avoiding unnecessary checks and potential issues. This approach also helps to improve the overall build performance and reduces the risk of errors.

Q: How can I customize CMake dependency checks for submodules in a way that is safe and effective?

A: To customize CMake dependency checks for submodules in a way that is safe and effective, you should follow best practices, such as testing thoroughly, using version control, documenting changes, and following CMake guidelines.

Q: What are some best practices for customizing CMake dependency checks for submodules?

A: Some best practices for customizing CMake dependency checks for submodules include:

Test thoroughly: Before making any changes, test the build process thoroughly to ensure that the changes do not introduce any errors or issues.
Use version control: Use version control systems like Git to track changes and collaborate with other developers.
Document changes: Document the changes made to the CMake files to ensure that other developers understand the changes and can maintain the codebase.
Follow CMake guidelines: Follow CMake guidelines and best practices to ensure that the changes are safe and effective.