1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 use rustc::ty::TyCtxt;
13 use rustc_data_structures::bitvec::BitVector;
14 use rustc_data_structures::indexed_vec::Idx;
15 use transform::{MirPass, MirSource};
16 use util::patch::MirPatch;
18 /// A pass that removes no-op landing pads and replaces jumps to them with
19 /// `None`. This is important because otherwise LLVM generates terrible
21 pub struct RemoveNoopLandingPads;
23 impl MirPass for RemoveNoopLandingPads {
24 fn run_pass<'a, 'tcx>(&self,
25 tcx: TyCtxt<'a, 'tcx, 'tcx>,
27 mir: &mut Mir<'tcx>) {
28 if tcx.sess.no_landing_pads() {
32 debug!("remove_noop_landing_pads({:?})", mir);
33 self.remove_nop_landing_pads(mir);
37 impl RemoveNoopLandingPads {
38 fn is_nop_landing_pad(&self, bb: BasicBlock, mir: &Mir, nop_landing_pads: &BitVector)
41 for stmt in &mir[bb].statements {
43 StatementKind::StorageLive(_) |
44 StatementKind::StorageDead(_) |
45 StatementKind::EndRegion(_) |
46 StatementKind::Nop => {
47 // These are all nops in a landing pad (there's some
48 // borrowck interaction between EndRegion and storage
49 // instructions, but this should all run after borrowck).
52 StatementKind::Assign(Place::Local(_), Rvalue::Use(_)) => {
53 // Writing to a local (e.g. a drop flag) does not
54 // turn a landing pad to a non-nop
57 StatementKind::Assign(_, _) |
58 StatementKind::SetDiscriminant { .. } |
59 StatementKind::InlineAsm { .. } |
60 StatementKind::Validate { .. } => {
66 let terminator = mir[bb].terminator();
67 match terminator.kind {
68 TerminatorKind::Goto { .. } |
69 TerminatorKind::Resume |
70 TerminatorKind::SwitchInt { .. } |
71 TerminatorKind::FalseEdges { .. } => {
72 terminator.successors().iter().all(|succ| {
73 nop_landing_pads.contains(succ.index())
76 TerminatorKind::GeneratorDrop |
77 TerminatorKind::Yield { .. } |
78 TerminatorKind::Return |
79 TerminatorKind::Unreachable |
80 TerminatorKind::Call { .. } |
81 TerminatorKind::Assert { .. } |
82 TerminatorKind::DropAndReplace { .. } |
83 TerminatorKind::Drop { .. } => {
89 fn remove_nop_landing_pads(&self, mir: &mut Mir) {
90 // make sure there's a single resume block
92 let patch = MirPatch::new(mir);
93 let resume_block = patch.resume_block();
97 debug!("remove_noop_landing_pads: resume block is {:?}", resume_block);
99 let mut jumps_folded = 0;
100 let mut landing_pads_removed = 0;
101 let mut nop_landing_pads = BitVector::new(mir.basic_blocks().len());
103 // This is a post-order traversal, so that if A post-dominates B
104 // then A will be visited before B.
105 let postorder: Vec<_> = traversal::postorder(mir).map(|(bb, _)| bb).collect();
106 for bb in postorder {
107 debug!(" processing {:?}", bb);
108 for target in mir[bb].terminator_mut().successors_mut() {
109 if *target != resume_block && nop_landing_pads.contains(target.index()) {
110 debug!(" folding noop jump to {:?} to resume block", target);
111 *target = resume_block;
116 match mir[bb].terminator_mut().unwind_mut() {
118 if *unwind == Some(resume_block) {
119 debug!(" removing noop landing pad");
121 landing_pads_removed += 1;
128 let is_nop_landing_pad = self.is_nop_landing_pad(bb, mir, &nop_landing_pads);
129 if is_nop_landing_pad {
130 nop_landing_pads.insert(bb.index());
132 debug!(" is_nop_landing_pad({:?}) = {}", bb, is_nop_landing_pad);
135 debug!("removed {:?} jumps and {:?} landing pads", jumps_folded, landing_pads_removed);