Optimisation-dependent IR decisions in Clang
Clang's Intermediate Representation varies with optimization levels; disabling optimization adds debugging aids, while enabling it introduces performance enhancements like lifetime markers and TBAA metadata, impacting compiler usage and performance tuning.
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The article discusses the differences in Intermediate Representation (IR) emitted by Clang based on the optimization level during compilation. Initially, the author assumed that Clang produced a consistent IR for the LLVM optimization pipeline, but further investigation revealed significant variations depending on whether optimization is enabled or disabled. The author conducted a survey of the IR emission code paths in Clang, focusing on the changes introduced at different optimization levels while excluding specialized language specifics and ABI details. When optimization is disabled, Clang adds certain features to aid debugging, such as distinct destructors and trap calls for unreachable blocks. Conversely, when optimization is enabled, Clang introduces various enhancements, including lifetime markers, type-based alias analysis (TBAA) metadata, and optimizations like loop unrolling and condition likelihood tracking. The author provides an example comparing IR outputs at different optimization levels, highlighting that lifetime markers and TBAA metadata are primarily responsible for the observed differences. The article emphasizes the importance of understanding these optimization-dependent decisions in Clang for better compiler usage and performance tuning.
- Clang's IR output varies significantly based on optimization levels.
- Disabling optimization adds debugging aids, while enabling it introduces performance enhancements.
- Key features activated during optimization include lifetime markers and TBAA metadata.
- The author provides a practical example to illustrate the differences in IR outputs.
- Understanding these differences can aid in compiler usage and performance optimization.
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