Address review comments

* Add lazily computed `switch_sources` data structure
* Don't assume a target has only one associated value

compiler/rustc_middle/src/mir/mod.rs

@@ -45,6 +45,7 @@ use std::{iter, mem, option};
 use self::graph_cyclic_cache::GraphIsCyclicCache;
 use self::predecessors::{PredecessorCache, Predecessors};
 pub use self::query::*;
+use self::switch_sources::{SwitchSourceCache, SwitchSources};
 
 pub mod coverage;
 mod generic_graph;
@@ -58,6 +59,7 @@ mod predecessors;
 pub mod pretty;
 mod query;
 pub mod spanview;
+mod switch_sources;
 pub mod tcx;
 pub mod terminator;
 pub use terminator::*;
@@ -284,6 +286,7 @@ pub struct Body<'tcx> {
     pub is_polymorphic: bool,
 
     predecessor_cache: PredecessorCache,
+    switch_source_cache: SwitchSourceCache,
     is_cyclic: GraphIsCyclicCache,
 
     pub tainted_by_errors: Option<ErrorGuaranteed>,
@@ -332,6 +335,7 @@ impl<'tcx> Body<'tcx> {
             required_consts: Vec::new(),
             is_polymorphic: false,
             predecessor_cache: PredecessorCache::new(),
+            switch_source_cache: SwitchSourceCache::new(),
             is_cyclic: GraphIsCyclicCache::new(),
             tainted_by_errors,
         };
@@ -360,6 +364,7 @@ impl<'tcx> Body<'tcx> {
             var_debug_info: Vec::new(),
             is_polymorphic: false,
             predecessor_cache: PredecessorCache::new(),
+            switch_source_cache: SwitchSourceCache::new(),
             is_cyclic: GraphIsCyclicCache::new(),
             tainted_by_errors: None,
         };
@@ -380,6 +385,7 @@ impl<'tcx> Body<'tcx> {
         // FIXME: Use a finer-grained API for this, so only transformations that alter terminators
         // invalidate the caches.
         self.predecessor_cache.invalidate();
+        self.switch_source_cache.invalidate();
         self.is_cyclic.invalidate();
         &mut self.basic_blocks
     }
@@ -389,6 +395,7 @@ impl<'tcx> Body<'tcx> {
         &mut self,
     ) -> (&mut IndexVec<BasicBlock, BasicBlockData<'tcx>>, &mut LocalDecls<'tcx>) {
         self.predecessor_cache.invalidate();
+        self.switch_source_cache.invalidate();
         self.is_cyclic.invalidate();
         (&mut self.basic_blocks, &mut self.local_decls)
     }
@@ -402,6 +409,7 @@ impl<'tcx> Body<'tcx> {
         &mut Vec<VarDebugInfo<'tcx>>,
     ) {
         self.predecessor_cache.invalidate();
+        self.switch_source_cache.invalidate();
         self.is_cyclic.invalidate();
         (&mut self.basic_blocks, &mut self.local_decls, &mut self.var_debug_info)
     }
@@ -529,6 +537,11 @@ impl<'tcx> Body<'tcx> {
         self.predecessor_cache.compute(&self.basic_blocks)
     }
 
+    #[inline]
+    pub fn switch_sources(&self) -> &SwitchSources {
+        self.switch_source_cache.compute(&self.basic_blocks)
+    }
+
     #[inline]
     pub fn dominators(&self) -> Dominators<BasicBlock> {
         dominators(self)

compiler/rustc_middle/src/mir/switch_sources.rs

@@ -0,0 +1,82 @@
+//! Lazily compute the inverse of each `SwitchInt`'s switch targets. Modeled after
+//! `Predecessors`/`PredecessorCache`.
+
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::OnceCell;
+use rustc_index::vec::IndexVec;
+use rustc_serialize as serialize;
+use smallvec::SmallVec;
+
+use crate::mir::{BasicBlock, BasicBlockData, Terminator, TerminatorKind};
+
+pub type SwitchSources = IndexVec<BasicBlock, IndexVec<BasicBlock, SmallVec<[Option<u128>; 1]>>>;
+
+#[derive(Clone, Debug)]
+pub(super) struct SwitchSourceCache {
+    cache: OnceCell<SwitchSources>,
+}
+
+impl SwitchSourceCache {
+    #[inline]
+    pub(super) fn new() -> Self {
+        SwitchSourceCache { cache: OnceCell::new() }
+    }
+
+    /// Invalidates the switch source cache.
+    #[inline]
+    pub(super) fn invalidate(&mut self) {
+        self.cache = OnceCell::new();
+    }
+
+    /// Returns the switch sources for this MIR.
+    #[inline]
+    pub(super) fn compute(
+        &self,
+        basic_blocks: &IndexVec<BasicBlock, BasicBlockData<'_>>,
+    ) -> &SwitchSources {
+        self.cache.get_or_init(|| {
+            let mut switch_sources = IndexVec::from_elem(
+                IndexVec::from_elem(SmallVec::new(), basic_blocks),
+                basic_blocks,
+            );
+            for (bb, data) in basic_blocks.iter_enumerated() {
+                if let Some(Terminator {
+                    kind: TerminatorKind::SwitchInt { targets, .. }, ..
+                }) = &data.terminator
+                {
+                    for (value, target) in targets.iter() {
+                        switch_sources[target][bb].push(Some(value));
+                    }
+                    switch_sources[targets.otherwise()][bb].push(None);
+                }
+            }
+
+            switch_sources
+        })
+    }
+}
+
+impl<S: serialize::Encoder> serialize::Encodable<S> for SwitchSourceCache {
+    #[inline]
+    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
+        s.emit_unit()
+    }
+}
+
+impl<D: serialize::Decoder> serialize::Decodable<D> for SwitchSourceCache {
+    #[inline]
+    fn decode(_: &mut D) -> Self {
+        Self::new()
+    }
+}
+
+impl<CTX> HashStable<CTX> for SwitchSourceCache {
+    #[inline]
+    fn hash_stable(&self, _: &mut CTX, _: &mut StableHasher) {
+        // do nothing
+    }
+}
+
+TrivialTypeFoldableAndLiftImpls! {
+    SwitchSourceCache,
+}

compiler/rustc_mir_dataflow/src/framework/direction.rs

@@ -267,11 +267,11 @@ impl Direction for Backward {
                     propagate(pred, &tmp);
                 }
 
-                mir::TerminatorKind::SwitchInt { ref targets, ref discr, switch_ty: _ } => {
+                mir::TerminatorKind::SwitchInt { targets: _, ref discr, switch_ty: _ } => {
                     let mut applier = BackwardSwitchIntEdgeEffectsApplier {
                         pred,
                         exit_state,
-                        targets,
+                        values: &body.switch_sources()[bb][pred],
                         bb,
                         propagate: &mut propagate,
                         effects_applied: false,
@@ -309,7 +309,7 @@ impl Direction for Backward {
 struct BackwardSwitchIntEdgeEffectsApplier<'a, D, F> {
     pred: BasicBlock,
     exit_state: &'a mut D,
-    targets: &'a SwitchTargets,
+    values: &'a [Option<u128>],
     bb: BasicBlock,
     propagate: &'a mut F,
 
@@ -324,10 +324,14 @@ where
     fn apply(&mut self, mut apply_edge_effect: impl FnMut(&mut D, SwitchIntTarget)) {
         assert!(!self.effects_applied);
 
-        let value =
-            self.targets.iter().find_map(|(value, target)| (target == self.bb).then_some(value));
-        apply_edge_effect(self.exit_state, SwitchIntTarget { value, target: self.bb });
-        (self.propagate)(self.pred, self.exit_state);
+        let targets = self.values.iter().map(|&value| SwitchIntTarget { value, target: self.bb });
+
+        let mut tmp = None;
+        for target in targets {
+            let tmp = opt_clone_from_or_clone(&mut tmp, self.exit_state);
+            apply_edge_effect(tmp, target);
+            (self.propagate)(self.pred, tmp);
+        }
 
         self.effects_applied = true;
     }