1 files changed, 78 insertions, 0 deletions
diff --git a/compiler/rustc_middle/src/mir/predecessors.rs b/compiler/rustc_middle/src/mir/predecessors.rs
new file mode 100644
index 000000000..5f1fadaf3
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/predecessors.rs
@@ -0,0 +1,78 @@
+//! Lazily compute the reverse control-flow graph for the MIR.
+
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::OnceCell;
+use rustc_index::vec::IndexVec;
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+use smallvec::SmallVec;
+
+use crate::mir::{BasicBlock, BasicBlockData};
+
+// Typically 95%+ of basic blocks have 4 or fewer predecessors.
+pub type Predecessors = IndexVec<BasicBlock, SmallVec<[BasicBlock; 4]>>;
+
+#[derive(Clone, Debug)]
+pub(super) struct PredecessorCache {
+    cache: OnceCell<Predecessors>,
+}
+
+impl PredecessorCache {
+    #[inline]
+    pub(super) fn new() -> Self {
+        PredecessorCache { cache: OnceCell::new() }
+    }
+
+    /// Invalidates the predecessor cache.
+    #[inline]
+    pub(super) fn invalidate(&mut self) {
+        // Invalidating the predecessor cache requires mutating the MIR, which in turn requires a
+        // unique reference (`&mut`) to the `mir::Body`. Because of this, we can assume that all
+        // callers of `invalidate` have a unique reference to the MIR and thus to the predecessor
+        // cache. This means we never need to do synchronization when `invalidate` is called, we can
+        // simply reinitialize the `OnceCell`.
+        self.cache = OnceCell::new();
+    }
+
+    /// Returns the predecessor graph for this MIR.
+    #[inline]
+    pub(super) fn compute(
+        &self,
+        basic_blocks: &IndexVec<BasicBlock, BasicBlockData<'_>>,
+    ) -> &Predecessors {
+        self.cache.get_or_init(|| {
+            let mut preds = IndexVec::from_elem(SmallVec::new(), basic_blocks);
+            for (bb, data) in basic_blocks.iter_enumerated() {
+                if let Some(term) = &data.terminator {
+                    for succ in term.successors() {
+                        preds[succ].push(bb);
+                    }
+                }
+            }
+
+            preds
+        })
+    }
+}
+
+impl<S: Encoder> Encodable<S> for PredecessorCache {
+    #[inline]
+    fn encode(&self, _s: &mut S) {}
+}
+
+impl<D: Decoder> Decodable<D> for PredecessorCache {
+    #[inline]
+    fn decode(_: &mut D) -> Self {
+        Self::new()
+    }
+}
+
+impl<CTX> HashStable<CTX> for PredecessorCache {
+    #[inline]
+    fn hash_stable(&self, _: &mut CTX, _: &mut StableHasher) {
+        // do nothing
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! {
+    PredecessorCache,
+}