Use SmallVec within MoveData.

This reduces allocation counts significantly in a few benchmarks,
reducing instruction counts by up to 2%.

src/librustc_mir/dataflow/move_paths/builder.rs

@@ -12,12 +12,12 @@ use rustc::ty::{self, TyCtxt};
 use rustc::mir::*;
 use rustc::mir::tcx::RvalueInitializationState;
 use rustc_data_structures::indexed_vec::{IndexVec};
+use smallvec::{SmallVec, smallvec};
 
 use std::collections::hash_map::Entry;
 use std::mem;
 
 use super::abs_domain::Lift;
-
 use super::{LocationMap, MoveData, MovePath, MovePathLookup, MovePathIndex, MoveOut, MoveOutIndex};
 use super::{MoveError, InitIndex, Init, InitLocation, LookupResult, InitKind};
 use super::IllegalMoveOriginKind::*;
@@ -64,8 +64,8 @@ impl<'a, 'gcx, 'tcx> MoveDataBuilder<'a, 'gcx, 'tcx> {
     }
 
     fn new_move_path(move_paths: &mut IndexVec<MovePathIndex, MovePath<'tcx>>,
-                     path_map: &mut IndexVec<MovePathIndex, Vec<MoveOutIndex>>,
-                     init_path_map: &mut IndexVec<MovePathIndex, Vec<InitIndex>>,
+                     path_map: &mut IndexVec<MovePathIndex, SmallVec<[MoveOutIndex; 4]>>,
+                     init_path_map: &mut IndexVec<MovePathIndex, SmallVec<[InitIndex; 4]>>,
                      parent: Option<MovePathIndex>,
                      place: Place<'tcx>)
                      -> MovePathIndex
@@ -83,10 +83,10 @@ impl<'a, 'gcx, 'tcx> MoveDataBuilder<'a, 'gcx, 'tcx> {
             move_paths[move_path].next_sibling = next_sibling;
         }
 
-        let path_map_ent = path_map.push(vec![]);
+        let path_map_ent = path_map.push(smallvec![]);
         assert_eq!(path_map_ent, move_path);
 
-        let init_path_map_ent = init_path_map.push(vec![]);
+        let init_path_map_ent = init_path_map.push(smallvec![]);
         assert_eq!(init_path_map_ent, move_path);
 
         move_path

src/librustc_mir/dataflow/move_paths/mod.rs

@@ -13,6 +13,7 @@ use rustc::ty::{self, TyCtxt};
 use rustc::mir::*;
 use rustc::util::nodemap::FxHashMap;
 use rustc_data_structures::indexed_vec::{IndexVec};
+use smallvec::SmallVec;
 use syntax_pos::{Span};
 
 use std::fmt;
@@ -141,14 +142,14 @@ pub struct MoveData<'tcx> {
     /// of executing the code at `l`. (There can be multiple MoveOut's
     /// for a given `l` because each MoveOut is associated with one
     /// particular path being moved.)
-    pub loc_map: LocationMap<Vec<MoveOutIndex>>,
-    pub path_map: IndexVec<MovePathIndex, Vec<MoveOutIndex>>,
+    pub loc_map: LocationMap<SmallVec<[MoveOutIndex; 4]>>,
+    pub path_map: IndexVec<MovePathIndex, SmallVec<[MoveOutIndex; 4]>>,
     pub rev_lookup: MovePathLookup<'tcx>,
     pub inits: IndexVec<InitIndex, Init>,
     /// Each Location `l` is mapped to the Inits that are effects
     /// of executing the code at `l`.
-    pub init_loc_map: LocationMap<Vec<InitIndex>>,
-    pub init_path_map: IndexVec<MovePathIndex, Vec<InitIndex>>,
+    pub init_loc_map: LocationMap<SmallVec<[InitIndex; 4]>>,
+    pub init_path_map: IndexVec<MovePathIndex, SmallVec<[InitIndex; 4]>>,
 }
 
 pub trait HasMoveData<'tcx> {