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A colleague contacted me and asked why Rust's counters start at 1, when Clangs appear to start at 0. There is a reason why Rust's internal counters start at 1 (see the docs), and I tried to keep them consistent when codegenned to LLVM's coverage mapping format. LLVM should be tolerant of missing counters, but as my colleague pointed out, `llvm-cov` will silently fail to generate a coverage report for a function based on LLVM's assumption that the counters are 0-based. See: https://github.com/llvm/llvm-project/blob/main/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp#L170 Apparently, if, for example, a function has no branches, it would have exactly 1 counter. `CounterValues.size()` would be 1, and (with the 1-based index), the counter ID would be 1. This would fail the check and abort reporting coverage for the function. It turns out that by correcting for this during coverage map generation, by subtracting 1 from the Rust Counter ID (both when generating the counter increment intrinsic call, and when adding counters to the map), some uncovered functions (including in tests) now appear covered! This corrects the coverage for a few tests! |
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For more information about how rustc works, see the rustc dev guide.