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| 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. |
| 4 | +// |
| 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. |
| 10 | + |
| 11 | +use rustc::ty::TyCtxt; |
| 12 | +use rustc::mir::*; |
| 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; |
| 17 | + |
| 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 |
| 20 | +/// code for these. |
| 21 | +pub struct RemoveNoopLandingPads; |
| 22 | + |
| 23 | +impl MirPass for RemoveNoopLandingPads { |
| 24 | + fn run_pass<'a, 'tcx>(&self, |
| 25 | + tcx: TyCtxt<'a, 'tcx, 'tcx>, |
| 26 | + _src: MirSource, |
| 27 | + mir: &mut Mir<'tcx>) { |
| 28 | + if tcx.sess.no_landing_pads() { |
| 29 | + return |
| 30 | + } |
| 31 | + |
| 32 | + debug!("remove_noop_landing_pads({:?})", mir); |
| 33 | + self.remove_nop_landing_pads(mir); |
| 34 | + } |
| 35 | +} |
| 36 | + |
| 37 | +impl RemoveNoopLandingPads { |
| 38 | + fn is_nop_landing_pad(&self, bb: BasicBlock, mir: &Mir, nop_landing_pads: &BitVector) |
| 39 | + -> bool |
| 40 | + { |
| 41 | + for stmt in &mir[bb].statements { |
| 42 | + match stmt.kind { |
| 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). |
| 50 | + } |
| 51 | + |
| 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 |
| 55 | + } |
| 56 | + |
| 57 | + StatementKind::Assign(_, _) | |
| 58 | + StatementKind::SetDiscriminant { .. } | |
| 59 | + StatementKind::InlineAsm { .. } | |
| 60 | + StatementKind::Validate { .. } => { |
| 61 | + return false; |
| 62 | + } |
| 63 | + } |
| 64 | + } |
| 65 | + |
| 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()) |
| 74 | + }) |
| 75 | + }, |
| 76 | + TerminatorKind::GeneratorDrop | |
| 77 | + TerminatorKind::Yield { .. } | |
| 78 | + TerminatorKind::Return | |
| 79 | + TerminatorKind::Unreachable | |
| 80 | + TerminatorKind::Call { .. } | |
| 81 | + TerminatorKind::Assert { .. } | |
| 82 | + TerminatorKind::DropAndReplace { .. } | |
| 83 | + TerminatorKind::Drop { .. } => { |
| 84 | + false |
| 85 | + } |
| 86 | + } |
| 87 | + } |
| 88 | + |
| 89 | + fn remove_nop_landing_pads(&self, mir: &mut Mir) { |
| 90 | + // make sure there's a single resume block |
| 91 | + let resume_block = { |
| 92 | + let patch = MirPatch::new(mir); |
| 93 | + let resume_block = patch.resume_block(); |
| 94 | + patch.apply(mir); |
| 95 | + resume_block |
| 96 | + }; |
| 97 | + debug!("remove_noop_landing_pads: resume block is {:?}", resume_block); |
| 98 | + |
| 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()); |
| 102 | + |
| 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; |
| 112 | + jumps_folded += 1; |
| 113 | + } |
| 114 | + } |
| 115 | + |
| 116 | + match mir[bb].terminator_mut().unwind_mut() { |
| 117 | + Some(unwind) => { |
| 118 | + if *unwind == Some(resume_block) { |
| 119 | + debug!(" removing noop landing pad"); |
| 120 | + jumps_folded -= 1; |
| 121 | + landing_pads_removed += 1; |
| 122 | + *unwind = None; |
| 123 | + } |
| 124 | + } |
| 125 | + _ => {} |
| 126 | + } |
| 127 | + |
| 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()); |
| 131 | + } |
| 132 | + debug!(" is_nop_landing_pad({:?}) = {}", bb, is_nop_landing_pad); |
| 133 | + } |
| 134 | + |
| 135 | + debug!("removed {:?} jumps and {:?} landing pads", jumps_folded, landing_pads_removed); |
| 136 | + } |
| 137 | +} |
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