2026 EuroLLVM - Coverage directed codebase reduction for the procedural generation of LIT tests

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2026 EuroLLVM Developers' Meeting

https://llvm.org/devmtg/2026-04/

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Title: Coverage directed codebase reduction for the procedural generation of LIT tests

Speaker: Freya Fewtrell

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Slides: https://llvm.org/devmtg/2026-04/slides/lightning_talk/lightning_talk_fewtrell.pdf

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The LIT test suite, whilst extensive, still leaves many parts of the LLVM codebase uncovered. Compiling large real-world C++ code from Sony Interactive Entertainment's downstream integration test suite routinely exercises code paths that the upstream LIT suite never reaches. To shift this testing leftwards, we have built a tool that uses coverage-directed reduction to automatically turn such high-coverage sources into minimal, self-contained LLVM IR fragments suitable for inclusion as upstream LIT tests. This lightning talk describes how the tool works, the challenges encountered when trying to automate test reduction at scale, and whether increasing coverage alone is sufficient motivation for new upstream LIT tests.

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