src/librustc_mir/transform/remove_noop_landing_pads.rs

   1 use crate::transform::{MirPass, MirSource};
   2 use crate::util::patch::MirPatch;
   3 use rustc::mir::*;
   4 use rustc::ty::TyCtxt;
   5 use rustc_index::bit_set::BitSet;
   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 BodyAndCache<'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 BodyAndCache<'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(box (place, Rvalue::Use(_))) => {
  45                     if place.as_local().is_some() {
  46                         // Writing to a local (e.g., a drop flag) does not
  47                         // turn a landing pad to a non-nop
  48                     } else {
  49                         return false;
  50                     }
  51                 }
  52
  53                 StatementKind::Assign { .. }
  54                 | StatementKind::SetDiscriminant { .. }
  55                 | StatementKind::InlineAsm { .. }
  56                 | StatementKind::Retag { .. } => {
  57                     return false;
  58                 }
  59             }
  60         }
  61
  62         let terminator = body[bb].terminator();
  63         match terminator.kind {
  64             TerminatorKind::Goto { .. }
  65             | TerminatorKind::Resume
  66             | TerminatorKind::SwitchInt { .. }
  67             | TerminatorKind::FalseEdges { .. }
  68             | TerminatorKind::FalseUnwind { .. } => {
  69                 terminator.successors().all(|&succ| nop_landing_pads.contains(succ))
  70             }
  71             TerminatorKind::GeneratorDrop
  72             | TerminatorKind::Yield { .. }
  73             | TerminatorKind::Return
  74             | TerminatorKind::Abort
  75             | TerminatorKind::Unreachable
  76             | TerminatorKind::Call { .. }
  77             | TerminatorKind::Assert { .. }
  78             | TerminatorKind::DropAndReplace { .. }
  79             | TerminatorKind::Drop { .. } => false,
  80         }
  81     }
  82
  83     fn remove_nop_landing_pads(&self, body: &mut BodyAndCache<'_>) {
  84         // make sure there's a single resume block
  85         let resume_block = {
  86             let patch = MirPatch::new(body);
  87             let resume_block = patch.resume_block();
  88             patch.apply(body);
  89             resume_block
  90         };
  91         debug!("remove_noop_landing_pads: resume block is {:?}", resume_block);
  92
  93         let mut jumps_folded = 0;
  94         let mut landing_pads_removed = 0;
  95         let mut nop_landing_pads = BitSet::new_empty(body.basic_blocks().len());
  96
  97         // This is a post-order traversal, so that if A post-dominates B
  98         // then A will be visited before B.
  99         let postorder: Vec<_> = traversal::postorder(body).map(|(bb, _)| bb).collect();
 100         for bb in postorder {
 101             debug!("  processing {:?}", bb);
 102             for target in body[bb].terminator_mut().successors_mut() {
 103                 if *target != resume_block && nop_landing_pads.contains(*target) {
 104                     debug!("    folding noop jump to {:?} to resume block", target);
 105                     *target = resume_block;
 106                     jumps_folded += 1;
 107                 }
 108             }
 109
 110             match body[bb].terminator_mut().unwind_mut() {
 111                 Some(unwind) => {
 112                     if *unwind == Some(resume_block) {
 113                         debug!("    removing noop landing pad");
 114                         jumps_folded -= 1;
 115                         landing_pads_removed += 1;
 116                         *unwind = None;
 117                     }
 118                 }
 119                 _ => {}
 120             }
 121
 122             let is_nop_landing_pad = self.is_nop_landing_pad(bb, body, &nop_landing_pads);
 123             if is_nop_landing_pad {
 124                 nop_landing_pads.insert(bb);
 125             }
 126             debug!("    is_nop_landing_pad({:?}) = {}", bb, is_nop_landing_pad);
 127         }
 128
 129         debug!("removed {:?} jumps and {:?} landing pads", jumps_folded, landing_pads_removed);
 130     }
 131 }