src/librustc_mir/transform/remove_noop_landing_pads.rs

   1 use rustc::ty::TyCtxt;
   2 use rustc::mir::*;
   3 use rustc_data_structures::bit_set::BitSet;
   4 use crate::transform::{MirPass, MirSource};
   5 use crate::util::patch::MirPatch;
   6
   7 /// A pass that removes noop landing pads and replaces jumps to them with
   8 /// `None`. This is important because otherwise LLVM generates terrible
   9 /// code for these.
  10 pub struct RemoveNoopLandingPads;
  11
  12 pub fn remove_noop_landing_pads<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  13     if tcx.sess.no_landing_pads() {
  14         return
  15     }
  16     debug!("remove_noop_landing_pads({:?})", body);
  17
  18     RemoveNoopLandingPads.remove_nop_landing_pads(body)
  19 }
  20
  21 impl<'tcx> MirPass<'tcx> for RemoveNoopLandingPads {
  22     fn run_pass(&self, tcx: TyCtxt<'tcx>, _src: MirSource<'tcx>, body: &mut Body<'tcx>) {
  23         remove_noop_landing_pads(tcx, body);
  24     }
  25 }
  26
  27 impl RemoveNoopLandingPads {
  28     fn is_nop_landing_pad(
  29         &self,
  30         bb: BasicBlock,
  31         body: &Body<'_>,
  32         nop_landing_pads: &BitSet<BasicBlock>,
  33     ) -> bool {
  34         for stmt in &body[bb].statements {
  35             match stmt.kind {
  36                 StatementKind::FakeRead(..) |
  37                 StatementKind::StorageLive(_) |
  38                 StatementKind::StorageDead(_) |
  39                 StatementKind::AscribeUserType(..) |
  40                 StatementKind::Nop => {
  41                     // These are all nops in a landing pad
  42                 }
  43
  44                 StatementKind::Assign(Place {
  45                     base: PlaceBase::Local(_),
  46                     projection: None,
  47                 }, box Rvalue::Use(_)) => {
  48                     // Writing to a local (e.g., a drop flag) does not
  49                     // turn a landing pad to a non-nop
  50                 }
  51
  52                 StatementKind::Assign { .. } |
  53                 StatementKind::SetDiscriminant { .. } |
  54                 StatementKind::InlineAsm { .. } |
  55                 StatementKind::Retag { .. } => {
  56                     return false;
  57                 }
  58             }
  59         }
  60
  61         let terminator = body[bb].terminator();
  62         match terminator.kind {
  63             TerminatorKind::Goto { .. } |
  64             TerminatorKind::Resume |
  65             TerminatorKind::SwitchInt { .. } |
  66             TerminatorKind::FalseEdges { .. } |
  67             TerminatorKind::FalseUnwind { .. } => {
  68                 terminator.successors().all(|&succ| {
  69                     nop_landing_pads.contains(succ)
  70                 })
  71             },
  72             TerminatorKind::GeneratorDrop |
  73             TerminatorKind::Yield { .. } |
  74             TerminatorKind::Return |
  75             TerminatorKind::Abort |
  76             TerminatorKind::Unreachable |
  77             TerminatorKind::Call { .. } |
  78             TerminatorKind::Assert { .. } |
  79             TerminatorKind::DropAndReplace { .. } |
  80             TerminatorKind::Drop { .. } => {
  81                 false
  82             }
  83         }
  84     }
  85
  86     fn remove_nop_landing_pads(&self, body: &mut Body<'_>) {
  87         // make sure there's a single resume block
  88         let resume_block = {
  89             let patch = MirPatch::new(body);
  90             let resume_block = patch.resume_block();
  91             patch.apply(body);
  92             resume_block
  93         };
  94         debug!("remove_noop_landing_pads: resume block is {:?}", resume_block);
  95
  96         let mut jumps_folded = 0;
  97         let mut landing_pads_removed = 0;
  98         let mut nop_landing_pads = BitSet::new_empty(body.basic_blocks().len());
  99
 100         // This is a post-order traversal, so that if A post-dominates B
 101         // then A will be visited before B.
 102         let postorder: Vec<_> = traversal::postorder(body).map(|(bb, _)| bb).collect();
 103         for bb in postorder {
 104             debug!("  processing {:?}", bb);
 105             for target in body[bb].terminator_mut().successors_mut() {
 106                 if *target != resume_block && nop_landing_pads.contains(*target) {
 107                     debug!("    folding noop jump to {:?} to resume block", target);
 108                     *target = resume_block;
 109                     jumps_folded += 1;
 110                 }
 111             }
 112
 113             match body[bb].terminator_mut().unwind_mut() {
 114                 Some(unwind) => {
 115                     if *unwind == Some(resume_block) {
 116                         debug!("    removing noop landing pad");
 117                         jumps_folded -= 1;
 118                         landing_pads_removed += 1;
 119                         *unwind = None;
 120                     }
 121                 }
 122                 _ => {}
 123             }
 124
 125             let is_nop_landing_pad = self.is_nop_landing_pad(bb, body, &nop_landing_pads);
 126             if is_nop_landing_pad {
 127                 nop_landing_pads.insert(bb);
 128             }
 129             debug!("    is_nop_landing_pad({:?}) = {}", bb, is_nop_landing_pad);
 130         }
 131
 132         debug!("removed {:?} jumps and {:?} landing pads", jumps_folded, landing_pads_removed);
 133     }
 134 }