Prefer Struct-of-array

Author: tesuji

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -58,9 +58,10 @@ impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
         }
 
         let ccx = ConstCx::new(tcx, body);
-        let (mut temps, all_candidates) = collect_temps_and_candidates(&ccx);
+        let (mut temps, mut all_candidates) = collect_temps_and_candidates(&ccx);
 
-        let promotable_candidates = validate_candidates(&ccx, &mut temps, &all_candidates);
+        validate_candidates(&ccx, &mut temps, &mut all_candidates);
+        let promotable_candidates = &all_candidates.0[..];
 
         let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
         self.promoted_fragments.set(promoted);
@@ -83,18 +84,16 @@ enum TempState {
     PromotedOut,
 }
 
-/// A "root candidate" for promotion, which will become the
-/// returned value in a promoted MIR, unless it's a subset
-/// of a larger candidate.
-#[derive(Copy, Clone, PartialEq, Eq, Debug)]
-struct Candidate {
-    location: Location,
-}
+/// Each `[Location]` is a "root candidate" for promotion, which will become
+/// the returned value in a promoted MIR, unless it's a subset of a larger
+/// candidate.
+#[derive(Clone, Debug)]
+struct Candidates(Vec<Location>);
 
 struct Collector<'a, 'tcx> {
     ccx: &'a ConstCx<'a, 'tcx>,
     temps: IndexVec<Local, TempState>,
-    candidates: Vec<Candidate>,
+    candidates: Candidates,
 }
 
 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
@@ -141,24 +140,23 @@ impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
             }
             TempState::Unpromotable | TempState::PromotedOut => TempState::Unpromotable,
         };
-        debug!(?temp);
     }
 
     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
         self.super_rvalue(rvalue, location);
 
         if let Rvalue::Ref(..) = *rvalue {
-            self.candidates.push(Candidate { location });
+            self.candidates.0.push(location);
         }
     }
 }
 
 fn collect_temps_and_candidates<'tcx>(
     ccx: &ConstCx<'_, 'tcx>,
-) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
+) -> (IndexVec<Local, TempState>, Candidates) {
     let mut collector = Collector {
         temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
-        candidates: vec![],
+        candidates: Candidates(vec![]),
         ccx,
     };
     for (bb, data) in traversal::reverse_postorder(ccx.body) {
@@ -192,8 +190,8 @@ impl<'a, 'tcx> std::ops::Deref for Validator<'a, 'tcx> {
 struct Unpromotable;
 
 impl<'tcx> Validator<'_, 'tcx> {
-    fn validate_candidate(&mut self, candidate: Candidate) -> Result<(), Unpromotable> {
-        let Left(statement) = self.body.stmt_at(candidate.location) else { bug!() };
+    fn validate_candidate(&mut self, candidate: Location) -> Result<(), Unpromotable> {
+        let Left(statement) = self.body.stmt_at(candidate) else { bug!() };
         let Some((_, Rvalue::Ref(_, kind, place))) = statement.kind.as_assign() else { bug!() };
 
         // We can only promote interior borrows of promotable temps (non-temps
@@ -691,15 +689,11 @@ impl<'tcx> Validator<'_, 'tcx> {
 fn validate_candidates(
     ccx: &ConstCx<'_, '_>,
     temps: &mut IndexSlice<Local, TempState>,
-    candidates: &[Candidate],
-) -> Vec<Candidate> {
+    candidates: &mut Candidates,
+) {
     let mut validator = Validator { ccx, temps, promotion_safe_blocks: None };
 
-    candidates
-        .iter()
-        .copied()
-        .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
-        .collect()
+    candidates.0.retain(|&candidate| validator.validate_candidate(candidate).is_ok());
 }
 
 struct Promoter<'a, 'tcx> {
@@ -854,7 +848,7 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
         new_temp
     }
 
-    fn promote_candidate(mut self, candidate: Candidate, next_promoted_id: usize) -> Body<'tcx> {
+    fn promote_candidate(mut self, candidate: Location, next_promoted_id: usize) -> Body<'tcx> {
         let def = self.source.source.def_id();
         let (mut rvalue, promoted_op) = {
             let promoted = &mut self.promoted;
@@ -871,7 +865,7 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
 
             let blocks = self.source.basic_blocks.as_mut();
             let local_decls = &mut self.source.local_decls;
-            let loc = candidate.location;
+            let loc = candidate;
             let statement = &mut blocks[loc.block].statements[loc.statement_index];
             let StatementKind::Assign(box (_, Rvalue::Ref(region, borrow_kind, place))) =
                 &mut statement.kind
@@ -965,7 +959,7 @@ fn promote_candidates<'tcx>(
     body: &mut Body<'tcx>,
     tcx: TyCtxt<'tcx>,
     mut temps: IndexVec<Local, TempState>,
-    candidates: Vec<Candidate>,
+    candidates: &[Location],
 ) -> IndexVec<Promoted, Body<'tcx>> {
     // Visit candidates in reverse, in case they're nested.
     debug!(promote_candidates = ?candidates);
@@ -978,8 +972,8 @@ fn promote_candidates<'tcx>(
     let mut promotions = IndexVec::new();
 
     let mut extra_statements = vec![];
-    for candidate in candidates.into_iter().rev() {
-        let Location { block, statement_index } = candidate.location;
+    for candidate in candidates.iter().copied().rev() {
+        let Location { block, statement_index } = candidate;
         if let StatementKind::Assign(box (place, _)) = &body[block].statements[statement_index].kind
         {
             if let Some(local) = place.as_local() {
@@ -993,7 +987,7 @@ fn promote_candidates<'tcx>(
         // Declare return place local so that `mir::Body::new` doesn't complain.
         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
 
-        let mut scope = body.source_scopes[body.source_info(candidate.location).scope].clone();
+        let mut scope = body.source_scopes[body.source_info(candidate).scope].clone();
         scope.parent_scope = None;
 
         let mut promoted = Body::new(