rustc_mir: don't pass on_entry when building transfer functions.

This commit makes `sets.on_entry` inaccessible in
`{before_,}{statement,terminator}_effect`. This field was meant to allow
implementors of `BitDenotation` to access the initial state for each
block (optionally with the effect of all previous statements applied via
`accumulates_intrablock_state`) while defining transfer functions.
However, the ability to set the initial value for the entry set of each
basic block (except for START_BLOCK) no longer exists. As a result, this
functionality is mostly useless, and when it *was* used it was used
erroneously (see rust-lang#62007).

Since `on_entry` is now useless, we can also remove `BlockSets`, which
held the `gen`, `kill`, and `on_entry` bitvectors and replace it with a
`GenKill` struct. Variables of this type are called `trans` since they
represent a transfer function. `GenKill`s are stored contiguously in
`AllSets`, which reduces the number of bounds checks and may improve
cache performance: one is almost never accessed without the other.

Replacing `BlockSets` with `GenKill` allows us to define some new helper
functions which streamline dataflow iteration and the
dataflow-at-location APIs. Notably, `state_for_location` used a subtle
side-effect of the `kill`/`kill_all` setters to apply the transfer
function, and could be incorrect if a transfer function depended on
effects of previous statements in the block on `gen_set`.

Author: ecstatic-morse

src/librustc_mir/dataflow/at_location.rs

@@ -4,7 +4,7 @@
 use rustc::mir::{BasicBlock, Location};
 use rustc_data_structures::bit_set::{BitIter, BitSet, HybridBitSet};
 
-use crate::dataflow::{BitDenotation, BlockSets, DataflowResults};
+use crate::dataflow::{BitDenotation, DataflowResults, GenKillSet};
 use crate::dataflow::move_paths::{HasMoveData, MovePathIndex};
 
 use std::iter;
@@ -66,8 +66,7 @@ where
 {
     base_results: DataflowResults<'tcx, BD>,
     curr_state: BitSet<BD::Idx>,
-    stmt_gen: HybridBitSet<BD::Idx>,
-    stmt_kill: HybridBitSet<BD::Idx>,
+    stmt_trans: GenKillSet<BD::Idx>,
 }
 
 impl<'tcx, BD> FlowAtLocation<'tcx, BD>
@@ -89,19 +88,17 @@ where
     where
         F: FnMut(BD::Idx),
     {
-        self.stmt_gen.iter().for_each(f)
+        self.stmt_trans.gen_set.iter().for_each(f)
     }
 
     pub fn new(results: DataflowResults<'tcx, BD>) -> Self {
         let bits_per_block = results.sets().bits_per_block();
         let curr_state = BitSet::new_empty(bits_per_block);
-        let stmt_gen = HybridBitSet::new_empty(bits_per_block);
-        let stmt_kill = HybridBitSet::new_empty(bits_per_block);
+        let stmt_trans = GenKillSet::from_elem(HybridBitSet::new_empty(bits_per_block));
         FlowAtLocation {
             base_results: results,
-            curr_state: curr_state,
-            stmt_gen: stmt_gen,
-            stmt_kill: stmt_kill,
+            curr_state,
+            stmt_trans,
         }
     }
 
@@ -127,8 +124,7 @@ where
         F: FnOnce(BitIter<'_, BD::Idx>),
     {
         let mut curr_state = self.curr_state.clone();
-        curr_state.union(&self.stmt_gen);
-        curr_state.subtract(&self.stmt_kill);
+        self.stmt_trans.apply(&mut curr_state);
         f(curr_state.iter());
     }
 
@@ -142,68 +138,41 @@ impl<'tcx, BD> FlowsAtLocation for FlowAtLocation<'tcx, BD>
     where BD: BitDenotation<'tcx>
 {
     fn reset_to_entry_of(&mut self, bb: BasicBlock) {
-        self.curr_state.overwrite(self.base_results.sets().on_entry_set_for(bb.index()));
+        self.curr_state.overwrite(self.base_results.sets().entry_set_for(bb.index()));
     }
 
     fn reset_to_exit_of(&mut self, bb: BasicBlock) {
         self.reset_to_entry_of(bb);
-        self.curr_state.union(self.base_results.sets().gen_set_for(bb.index()));
-        self.curr_state.subtract(self.base_results.sets().kill_set_for(bb.index()));
+        let trans = self.base_results.sets().trans_for(bb.index());
+        trans.apply(&mut self.curr_state)
     }
 
     fn reconstruct_statement_effect(&mut self, loc: Location) {
-        self.stmt_gen.clear();
-        self.stmt_kill.clear();
-        {
-            let mut sets = BlockSets {
-                on_entry: &mut self.curr_state,
-                gen_set: &mut self.stmt_gen,
-                kill_set: &mut self.stmt_kill,
-            };
-            self.base_results
-                .operator()
-                .before_statement_effect(&mut sets, loc);
-        }
-        self.apply_local_effect(loc);
+        self.stmt_trans.clear();
+        self.base_results
+            .operator()
+            .before_statement_effect(&mut self.stmt_trans, loc);
+        self.stmt_trans.apply(&mut self.curr_state);
 
-        let mut sets = BlockSets {
-            on_entry: &mut self.curr_state,
-            gen_set: &mut self.stmt_gen,
-            kill_set: &mut self.stmt_kill,
-        };
         self.base_results
             .operator()
-            .statement_effect(&mut sets, loc);
+            .statement_effect(&mut self.stmt_trans, loc);
     }
 
     fn reconstruct_terminator_effect(&mut self, loc: Location) {
-        self.stmt_gen.clear();
-        self.stmt_kill.clear();
-        {
-            let mut sets = BlockSets {
-                on_entry: &mut self.curr_state,
-                gen_set: &mut self.stmt_gen,
-                kill_set: &mut self.stmt_kill,
-            };
-            self.base_results
-                .operator()
-                .before_terminator_effect(&mut sets, loc);
-        }
-        self.apply_local_effect(loc);
+        self.stmt_trans.clear();
+        self.base_results
+            .operator()
+            .before_terminator_effect(&mut self.stmt_trans, loc);
+        self.stmt_trans.apply(&mut self.curr_state);
 
-        let mut sets = BlockSets {
-            on_entry: &mut self.curr_state,
-            gen_set: &mut self.stmt_gen,
-            kill_set: &mut self.stmt_kill,
-        };
         self.base_results
             .operator()
-            .terminator_effect(&mut sets, loc);
+            .terminator_effect(&mut self.stmt_trans, loc);
     }
 
     fn apply_local_effect(&mut self, _loc: Location) {
-        self.curr_state.union(&self.stmt_gen);
-        self.curr_state.subtract(&self.stmt_kill);
+        self.stmt_trans.apply(&mut self.curr_state)
     }
 }
 

src/librustc_mir/dataflow/graphviz.rs

@@ -170,7 +170,7 @@ where MWF: MirWithFlowState<'tcx>,
 
         write!(w, "<tr>")?;
         // Entry
-        dump_set_for!(on_entry_set_for, interpret_set);
+        dump_set_for!(entry_set_for, interpret_set);
 
         // MIR statements
         write!(w, "<td>")?;
@@ -208,7 +208,7 @@ where MWF: MirWithFlowState<'tcx>,
         write!(w, "<tr>")?;
 
         // Entry
-        let set = flow.sets.on_entry_set_for(i);
+        let set = flow.sets.entry_set_for(i);
         write!(w, "<td>{:?}</td>", dot::escape_html(&set.to_string()))?;
 
         // Terminator
@@ -221,13 +221,10 @@ where MWF: MirWithFlowState<'tcx>,
         }
         write!(w, "</td>")?;
 
-        // Gen
-        let set = flow.sets.gen_set_for(i);
-        write!(w, "<td>{:?}</td>", dot::escape_html(&format!("{:?}", set)))?;
-
-        // Kill
-        let set = flow.sets.kill_set_for(i);
-        write!(w, "<td>{:?}</td>", dot::escape_html(&format!("{:?}", set)))?;
+        // Gen/Kill
+        let trans = flow.sets.trans_for(i);
+        write!(w, "<td>{:?}</td>", dot::escape_html(&format!("{:?}", trans.gen_set)))?;
+        write!(w, "<td>{:?}</td>", dot::escape_html(&format!("{:?}", trans.kill_set)))?;
 
         write!(w, "</tr>")?;
 

src/librustc_mir/dataflow/impls/borrowed_locals.rs

@@ -2,7 +2,7 @@ pub use super::*;
 
 use rustc::mir::*;
 use rustc::mir::visit::Visitor;
-use crate::dataflow::BitDenotation;
+use crate::dataflow::{BitDenotation, GenKillSet};
 
 /// This calculates if any part of a MIR local could have previously been borrowed.
 /// This means that once a local has been borrowed, its bit will be set
@@ -33,39 +33,39 @@ impl<'a, 'tcx> BitDenotation<'tcx> for HaveBeenBorrowedLocals<'a, 'tcx> {
         self.body.local_decls.len()
     }
 
-    fn start_block_effect(&self, _sets: &mut BitSet<Local>) {
+    fn start_block_effect(&self, _on_entry: &mut BitSet<Local>) {
         // Nothing is borrowed on function entry
     }
 
     fn statement_effect(&self,
-                        sets: &mut BlockSets<'_, Local>,
+                        trans: &mut GenKillSet<Local>,
                         loc: Location) {
         let stmt = &self.body[loc.block].statements[loc.statement_index];
 
         BorrowedLocalsVisitor {
-            sets,
+            trans,
         }.visit_statement(stmt, loc);
 
         // StorageDead invalidates all borrows and raw pointers to a local
         match stmt.kind {
-            StatementKind::StorageDead(l) => sets.kill(l),
+            StatementKind::StorageDead(l) => trans.kill(l),
             _ => (),
         }
     }
 
     fn terminator_effect(&self,
-                         sets: &mut BlockSets<'_, Local>,
+                         trans: &mut GenKillSet<Local>,
                          loc: Location) {
         let terminator = self.body[loc.block].terminator();
         BorrowedLocalsVisitor {
-            sets,
+            trans,
         }.visit_terminator(terminator, loc);
         match &terminator.kind {
             // Drop terminators borrows the location
             TerminatorKind::Drop { location, .. } |
             TerminatorKind::DropAndReplace { location, .. } => {
                 if let Some(local) = find_local(location) {
-                    sets.gen(local);
+                    trans.gen(local);
                 }
             }
             _ => (),
@@ -97,8 +97,8 @@ impl<'a, 'tcx> InitialFlow for HaveBeenBorrowedLocals<'a, 'tcx> {
     }
 }
 
-struct BorrowedLocalsVisitor<'b, 'c> {
-    sets: &'b mut BlockSets<'c, Local>,
+struct BorrowedLocalsVisitor<'gk> {
+    trans: &'gk mut GenKillSet<Local>,
 }
 
 fn find_local<'tcx>(place: &Place<'tcx>) -> Option<Local> {
@@ -117,13 +117,13 @@ fn find_local<'tcx>(place: &Place<'tcx>) -> Option<Local> {
     })
 }
 
-impl<'tcx, 'b, 'c> Visitor<'tcx> for BorrowedLocalsVisitor<'b, 'c> {
+impl<'tcx> Visitor<'tcx> for BorrowedLocalsVisitor<'_> {
     fn visit_rvalue(&mut self,
                     rvalue: &Rvalue<'tcx>,
                     location: Location) {
         if let Rvalue::Ref(_, _, ref place) = *rvalue {
             if let Some(local) = find_local(place) {
-                self.sets.gen(local);
+                self.trans.gen(local);
             }
         }
 

src/librustc_mir/dataflow/impls/borrows.rs

@@ -9,7 +9,7 @@ use rustc_data_structures::bit_set::{BitSet, BitSetOperator};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
 
-use crate::dataflow::{BitDenotation, BlockSets, InitialFlow};
+use crate::dataflow::{BitDenotation, InitialFlow, GenKillSet};
 use crate::borrow_check::nll::region_infer::RegionInferenceContext;
 use crate::borrow_check::nll::ToRegionVid;
 use crate::borrow_check::places_conflict;
@@ -168,7 +168,7 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
     /// Add all borrows to the kill set, if those borrows are out of scope at `location`.
     /// That means they went out of a nonlexical scope
     fn kill_loans_out_of_scope_at_location(&self,
-                                           sets: &mut BlockSets<'_, BorrowIndex>,
+                                           trans: &mut GenKillSet<BorrowIndex>,
                                            location: Location) {
         // NOTE: The state associated with a given `location`
         // reflects the dataflow on entry to the statement.
@@ -182,14 +182,14 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
         // region, then setting that gen-bit will override any
         // potential kill introduced here.
         if let Some(indices) = self.borrows_out_of_scope_at_location.get(&location) {
-            sets.kill_all(indices);
+            trans.kill_all(indices);
         }
     }
 
     /// Kill any borrows that conflict with `place`.
     fn kill_borrows_on_place(
         &self,
-        sets: &mut BlockSets<'_, BorrowIndex>,
+        trans: &mut GenKillSet<BorrowIndex>,
         place: &Place<'tcx>
     ) {
         debug!("kill_borrows_on_place: place={:?}", place);
@@ -206,7 +206,7 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
             // local must conflict. This is purely an optimization so we don't have to call
             // `places_conflict` for every borrow.
             if let Place::Base(PlaceBase::Local(_)) = place {
-                sets.kill_all(other_borrows_of_local);
+                trans.kill_all(other_borrows_of_local);
                 return;
             }
 
@@ -224,7 +224,7 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
                         places_conflict::PlaceConflictBias::NoOverlap)
                 });
 
-            sets.kill_all(definitely_conflicting_borrows);
+            trans.kill_all(definitely_conflicting_borrows);
         }
     }
 }
@@ -236,21 +236,24 @@ impl<'a, 'tcx> BitDenotation<'tcx> for Borrows<'a, 'tcx> {
         self.borrow_set.borrows.len() * 2
     }
 
-    fn start_block_effect(&self, _entry_set: &mut BitSet<BorrowIndex>) {
+    fn start_block_effect(&self, _entry_set: &mut BitSet<Self::Idx>) {
         // no borrows of code region_scopes have been taken prior to
-        // function execution, so this method has no effect on
-        // `_sets`.
+        // function execution, so this method has no effect.
     }
 
     fn before_statement_effect(&self,
-                               sets: &mut BlockSets<'_, BorrowIndex>,
+                               trans: &mut GenKillSet<Self::Idx>,
                                location: Location) {
-        debug!("Borrows::before_statement_effect sets: {:?} location: {:?}", sets, location);
-        self.kill_loans_out_of_scope_at_location(sets, location);
+        debug!("Borrows::before_statement_effect trans: {:?} location: {:?}",
+               trans, location);
+        self.kill_loans_out_of_scope_at_location(trans, location);
     }
 
-    fn statement_effect(&self, sets: &mut BlockSets<'_, BorrowIndex>, location: Location) {
-        debug!("Borrows::statement_effect: sets={:?} location={:?}", sets, location);
+    fn statement_effect(&self,
+                        trans: &mut GenKillSet<Self::Idx>,
+                        location: Location) {
+        debug!("Borrows::statement_effect: trans={:?} location={:?}",
+               trans, location);
 
         let block = &self.body.basic_blocks().get(location.block).unwrap_or_else(|| {
             panic!("could not find block at location {:?}", location);
@@ -264,7 +267,7 @@ impl<'a, 'tcx> BitDenotation<'tcx> for Borrows<'a, 'tcx> {
             mir::StatementKind::Assign(ref lhs, ref rhs) => {
                 // Make sure there are no remaining borrows for variables
                 // that are assigned over.
-                self.kill_borrows_on_place(sets, lhs);
+                self.kill_borrows_on_place(trans, lhs);
 
                 if let mir::Rvalue::Ref(_, _, ref place) = **rhs {
                     if place.ignore_borrow(
@@ -278,20 +281,20 @@ impl<'a, 'tcx> BitDenotation<'tcx> for Borrows<'a, 'tcx> {
                         panic!("could not find BorrowIndex for location {:?}", location);
                     });
 
-                    sets.gen(*index);
+                    trans.gen(*index);
                 }
             }
 
             mir::StatementKind::StorageDead(local) => {
                 // Make sure there are no remaining borrows for locals that
                 // are gone out of scope.
-                self.kill_borrows_on_place(sets, &Place::Base(PlaceBase::Local(local)));
+                self.kill_borrows_on_place(trans, &Place::Base(PlaceBase::Local(local)));
             }
 
             mir::StatementKind::InlineAsm(ref asm) => {
                 for (output, kind) in asm.outputs.iter().zip(&asm.asm.outputs) {
                     if !kind.is_indirect && !kind.is_rw {
-                        self.kill_borrows_on_place(sets, output);
+                        self.kill_borrows_on_place(trans, output);
                     }
                 }
             }
@@ -307,13 +310,16 @@ impl<'a, 'tcx> BitDenotation<'tcx> for Borrows<'a, 'tcx> {
     }
 
     fn before_terminator_effect(&self,
-                                sets: &mut BlockSets<'_, BorrowIndex>,
+                                trans: &mut GenKillSet<Self::Idx>,
                                 location: Location) {
-        debug!("Borrows::before_terminator_effect sets: {:?} location: {:?}", sets, location);
-        self.kill_loans_out_of_scope_at_location(sets, location);
+        debug!("Borrows::before_terminator_effect: trans={:?} location={:?}",
+               trans, location);
+        self.kill_loans_out_of_scope_at_location(trans, location);
     }
 
-    fn terminator_effect(&self, _: &mut BlockSets<'_, BorrowIndex>, _: Location) {}
+    fn terminator_effect(&self,
+                         _: &mut GenKillSet<Self::Idx>,
+                         _: Location) {}
 
     fn propagate_call_return(
         &self,