compiler/rustc_mir_transform/src/unreachable_prop.rs

   1 //! A pass that propagates the unreachable terminator of a block to its predecessors
   2 //! when all of their successors are unreachable. This is achieved through a
   3 //! post-order traversal of the blocks.
   4
   5 use crate::simplify;
   6 use crate::MirPass;
   7 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
   8 use rustc_middle::mir::*;
   9 use rustc_middle::ty::TyCtxt;
  10
  11 pub struct UnreachablePropagation;
  12
  13 impl MirPass<'_> for UnreachablePropagation {
  14     fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  15         if tcx.sess.mir_opt_level() < 4 {
  16             // Enable only under -Zmir-opt-level=4 as in some cases (check the deeply-nested-opt
  17             // perf benchmark) LLVM may spend quite a lot of time optimizing the generated code.
  18             return;
  19         }
  20
  21         let mut unreachable_blocks = FxHashSet::default();
  22         let mut replacements = FxHashMap::default();
  23
  24         for (bb, bb_data) in traversal::postorder(body) {
  25             let terminator = bb_data.terminator();
  26             // HACK: If the block contains any asm statement it is not regarded as unreachable.
  27             // This is a temporary solution that handles possibly diverging asm statements.
  28             // Accompanying testcases: mir-opt/unreachable_asm.rs and mir-opt/unreachable_asm_2.rs
  29             let asm_stmt_in_block = || {
  30                 bb_data.statements.iter().any(|stmt: &Statement<'_>| {
  31                     matches!(stmt.kind, StatementKind::LlvmInlineAsm(..))
  32                 })
  33             };
  34
  35             if terminator.kind == TerminatorKind::Unreachable && !asm_stmt_in_block() {
  36                 unreachable_blocks.insert(bb);
  37             } else {
  38                 let is_unreachable = |succ: BasicBlock| unreachable_blocks.contains(&succ);
  39                 let terminator_kind_opt = remove_successors(&terminator.kind, is_unreachable);
  40
  41                 if let Some(terminator_kind) = terminator_kind_opt {
  42                     if terminator_kind == TerminatorKind::Unreachable && !asm_stmt_in_block() {
  43                         unreachable_blocks.insert(bb);
  44                     }
  45                     replacements.insert(bb, terminator_kind);
  46                 }
  47             }
  48         }
  49
  50         let replaced = !replacements.is_empty();
  51         for (bb, terminator_kind) in replacements {
  52             if !tcx.consider_optimizing(|| {
  53                 format!("UnreachablePropagation {:?} ", body.source.def_id())
  54             }) {
  55                 break;
  56             }
  57
  58             body.basic_blocks_mut()[bb].terminator_mut().kind = terminator_kind;
  59         }
  60
  61         if replaced {
  62             simplify::remove_dead_blocks(tcx, body);
  63         }
  64     }
  65 }
  66
  67 fn remove_successors<F>(
  68     terminator_kind: &TerminatorKind<'tcx>,
  69     predicate: F,
  70 ) -> Option<TerminatorKind<'tcx>>
  71 where
  72     F: Fn(BasicBlock) -> bool,
  73 {
  74     let terminator = match *terminator_kind {
  75         TerminatorKind::Goto { target } if predicate(target) => TerminatorKind::Unreachable,
  76         TerminatorKind::SwitchInt { ref discr, switch_ty, ref targets } => {
  77             let otherwise = targets.otherwise();
  78
  79             let original_targets_len = targets.iter().len() + 1;
  80             let (mut values, mut targets): (Vec<_>, Vec<_>) =
  81                 targets.iter().filter(|(_, bb)| !predicate(*bb)).unzip();
  82
  83             if !predicate(otherwise) {
  84                 targets.push(otherwise);
  85             } else {
  86                 values.pop();
  87             }
  88
  89             let retained_targets_len = targets.len();
  90
  91             if targets.is_empty() {
  92                 TerminatorKind::Unreachable
  93             } else if targets.len() == 1 {
  94                 TerminatorKind::Goto { target: targets[0] }
  95             } else if original_targets_len != retained_targets_len {
  96                 TerminatorKind::SwitchInt {
  97                     discr: discr.clone(),
  98                     switch_ty,
  99                     targets: SwitchTargets::new(
 100                         values.iter().copied().zip(targets.iter().copied()),
 101                         *targets.last().unwrap(),
 102                     ),
 103                 }
 104             } else {
 105                 return None;
 106             }
 107         }
 108         _ => return None,
 109     };
 110     Some(terminator)
 111 }