Deduplicate vtables within a single evaluation

src/librustc_mir/interpret/eval_context.rs

@@ -27,6 +27,7 @@ use rustc::mir::interpret::{
     EvalResult, EvalErrorKind,
     truncate, sign_extend,
 };
+use rustc_data_structures::fx::FxHashMap;
 
 use syntax::source_map::{self, Span};
 
@@ -50,6 +51,9 @@ pub struct EvalContext<'a, 'mir, 'tcx: 'a + 'mir, M: Machine<'a, 'mir, 'tcx>> {
 
     /// The virtual call stack.
     pub(crate) stack: Vec<Frame<'mir, 'tcx, M::PointerTag>>,
+
+    /// A cache for deduplicating vtables
+    pub(super) vtables: FxHashMap<(Ty<'tcx>, ty::PolyTraitRef<'tcx>), AllocId>,
 }
 
 /// A stack frame.
@@ -209,6 +213,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
             param_env,
             memory: Memory::new(tcx, memory_data),
             stack: Vec::new(),
+            vtables: FxHashMap(),
         }
     }
 

src/librustc_mir/interpret/traits.rs

@@ -28,7 +28,9 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
     ) -> EvalResult<'tcx, Pointer<M::PointerTag>> {
         debug!("get_vtable(trait_ref={:?})", trait_ref);
 
-        // FIXME: Cache this!
+        if let Some(&vtable) = self.vtables.get(&(ty, trait_ref)) {
+            return Ok(Pointer::from(vtable).with_default_tag());
+        }
 
         let layout = self.layout_of(trait_ref.self_ty())?;
         assert!(!layout.is_unsized(), "can't create a vtable for an unsized type");
@@ -64,6 +66,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
         }
 
         self.memory.mark_immutable(vtable.alloc_id)?;
+        assert!(self.vtables.insert((ty, trait_ref), vtable.alloc_id).is_none());
 
         Ok(vtable)
     }