CMake Spack Compiler Flag Overrides Discussion And Solutions

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This article delves into a discussion surrounding CMake and Spack compiler flag overrides, focusing on a specific issue where flags defined in CMAKE_*_FLAGS_RELEASE take precedence over Spack compiler flag overrides. We will explore the context of this issue, the underlying causes, the implications, and potential solutions. This exploration aims to provide a comprehensive understanding of the problem and guide developers towards effective resolutions.

Background: CMake and Spack

Before diving into the specifics, let's establish a foundational understanding of the technologies involved.

  • CMake: CMake is a cross-platform, open-source build system generator. It manages the build process of software, allowing developers to define build configurations in a platform-independent manner. CMake generates native build files (e.g., Makefiles, Ninja build files) that are then used by the underlying build system to compile and link the software.
  • Spack: Spack is a package manager designed for scientific computing environments. It simplifies the installation and management of software packages, particularly those with complex dependencies. Spack allows users to specify compilers, compiler flags, and other build options, ensuring reproducibility and consistency across different systems.

The Issue: Spack Compiler Flag Overrides Precedence

The core problem lies in the order in which CMake processes compiler flags. Spack, when used, injects compiler flags into CMake's variables. Specifically, these flags are added to CMAKE_*_FLAGS, where * represents the language (e.g., C, C++, Fortran). However, CMake also has variables like CMAKE_*_FLAGS_RELEASE, which are intended for release builds and often contain optimization flags. The issue arises because CMAKE_*_FLAGS_RELEASE is appended after CMAKE_*_FLAGS, effectively giving it higher precedence. This means that any flags defined in CMAKE_*_FLAGS_RELEASE will override the flags injected by Spack, potentially leading to unexpected behavior or performance issues.

The Problematic Scenario: Overriding Optimization Flags

Compiler flag precedence becomes crucial when Spack is used to specify particular optimization levels or debugging flags. Consider a scenario where Spack is configured to build a package with debugging symbols (-g) for development purposes. If the CMAKE_*_FLAGS_RELEASE variables in the project's CMakeLists.txt contain optimization flags (e.g., -O3) without debugging symbols, the release flags will override Spack's debugging flags, resulting in a release build without debugging information. This makes debugging significantly more challenging.

Why This Matters: Reproducibility and Consistency

One of the primary goals of Spack is to ensure reproducibility and consistency in software builds. By allowing compiler flags to be overridden, this goal is undermined. If a user specifies a particular set of flags through Spack, they expect those flags to be honored during the build process. Overriding these flags can lead to builds that are not reproducible, as the final compiler flags used may differ from what was intended.

The ACCESS-NRI Context and the flag_handler

The ACCESS-NRI (Australian Community Climate and Earth System Simulator National Research Infrastructure) context adds another layer to this discussion. ACCESS-NRI utilizes Spack extensively for managing its software stack. A key aspect of this setup involves the flag_handler, which is responsible for injecting compiler flags into the build system.

The Shift to build_system_flags

Recently, the default flag_handler was changed to use build_system_flags for CMake and Autotools Spack packages. This change was implemented via pull requests #239 and #240 in the ACCESS-NRI/spack-packages repository.

  • Motivation Behind the Change: The primary motivation behind this change was to improve the handling of compiler flags in a more robust and consistent manner. The build_system_flags approach was intended to provide a more controlled way of injecting flags, ensuring that they are applied correctly within the build system's context.
  • How build_system_flags Works: The build_system_flags approach aims to directly inject flags into the build system's configuration, rather than relying on environment variables or other indirect mechanisms. This can lead to more predictable and reliable flag handling.

The Consequence: Exacerbating the Override Issue

Unfortunately, the shift to build_system_flags, while intended to improve flag handling, inadvertently exacerbated the issue of flag overrides in CMake. By injecting flags in a way that makes them susceptible to being overridden by CMAKE_*_FLAGS_RELEASE, the change effectively made the problem more pronounced.

Re-evaluating the Motivation

The original motivation for the build_system_flags change is now less critical due to the availability of Spack debug logs on Gadi, a high-performance computing system used by ACCESS-NRI. These debug logs provide detailed information about the build process, making it easier to diagnose and resolve issues without relying as heavily on specific flag handling mechanisms.

Potential Solutions and Mitigation Strategies

Given the complexities and implications of this issue, several potential solutions and mitigation strategies can be considered.

1. Reverting to the Default inject_flags Flag Handler

  • The Quickest Solution: The most straightforward solution is to revert to the default inject_flags flag handler. This would effectively undo the changes introduced in pull requests #239 and #240, mitigating the immediate issue of flag overrides.
  • Drawbacks: While this approach provides a quick fix, it doesn't address the underlying desire for a more robust flag handling mechanism. It essentially rolls back to the previous behavior, which may have its own limitations.

2. Developing a Solution for build_system_flags and CMake

  • The Preferred Approach: A more comprehensive solution would involve developing a mechanism that allows build_system_flags to work effectively with CMake-based packages. This would retain the benefits of the build_system_flags approach while addressing the flag override issue.
  • Possible Strategies: Several strategies could be employed to achieve this:
    • Modifying CMakeLists.txt: One approach is to modify the CMakeLists.txt files of affected packages to ensure that Spack-injected flags are given higher precedence. This could involve adjusting the order in which flags are processed or using CMake's features to manage flag precedence.
    • Enhancing Spack's CMake Integration: Another strategy is to enhance Spack's integration with CMake. This could involve adding new features to Spack that allow it to more directly control the flag injection process, ensuring that Spack-specified flags take precedence.
    • Custom Flag Handling Logic: A more advanced approach might involve developing custom flag handling logic within Spack that is specifically tailored to CMake. This could involve creating a custom module or function that manages flag injection in a way that respects precedence rules.

3. Educating Users and Providing Workarounds

  • A Practical Step: In the interim, while a long-term solution is being developed, it's crucial to educate users about the issue and provide workarounds.
  • Workaround Examples:
    • Modifying Spack Configuration: Users can modify their Spack configuration to explicitly set the desired compiler flags, ensuring that they are not overridden by CMake's default flags.
    • Adjusting CMake Variables: Users can also adjust the CMAKE_*_FLAGS_RELEASE variables in their build environment to include the necessary flags, effectively merging the Spack-specified flags with the release flags.

4. Implementing a Precedence System

  • A Structured Approach: A more structured solution would be to implement a clear precedence system for compiler flags. This system would define the order in which flags from different sources (e.g., Spack, CMakeLists.txt, environment variables) are applied.
  • Benefits of Precedence: This approach would provide a predictable and consistent way of managing flags, reducing the likelihood of unexpected overrides.

The Decision: Balancing Act

Choosing the best solution involves balancing several factors, including:

  • Time and Resources: Reverting to the default inject_flags is the quickest solution but doesn't address the underlying issue.
  • Long-Term Maintainability: Developing a solution for build_system_flags and CMake is more complex but offers a more robust long-term solution.
  • User Impact: Educating users and providing workarounds is essential in the short term, regardless of the chosen long-term solution.

Conclusion

The issue of Spack compiler flag overrides in CMake highlights the complexities of managing build systems and package managers in scientific computing environments. While the shift to build_system_flags aimed to improve flag handling, it inadvertently exacerbated the override issue. Moving forward, a comprehensive solution that allows build_system_flags to work effectively with CMake is desirable. In the meantime, educating users and providing workarounds are crucial steps. This discussion serves as a valuable guide for developers navigating similar challenges and emphasizes the importance of careful consideration when integrating different build tools and package managers.