compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs

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