rustc_mir: implement a "place unification" optimization (aka source/destination propagation).

Author: eddyb

src/librustc_mir/analysis/eventflow.rs

@@ -42,6 +42,10 @@ impl<I: Idx> SparseBitSet<I> {
         }
     }
 
+    pub fn capacity(&self) -> usize {
+        self.map.len() * 128
+    }
+
     pub fn contains(&self, index: I) -> bool {
         let (key, mask) = key_and_mask(index);
         self.map.get(&key).map_or(false, |bits| (bits & mask) != 0)

src/librustc_mir/analysis/local_paths/mod.rs

@@ -109,6 +109,23 @@ impl<'tcx> LocalPaths<'tcx> {
             }
         }
     }
+
+    /// Add a new local to the given `mir` (which is assumed to be the
+    /// one `self` was created from) and record a new path for it.
+    pub fn create_and_record_new_local(&mut self,
+                                       mir: &mut Mir<'tcx>,
+                                       decl: LocalDecl<'tcx>)
+                                       -> Local {
+        let path = self.data.push(PathData {
+            last_descendant: PathId::new(0),
+            ty: decl.ty
+        });
+        self.data[path].last_descendant = path;
+
+        let local = mir.local_decls.push(decl);
+        assert_eq!(self.locals.push(path), local);
+        local
+    }
 }
 
 pub struct Children<'a, 'tcx: 'a> {

src/librustc_mir/transform/mod.rs

@@ -44,6 +44,7 @@ pub mod copy_prop;
 pub mod generator;
 pub mod inline;
 pub mod lower_128bit;
+pub mod unify_places;
 
 pub(crate) fn provide(providers: &mut Providers) {
     self::qualify_consts::provide(providers);
@@ -255,14 +256,19 @@ fn optimized_mir<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> &'tcx
 
         // Optimizations begin.
         inline::Inline,
+
+        // Lowering generator control-flow and variables
+        // has to happen before we do anything else to them.
+        generator::StateTransform,
+
         instcombine::InstCombine,
         deaggregator::Deaggregator,
+        unify_places::UnifyPlaces,
         copy_prop::CopyPropagation,
         remove_noop_landing_pads::RemoveNoopLandingPads,
         simplify::SimplifyCfg::new("final"),
         simplify::SimplifyLocals,
 
-        generator::StateTransform,
         add_call_guards::CriticalCallEdges,
         dump_mir::Marker("PreTrans"),
     ];