compiler/rustc_mir/src/transform/remove_noop_landing_pads.rs

   1 use crate::transform::MirPass;
   2 use crate::util::patch::MirPatch;
   3 use rustc_index::bit_set::BitSet;
   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 pub fn remove_noop_landing_pads<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  14     if tcx.sess.panic_strategy() == PanicStrategy::Abort {
  15         return;
  16     }
  17     debug!("remove_noop_landing_pads({:?})", body);
  18
  19     RemoveNoopLandingPads.remove_nop_landing_pads(body)
  20 }
  21
  22 impl<'tcx> MirPass<'tcx> for RemoveNoopLandingPads {
  23     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  24         remove_noop_landing_pads(tcx, body);
  25     }
  26 }
  27
  28 impl RemoveNoopLandingPads {
  29     fn is_nop_landing_pad(
  30         &self,
  31         bb: BasicBlock,
  32         body: &Body<'_>,
  33         nop_landing_pads: &BitSet<BasicBlock>,
  34     ) -> bool {
  35         for stmt in &body[bb].statements {
  36             match &stmt.kind {
  37                 StatementKind::FakeRead(..)
  38                 | StatementKind::StorageLive(_)
  39                 | StatementKind::StorageDead(_)
  40                 | StatementKind::AscribeUserType(..)
  41                 | StatementKind::Coverage(..)
  42                 | StatementKind::Nop => {
  43                     // These are all nops in a landing pad
  44                 }
  45
  46                 StatementKind::Assign(box (place, Rvalue::Use(_) | Rvalue::Discriminant(_))) => {
  47                     if place.as_local().is_some() {
  48                         // Writing to a local (e.g., a drop flag) does not
  49                         // turn a landing pad to a non-nop
  50                     } else {
  51                         return false;
  52                     }
  53                 }
  54
  55                 StatementKind::Assign { .. }
  56                 | StatementKind::SetDiscriminant { .. }
  57                 | StatementKind::LlvmInlineAsm { .. }
  58                 | StatementKind::Retag { .. } => {
  59                     return false;
  60                 }
  61             }
  62         }
  63
  64         let terminator = body[bb].terminator();
  65         match terminator.kind {
  66             TerminatorKind::Goto { .. }
  67             | TerminatorKind::Resume
  68             | TerminatorKind::SwitchInt { .. }
  69             | TerminatorKind::FalseEdge { .. }
  70             | TerminatorKind::FalseUnwind { .. } => {
  71                 terminator.successors().all(|&succ| nop_landing_pads.contains(succ))
  72             }
  73             TerminatorKind::GeneratorDrop
  74             | TerminatorKind::Yield { .. }
  75             | TerminatorKind::Return
  76             | TerminatorKind::Abort
  77             | TerminatorKind::Unreachable
  78             | TerminatorKind::Call { .. }
  79             | TerminatorKind::Assert { .. }
  80             | TerminatorKind::DropAndReplace { .. }
  81             | TerminatorKind::Drop { .. }
  82             | TerminatorKind::InlineAsm { .. } => false,
  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             if let Some(unwind) = body[bb].terminator_mut().unwind_mut() {
 106                 if let Some(unwind_bb) = *unwind {
 107                     if nop_landing_pads.contains(unwind_bb) {
 108                         debug!("    removing noop landing pad");
 109                         landing_pads_removed += 1;
 110                         *unwind = None;
 111                     }
 112                 }
 113             }
 114
 115             for target in body[bb].terminator_mut().successors_mut() {
 116                 if *target != resume_block && nop_landing_pads.contains(*target) {
 117                     debug!("    folding noop jump to {:?} to resume block", target);
 118                     *target = resume_block;
 119                     jumps_folded += 1;
 120                 }
 121             }
 122
 123             let is_nop_landing_pad = self.is_nop_landing_pad(bb, body, &nop_landing_pads);
 124             if is_nop_landing_pad {
 125                 nop_landing_pads.insert(bb);
 126             }
 127             debug!("    is_nop_landing_pad({:?}) = {}", bb, is_nop_landing_pad);
 128         }
 129
 130         debug!("removed {:?} jumps and {:?} landing pads", jumps_folded, landing_pads_removed);
 131     }
 132 }