rustc_mir_transform/
remove_noop_landing_pads.rs

1use rustc_index::bit_set::DenseBitSet;
2use rustc_middle::mir::*;
3use rustc_middle::ty::TyCtxt;
4use rustc_target::spec::PanicStrategy;
5use tracing::debug;
6
7use crate::patch::MirPatch;
8
9/// A pass that removes noop landing pads and replaces jumps to them with
10/// `UnwindAction::Continue`. This is important because otherwise LLVM generates
11/// terrible code for these.
12pub(super) struct RemoveNoopLandingPads;
13
14impl<'tcx> crate::MirPass<'tcx> for RemoveNoopLandingPads {
15    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
16        sess.panic_strategy() != PanicStrategy::Abort
17    }
18
19    fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
20        let def_id = body.source.def_id();
21        debug!(?def_id);
22
23        // Skip the pass if there are no blocks with a resume terminator.
24        let has_resume = body
25            .basic_blocks
26            .iter_enumerated()
27            .any(|(_bb, block)| matches!(block.terminator().kind, TerminatorKind::UnwindResume));
28        if !has_resume {
29            debug!("remove_noop_landing_pads: no resume block in MIR");
30            return;
31        }
32
33        // make sure there's a resume block without any statements
34        let resume_block = {
35            let mut patch = MirPatch::new(body);
36            let resume_block = patch.resume_block();
37            patch.apply(body);
38            resume_block
39        };
40        debug!("remove_noop_landing_pads: resume block is {:?}", resume_block);
41
42        let mut jumps_folded = 0;
43        let mut landing_pads_removed = 0;
44        let mut nop_landing_pads = DenseBitSet::new_empty(body.basic_blocks.len());
45
46        // This is a post-order traversal, so that if A post-dominates B
47        // then A will be visited before B.
48        let postorder: Vec<_> = traversal::postorder(body).map(|(bb, _)| bb).collect();
49        for bb in postorder {
50            debug!("  processing {:?}", bb);
51            if let Some(unwind) = body[bb].terminator_mut().unwind_mut()
52                && let UnwindAction::Cleanup(unwind_bb) = *unwind
53                && nop_landing_pads.contains(unwind_bb)
54            {
55                debug!("    removing noop landing pad");
56                landing_pads_removed += 1;
57                *unwind = UnwindAction::Continue;
58            }
59
60            body[bb].terminator_mut().successors_mut(|target| {
61                if *target != resume_block && nop_landing_pads.contains(*target) {
62                    debug!("    folding noop jump to {:?} to resume block", target);
63                    *target = resume_block;
64                    jumps_folded += 1;
65                }
66            });
67
68            let is_nop_landing_pad = self.is_nop_landing_pad(bb, body, &nop_landing_pads);
69            if is_nop_landing_pad {
70                nop_landing_pads.insert(bb);
71            }
72            debug!("    is_nop_landing_pad({:?}) = {}", bb, is_nop_landing_pad);
73        }
74
75        debug!("removed {:?} jumps and {:?} landing pads", jumps_folded, landing_pads_removed);
76    }
77
78    fn is_required(&self) -> bool {
79        true
80    }
81}
82
83impl RemoveNoopLandingPads {
84    fn is_nop_landing_pad(
85        &self,
86        bb: BasicBlock,
87        body: &Body<'_>,
88        nop_landing_pads: &DenseBitSet<BasicBlock>,
89    ) -> bool {
90        for stmt in &body[bb].statements {
91            match &stmt.kind {
92                StatementKind::FakeRead(..)
93                | StatementKind::StorageLive(_)
94                | StatementKind::StorageDead(_)
95                | StatementKind::PlaceMention(..)
96                | StatementKind::AscribeUserType(..)
97                | StatementKind::Coverage(..)
98                | StatementKind::ConstEvalCounter
99                | StatementKind::BackwardIncompatibleDropHint { .. }
100                | StatementKind::Nop => {
101                    // These are all noops in a landing pad
102                }
103
104                StatementKind::Assign(box (place, Rvalue::Use(_) | Rvalue::Discriminant(_))) => {
105                    if place.as_local().is_some() {
106                        // Writing to a local (e.g., a drop flag) does not
107                        // turn a landing pad to a non-nop
108                    } else {
109                        return false;
110                    }
111                }
112
113                StatementKind::Assign { .. }
114                | StatementKind::SetDiscriminant { .. }
115                | StatementKind::Deinit(..)
116                | StatementKind::Intrinsic(..)
117                | StatementKind::Retag { .. } => {
118                    return false;
119                }
120            }
121        }
122
123        let terminator = body[bb].terminator();
124        match terminator.kind {
125            TerminatorKind::Goto { .. }
126            | TerminatorKind::UnwindResume
127            | TerminatorKind::SwitchInt { .. }
128            | TerminatorKind::FalseEdge { .. }
129            | TerminatorKind::FalseUnwind { .. } => {
130                terminator.successors().all(|succ| nop_landing_pads.contains(succ))
131            }
132            TerminatorKind::CoroutineDrop
133            | TerminatorKind::Yield { .. }
134            | TerminatorKind::Return
135            | TerminatorKind::UnwindTerminate(_)
136            | TerminatorKind::Unreachable
137            | TerminatorKind::Call { .. }
138            | TerminatorKind::TailCall { .. }
139            | TerminatorKind::Assert { .. }
140            | TerminatorKind::Drop { .. }
141            | TerminatorKind::InlineAsm { .. } => false,
142        }
143    }
144}
rustc_mir_transform/remove_noop_landing_pads.rs

rustc_mir_transform/
remove_noop_landing_pads.rs
