1 files changed, 102 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/unreachable_prop.rs b/compiler/rustc_mir_transform/src/unreachable_prop.rs
new file mode 100644
index 000000000..f916ca362
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/unreachable_prop.rs
@@ -0,0 +1,102 @@
+//! A pass that propagates the unreachable terminator of a block to its predecessors
+//! when all of their successors are unreachable. This is achieved through a
+//! post-order traversal of the blocks.
+
+use crate::simplify;
+use crate::MirPass;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+
+pub struct UnreachablePropagation;
+
+impl MirPass<'_> for UnreachablePropagation {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        // Enable only under -Zmir-opt-level=4 as in some cases (check the deeply-nested-opt
+        // perf benchmark) LLVM may spend quite a lot of time optimizing the generated code.
+        sess.mir_opt_level() >= 4
+    }
+
+    fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        let mut unreachable_blocks = FxHashSet::default();
+        let mut replacements = FxHashMap::default();
+
+        for (bb, bb_data) in traversal::postorder(body) {
+            let terminator = bb_data.terminator();
+            if terminator.kind == TerminatorKind::Unreachable {
+                unreachable_blocks.insert(bb);
+            } else {
+                let is_unreachable = |succ: BasicBlock| unreachable_blocks.contains(&succ);
+                let terminator_kind_opt = remove_successors(&terminator.kind, is_unreachable);
+
+                if let Some(terminator_kind) = terminator_kind_opt {
+                    if terminator_kind == TerminatorKind::Unreachable {
+                        unreachable_blocks.insert(bb);
+                    }
+                    replacements.insert(bb, terminator_kind);
+                }
+            }
+        }
+
+        let replaced = !replacements.is_empty();
+        for (bb, terminator_kind) in replacements {
+            if !tcx.consider_optimizing(|| {
+                format!("UnreachablePropagation {:?} ", body.source.def_id())
+            }) {
+                break;
+            }
+
+            body.basic_blocks_mut()[bb].terminator_mut().kind = terminator_kind;
+        }
+
+        if replaced {
+            simplify::remove_dead_blocks(tcx, body);
+        }
+    }
+}
+
+fn remove_successors<'tcx, F>(
+    terminator_kind: &TerminatorKind<'tcx>,
+    predicate: F,
+) -> Option<TerminatorKind<'tcx>>
+where
+    F: Fn(BasicBlock) -> bool,
+{
+    let terminator = match *terminator_kind {
+        TerminatorKind::Goto { target } if predicate(target) => TerminatorKind::Unreachable,
+        TerminatorKind::SwitchInt { ref discr, switch_ty, ref targets } => {
+            let otherwise = targets.otherwise();
+
+            let original_targets_len = targets.iter().len() + 1;
+            let (mut values, mut targets): (Vec<_>, Vec<_>) =
+                targets.iter().filter(|(_, bb)| !predicate(*bb)).unzip();
+
+            if !predicate(otherwise) {
+                targets.push(otherwise);
+            } else {
+                values.pop();
+            }
+
+            let retained_targets_len = targets.len();
+
+            if targets.is_empty() {
+                TerminatorKind::Unreachable
+            } else if targets.len() == 1 {
+                TerminatorKind::Goto { target: targets[0] }
+            } else if original_targets_len != retained_targets_len {
+                TerminatorKind::SwitchInt {
+                    discr: discr.clone(),
+                    switch_ty,
+                    targets: SwitchTargets::new(
+                        values.iter().copied().zip(targets.iter().copied()),
+                        *targets.last().unwrap(),
+                    ),
+                }
+            } else {
+                return None;
+            }
+        }
+        _ => return None,
+    };
+    Some(terminator)
+}