From 64df5a851e1cb8c2ae539aa5331d48b8f3646bf4 Mon Sep 17 00:00:00 2001 From: Zalathar Date: Mon, 25 Sep 2023 23:09:27 +1000 Subject: [PATCH] Skip MIR pass `UnreachablePropagation` when coverage is enabled When coverage instrumentation and MIR opts are both enabled, coverage relies on two assumptions: - MIR opts that would delete `StatementKind::Coverage` statements instead move them into bb0 and change them to `CoverageKind::Unreachable`. - MIR opts won't delete all `CoverageKind::Counter` statements 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. --- compiler/rustc_mir_transform/src/unreachable_prop.rs | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/compiler/rustc_mir_transform/src/unreachable_prop.rs b/compiler/rustc_mir_transform/src/unreachable_prop.rs index bd1724bf84241..0b9311a20efa3 100644 --- a/compiler/rustc_mir_transform/src/unreachable_prop.rs +++ b/compiler/rustc_mir_transform/src/unreachable_prop.rs @@ -13,7 +13,11 @@ pub struct UnreachablePropagation; impl MirPass<'_> for UnreachablePropagation { fn is_enabled(&self, sess: &rustc_session::Session) -> bool { // Enable only under -Zmir-opt-level=2 as this can make programs less debuggable. - sess.mir_opt_level() >= 2 + + // FIXME(#116171) Coverage gets confused by MIR passes that can remove all + // coverage statements from an instrumented function. This pass can be + // re-enabled when coverage codegen is robust against that happening. + sess.mir_opt_level() >= 2 && !sess.instrument_coverage() } fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {