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.
7 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
8 use rustc_middle::mir::*;
9 use rustc_middle::ty::TyCtxt;
11 pub struct UnreachablePropagation;
13 impl MirPass<'_> for UnreachablePropagation {
14 fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
15 // Enable only under -Zmir-opt-level=4 as in some cases (check the deeply-nested-opt
16 // perf benchmark) LLVM may spend quite a lot of time optimizing the generated code.
17 sess.mir_opt_level() >= 4
20 fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
21 let mut unreachable_blocks = FxHashSet::default();
22 let mut replacements = FxHashMap::default();
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(..))
35 if terminator.kind == TerminatorKind::Unreachable && !asm_stmt_in_block() {
36 unreachable_blocks.insert(bb);
38 let is_unreachable = |succ: BasicBlock| unreachable_blocks.contains(&succ);
39 let terminator_kind_opt = remove_successors(&terminator.kind, is_unreachable);
41 if let Some(terminator_kind) = terminator_kind_opt {
42 if terminator_kind == TerminatorKind::Unreachable && !asm_stmt_in_block() {
43 unreachable_blocks.insert(bb);
45 replacements.insert(bb, terminator_kind);
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())
58 body.basic_blocks_mut()[bb].terminator_mut().kind = terminator_kind;
62 simplify::remove_dead_blocks(tcx, body);
67 fn remove_successors<'tcx, F>(
68 terminator_kind: &TerminatorKind<'tcx>,
70 ) -> Option<TerminatorKind<'tcx>>
72 F: Fn(BasicBlock) -> bool,
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();
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();
83 if !predicate(otherwise) {
84 targets.push(otherwise);
89 let retained_targets_len = targets.len();
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 {
99 targets: SwitchTargets::new(
100 values.iter().copied().zip(targets.iter().copied()),
101 *targets.last().unwrap(),