Skip MIR pass `UnreachablePropagation` when coverage is enabled by Zalathar · Pu...
source link: https://github.com/rust-lang/rust/pull/116166
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When coverage instrumentation and MIR opts are both enabled, coverage relies on two assumptions:
-
MIR opts that would delete
StatementKind::Coveragestatements instead move them into bb0 and change them toCoverageKind::Unreachable. -
MIR opts won't delete all
CoverageKind::Counterstatements from an instrumented function.
Most MIR opts naturally satisfy the second assumption, because they won't remove coverage statements from bb0, but UnreachablePropagation can do so if it finds that bb0 is unreachable. If this happens, LLVM thinks the function isn't instrumented, and it vanishes from coverage reports.
A proper solution won't be possible until after per-function coverage info lands in #116046, but for now we can avoid the problem by turning off this particular pass when coverage instrumentation is enabled.
cc @cjgillot since I found this while investigating coverage problems encountered by #113970
@rustbot label +A-code-coverage +A-mir-opt
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