rustc_mir_transform/
remove_noop_landing_pads.rs1use rustc_index::bit_set::DenseBitSet;
2use rustc_middle::mir::*;
3use rustc_middle::ty::TyCtxt;
4use tracing::{debug, instrument};
5
6use crate::patch::MirPatch;
7
8pub(super) struct RemoveNoopLandingPads;
12
13impl<'tcx> crate::MirPass<'tcx> for RemoveNoopLandingPads {
14 fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
15 sess.panic_strategy().unwinds()
16 }
17
18 #[instrument(level = "debug", skip(self, _tcx, body))]
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 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!("no resume block in MIR");
30 return;
31 }
32
33 let mut nop_landing_pads = DenseBitSet::new_empty(body.basic_blocks.len());
34
35 for (bb, bbdata) in traversal::postorder(body) {
38 let is_nop_landing_pad = self.is_nop_landing_pad(bbdata, &nop_landing_pads);
39 debug!("is_nop_landing_pad({bb:?}) = {is_nop_landing_pad}");
40 if is_nop_landing_pad {
41 nop_landing_pads.insert(bb);
42 }
43 }
44
45 if nop_landing_pads.is_empty() {
46 debug!("no nop landing pads in MIR");
47 return;
48 }
49
50 let resume_block = {
52 let mut patch = MirPatch::new(body);
53 let resume_block = patch.resume_block();
54 patch.apply(body);
55 resume_block
56 };
57 debug!(?resume_block);
58
59 let basic_blocks = body.basic_blocks.as_mut();
60 for (bb, bbdata) in basic_blocks.iter_enumerated_mut() {
61 debug!("processing {:?}", bb);
62
63 if let Some(unwind) = bbdata.terminator_mut().unwind_mut()
64 && let UnwindAction::Cleanup(unwind_bb) = *unwind
65 && nop_landing_pads.contains(unwind_bb)
66 {
67 debug!(" removing noop landing pad");
68 *unwind = UnwindAction::Continue;
69 }
70
71 bbdata.terminator_mut().successors_mut(|target| {
72 if *target != resume_block && nop_landing_pads.contains(*target) {
73 debug!(" folding noop jump to {:?} to resume block", target);
74 *target = resume_block;
75 }
76 });
77 }
78 }
79
80 fn is_required(&self) -> bool {
81 true
82 }
83}
84
85impl RemoveNoopLandingPads {
86 fn is_nop_landing_pad(
87 &self,
88 bbdata: &BasicBlockData<'_>,
89 nop_landing_pads: &DenseBitSet<BasicBlock>,
90 ) -> bool {
91 for stmt in &bbdata.statements {
92 match &stmt.kind {
93 StatementKind::FakeRead(..)
94 | StatementKind::StorageLive(_)
95 | StatementKind::StorageDead(_)
96 | StatementKind::PlaceMention(..)
97 | StatementKind::AscribeUserType(..)
98 | StatementKind::Coverage(..)
99 | StatementKind::ConstEvalCounter
100 | StatementKind::BackwardIncompatibleDropHint { .. }
101 | StatementKind::Nop => {
102 }
104
105 StatementKind::Assign(box (place, Rvalue::Use(_) | Rvalue::Discriminant(_))) => {
106 if place.as_local().is_some() {
107 } else {
110 return false;
111 }
112 }
113
114 StatementKind::Assign { .. }
115 | StatementKind::SetDiscriminant { .. }
116 | StatementKind::Intrinsic(..)
117 | StatementKind::Retag { .. } => {
118 return false;
119 }
120 }
121 }
122
123 let terminator = bbdata.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}