1 use rustc_data_structures::vec_linked_list as vll;
2 use rustc_index::vec::IndexVec;
3 use rustc_middle::mir::visit::{PlaceContext, Visitor};
4 use rustc_middle::mir::{Body, Local, Location};
6 use crate::def_use::{self, DefUse};
7 use crate::region_infer::values::{PointIndex, RegionValueElements};
9 /// A map that cross references each local with the locations where it
10 /// is defined (assigned), used, or dropped. Used during liveness
13 /// We keep track only of `Local`s we'll do the liveness analysis later,
14 /// this means that our internal `IndexVec`s will only be sparsely populated.
15 /// In the time-memory trade-off between keeping compact vectors with new
16 /// indexes (and needing to continuously map the `Local` index to its compact
17 /// counterpart) and having `IndexVec`s that we only use a fraction of, time
18 /// (and code simplicity) was favored. The rationale is that we only keep
19 /// a small number of `IndexVec`s throughout the entire analysis while, in
20 /// contrast, we're accessing each `Local` *many* times.
21 crate struct LocalUseMap {
22 /// Head of a linked list of **definitions** of each variable --
23 /// definition in this context means assignment, e.g., `x` is
24 /// defined in `x = y` but not `y`; that first def is the head of
25 /// a linked list that lets you enumerate all places the variable
27 first_def_at: IndexVec<Local, Option<AppearanceIndex>>,
29 /// Head of a linked list of **uses** of each variable -- use in
30 /// this context means that the existing value of the variable is
31 /// read or modified. e.g., `y` is used in `x = y` but not `x`.
32 /// Note that `DROP(x)` terminators are excluded from this list.
33 first_use_at: IndexVec<Local, Option<AppearanceIndex>>,
35 /// Head of a linked list of **drops** of each variable -- these
36 /// are a special category of uses corresponding to the drop that
37 /// we add for each local variable.
38 first_drop_at: IndexVec<Local, Option<AppearanceIndex>>,
40 appearances: IndexVec<AppearanceIndex, Appearance>,
44 point_index: PointIndex,
45 next: Option<AppearanceIndex>,
48 rustc_index::newtype_index! {
49 pub struct AppearanceIndex { .. }
52 impl vll::LinkElem for Appearance {
53 type LinkIndex = AppearanceIndex;
55 fn next(elem: &Self) -> Option<AppearanceIndex> {
61 crate fn build(live_locals: &[Local], elements: &RegionValueElements, body: &Body<'_>) -> Self {
62 let nones = IndexVec::from_elem_n(None, body.local_decls.len());
63 let mut local_use_map = LocalUseMap {
64 first_def_at: nones.clone(),
65 first_use_at: nones.clone(),
67 appearances: IndexVec::new(),
70 if live_locals.is_empty() {
74 let mut locals_with_use_data: IndexVec<Local, bool> =
75 IndexVec::from_elem_n(false, body.local_decls.len());
76 live_locals.iter().for_each(|&local| locals_with_use_data[local] = true);
78 LocalUseMapBuild { local_use_map: &mut local_use_map, elements, locals_with_use_data }
84 crate fn defs(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
85 vll::iter(self.first_def_at[local], &self.appearances)
86 .map(move |aa| self.appearances[aa].point_index)
89 crate fn uses(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
90 vll::iter(self.first_use_at[local], &self.appearances)
91 .map(move |aa| self.appearances[aa].point_index)
94 crate fn drops(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
95 vll::iter(self.first_drop_at[local], &self.appearances)
96 .map(move |aa| self.appearances[aa].point_index)
100 struct LocalUseMapBuild<'me> {
101 local_use_map: &'me mut LocalUseMap,
102 elements: &'me RegionValueElements,
104 // Vector used in `visit_local` to signal which `Local`s do we need
105 // def/use/drop information on, constructed from `live_locals` (that
106 // contains the variables we'll do the liveness analysis for).
107 // This vector serves optimization purposes only: we could have
108 // obtained the same information from `live_locals` but we want to
109 // avoid repeatedly calling `Vec::contains()` (see `LocalUseMap` for
110 // the rationale on the time-memory trade-off we're favoring here).
111 locals_with_use_data: IndexVec<Local, bool>,
114 impl LocalUseMapBuild<'_> {
115 fn insert_def(&mut self, local: Local, location: Location) {
118 &mut self.local_use_map.first_def_at[local],
119 &mut self.local_use_map.appearances,
124 fn insert_use(&mut self, local: Local, location: Location) {
127 &mut self.local_use_map.first_use_at[local],
128 &mut self.local_use_map.appearances,
133 fn insert_drop(&mut self, local: Local, location: Location) {
136 &mut self.local_use_map.first_drop_at[local],
137 &mut self.local_use_map.appearances,
143 elements: &RegionValueElements,
144 first_appearance: &mut Option<AppearanceIndex>,
145 appearances: &mut IndexVec<AppearanceIndex, Appearance>,
148 let point_index = elements.point_from_location(location);
149 let appearance_index =
150 appearances.push(Appearance { point_index, next: *first_appearance });
151 *first_appearance = Some(appearance_index);
155 impl Visitor<'_> for LocalUseMapBuild<'_> {
156 fn visit_local(&mut self, &local: &Local, context: PlaceContext, location: Location) {
157 if self.locals_with_use_data[local] {
158 match def_use::categorize(context) {
159 Some(DefUse::Def) => self.insert_def(local, location),
160 Some(DefUse::Use) => self.insert_use(local, location),
161 Some(DefUse::Drop) => self.insert_drop(local, location),