Auto merge of #71956 - ecstatic-morse:remove-requires-storage-analysis, r=tmandry

Clean up logic around live locals in generator analysis

Resolves #69902. Requires #71893.

I've found it difficult to make changes in the logic around live locals in `generator/transform.rs`. It uses a custom dataflow analysis, `MaybeRequiresStorage`, that AFAICT computes whether a local is either initialized or borrowed. That analysis is using `before` effects, which we should try to avoid if possible because they are harder to reason about than ones only using the unprefixed effects. @pnkfelix has suggested removing "before" effects entirely to simplify the dataflow framework, which I might pursue someday.

This PR replaces `MaybeRequiresStorage` with a combination of the existing `MaybeBorrowedLocals` and a new `MaybeInitializedLocals`. `MaybeInitializedLocals` is just `MaybeInitializedPlaces` with a coarser resolution: it works on whole locals instead of move paths. As a result, I was able to simplify the logic in `compute_storage_conflicts` and `locals_live_across_suspend_points`.

This is not exactly equivalent to the old logic; some generators are now smaller than before. I believe this was because the old logic was too conservative, but I'm not as familiar with the constraints as the original implementers were, so I could be wrong. For example, I don't see a reason the size of the `mixed_sizes` future couldn't be 5K. It went from 7K to 6K in this PR.

r? @jonas-schievink @tmandry

7 files changed, 254 insertions, 372 deletions

diff --git a/src/librustc_mir/dataflow/impls/borrowed_locals.rs b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
index f929b2ddde0..b61dc56407e 100644
--- a/src/librustc_mir/dataflow/impls/borrowed_locals.rs
+++ b/src/librustc_mir/dataflow/impls/borrowed_locals.rs
@@ -99,6 +99,9 @@ impl<K> GenKillAnalysis<'tcx> for MaybeBorrowedLocals<K>
 where
     K: BorrowAnalysisKind<'tcx>,
 {
+    // The generator transform relies on the fact that this analysis does **not** use "before"
+    // effects.
+
     fn statement_effect(
         &self,
         trans: &mut impl GenKill<Self::Idx>,
diff --git a/src/librustc_mir/dataflow/impls/init_locals.rs b/src/librustc_mir/dataflow/impls/init_locals.rs
new file mode 100644
index 00000000000..726330b1f03
--- /dev/null
+++ b/src/librustc_mir/dataflow/impls/init_locals.rs
@@ -0,0 +1,118 @@
+//! A less precise version of `MaybeInitializedPlaces` whose domain is entire locals.
+//!
+//! A local will be maybe initialized if *any* projections of that local might be initialized.
+
+use crate::dataflow::{self, BottomValue, GenKill};
+
+use rustc_index::bit_set::BitSet;
+use rustc_middle::mir::visit::{PlaceContext, Visitor};
+use rustc_middle::mir::{self, BasicBlock, Local, Location};
+
+pub struct MaybeInitializedLocals;
+
+impl BottomValue for MaybeInitializedLocals {
+    /// bottom = uninit
+    const BOTTOM_VALUE: bool = false;
+}
+
+impl dataflow::AnalysisDomain<'tcx> for MaybeInitializedLocals {
+    type Idx = Local;
+
+    const NAME: &'static str = "maybe_init_locals";
+
+    fn bits_per_block(&self, body: &mir::Body<'tcx>) -> usize {
+        body.local_decls.len()
+    }
+
+    fn initialize_start_block(&self, body: &mir::Body<'tcx>, entry_set: &mut BitSet<Self::Idx>) {
+        // Function arguments are initialized to begin with.
+        for arg in body.args_iter() {
+            entry_set.insert(arg);
+        }
+    }
+}
+
+impl dataflow::GenKillAnalysis<'tcx> for MaybeInitializedLocals {
+    // The generator transform relies on the fact that this analysis does **not** use "before"
+    // effects.
+
+    fn statement_effect(
+        &self,
+        trans: &mut impl GenKill<Self::Idx>,
+        statement: &mir::Statement<'tcx>,
+        loc: Location,
+    ) {
+        TransferFunction { trans }.visit_statement(statement, loc)
+    }
+
+    fn terminator_effect(
+        &self,
+        trans: &mut impl GenKill<Self::Idx>,
+        terminator: &mir::Terminator<'tcx>,
+        loc: Location,
+    ) {
+        TransferFunction { trans }.visit_terminator(terminator, loc)
+    }
+
+    fn call_return_effect(
+        &self,
+        trans: &mut impl GenKill<Self::Idx>,
+        _block: BasicBlock,
+        _func: &mir::Operand<'tcx>,
+        _args: &[mir::Operand<'tcx>],
+        return_place: mir::Place<'tcx>,
+    ) {
+        trans.gen(return_place.local)
+    }
+
+    /// See `Analysis::apply_yield_resume_effect`.
+    fn yield_resume_effect(
+        &self,
+        trans: &mut impl GenKill<Self::Idx>,
+        _resume_block: BasicBlock,
+        resume_place: mir::Place<'tcx>,
+    ) {
+        trans.gen(resume_place.local)
+    }
+}
+
+struct TransferFunction<'a, T> {
+    trans: &'a mut T,
+}
+
+impl<T> Visitor<'tcx> for TransferFunction<'a, T>
+where
+    T: GenKill<Local>,
+{
+    fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
+        use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, NonUseContext};
+        match context {
+            // These are handled specially in `call_return_effect` and `yield_resume_effect`.
+            PlaceContext::MutatingUse(MutatingUseContext::Call | MutatingUseContext::Yield) => {}
+
+            // Otherwise, when a place is mutated, we must consider it possibly initialized.
+            PlaceContext::MutatingUse(_) => self.trans.gen(local),
+
+            // If the local is moved out of, or if it gets marked `StorageDead`, consider it no
+            // longer initialized.
+            PlaceContext::NonUse(NonUseContext::StorageDead)
+            | PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => self.trans.kill(local),
+
+            // All other uses do not affect this analysis.
+            PlaceContext::NonUse(
+                NonUseContext::StorageLive
+                | NonUseContext::AscribeUserTy
+                | NonUseContext::VarDebugInfo,
+            )
+            | PlaceContext::NonMutatingUse(
+                NonMutatingUseContext::Inspect
+                | NonMutatingUseContext::Copy
+                | NonMutatingUseContext::SharedBorrow
+                | NonMutatingUseContext::ShallowBorrow
+                | NonMutatingUseContext::UniqueBorrow
+                | NonMutatingUseContext::AddressOf
+                | NonMutatingUseContext::Projection,
+            ) => {}
+        }
+    }
+}
diff --git a/src/librustc_mir/dataflow/impls/mod.rs b/src/librustc_mir/dataflow/impls/mod.rs
index e199a174efb..ed01d6b01ea 100644
--- a/src/librustc_mir/dataflow/impls/mod.rs
+++ b/src/librustc_mir/dataflow/impls/mod.rs
@@ -22,13 +22,15 @@ use crate::dataflow::drop_flag_effects;
 
 mod borrowed_locals;
 pub(super) mod borrows;
+mod init_locals;
 mod liveness;
 mod storage_liveness;
 
 pub use self::borrowed_locals::{MaybeBorrowedLocals, MaybeMutBorrowedLocals};
 pub use self::borrows::Borrows;
+pub use self::init_locals::MaybeInitializedLocals;
 pub use self::liveness::MaybeLiveLocals;
-pub use self::storage_liveness::{MaybeRequiresStorage, MaybeStorageLive};
+pub use self::storage_liveness::MaybeStorageLive;
 
 /// `MaybeInitializedPlaces` tracks all places that might be
 /// initialized upon reaching a particular point in the control flow
diff --git a/src/librustc_mir/dataflow/impls/storage_liveness.rs b/src/librustc_mir/dataflow/impls/storage_liveness.rs
index bbc4942030e..2a2be069b1e 100644
--- a/src/librustc_mir/dataflow/impls/storage_liveness.rs
+++ b/src/librustc_mir/dataflow/impls/storage_liveness.rs
@@ -1,11 +1,9 @@
 pub use super::*;
 
 use crate::dataflow::BottomValue;
-use crate::dataflow::{self, GenKill, Results, ResultsRefCursor};
+use crate::dataflow::{self, GenKill};
 use crate::util::storage::AlwaysLiveLocals;
-use rustc_middle::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
-use std::cell::RefCell;
 
 #[derive(Clone)]
 pub struct MaybeStorageLive {
@@ -78,233 +76,3 @@ impl BottomValue for MaybeStorageLive {
     /// bottom = dead
     const BOTTOM_VALUE: bool = false;
 }
-
-type BorrowedLocalsResults<'a, 'tcx> = ResultsRefCursor<'a, 'a, 'tcx, MaybeBorrowedLocals>;
-
-/// Dataflow analysis that determines whether each local requires storage at a
-/// given location; i.e. whether its storage can go away without being observed.
-pub struct MaybeRequiresStorage<'mir, 'tcx> {
-    body: &'mir Body<'tcx>,
-    borrowed_locals: RefCell<BorrowedLocalsResults<'mir, 'tcx>>,
-}
-
-impl<'mir, 'tcx> MaybeRequiresStorage<'mir, 'tcx> {
-    pub fn new(
-        body: &'mir Body<'tcx>,
-        borrowed_locals: &'mir Results<'tcx, MaybeBorrowedLocals>,
-    ) -> Self {
-        MaybeRequiresStorage {
-            body,
-            borrowed_locals: RefCell::new(ResultsRefCursor::new(&body, borrowed_locals)),
-        }
-    }
-}
-
-impl<'mir, 'tcx> dataflow::AnalysisDomain<'tcx> for MaybeRequiresStorage<'mir, 'tcx> {
-    type Idx = Local;
-
-    const NAME: &'static str = "requires_storage";
-
-    fn bits_per_block(&self, body: &mir::Body<'tcx>) -> usize {
-        body.local_decls.len()
-    }
-
-    fn initialize_start_block(&self, body: &mir::Body<'tcx>, on_entry: &mut BitSet<Self::Idx>) {
-        // The resume argument is live on function entry (we don't care about
-        // the `self` argument)
-        for arg in body.args_iter().skip(1) {
-            on_entry.insert(arg);
-        }
-    }
-}
-
-impl<'mir, 'tcx> dataflow::GenKillAnalysis<'tcx> for MaybeRequiresStorage<'mir, 'tcx> {
-    fn before_statement_effect(
-        &self,
-        trans: &mut impl GenKill<Self::Idx>,
-        stmt: &mir::Statement<'tcx>,
-        loc: Location,
-    ) {
-        // If a place is borrowed in a statement, it needs storage for that statement.
-        self.borrowed_locals.borrow().analysis().statement_effect(trans, stmt, loc);
-
-        match &stmt.kind {
-            StatementKind::StorageDead(l) => trans.kill(*l),
-
-            // If a place is assigned to in a statement, it needs storage for that statement.
-            StatementKind::Assign(box (place, _))
-            | StatementKind::SetDiscriminant { box place, .. } => {
-                trans.gen(place.local);
-            }
-            StatementKind::LlvmInlineAsm(asm) => {
-                for place in &*asm.outputs {
-                    trans.gen(place.local);
-                }
-            }
-
-            // Nothing to do for these. Match exhaustively so this fails to compile when new
-            // variants are added.
-            StatementKind::AscribeUserType(..)
-            | StatementKind::FakeRead(..)
-            | StatementKind::Nop
-            | StatementKind::Retag(..)
-            | StatementKind::StorageLive(..) => {}
-        }
-    }
-
-    fn statement_effect(
-        &self,
-        trans: &mut impl GenKill<Self::Idx>,
-        _: &mir::Statement<'tcx>,
-        loc: Location,
-    ) {
-        // If we move from a place then only stops needing storage *after*
-        // that statement.
-        self.check_for_move(trans, loc);
-    }
-
-    fn before_terminator_effect(
-        &self,
-        trans: &mut impl GenKill<Self::Idx>,
-        terminator: &mir::Terminator<'tcx>,
-        loc: Location,
-    ) {
-        // If a place is borrowed in a terminator, it needs storage for that terminator.
-        self.borrowed_locals.borrow().analysis().terminator_effect(trans, terminator, loc);
-
-        match &terminator.kind {
-            TerminatorKind::Call { destination: Some((place, _)), .. } => {
-                trans.gen(place.local);
-            }
-
-            // Note that we do *not* gen the `resume_arg` of `Yield` terminators. The reason for
-            // that is that a `yield` will return from the function, and `resume_arg` is written
-            // only when the generator is later resumed. Unlike `Call`, this doesn't require the
-            // place to have storage *before* the yield, only after.
-            TerminatorKind::Yield { .. } => {}
-
-            TerminatorKind::InlineAsm { operands, .. } => {
-                for op in operands {
-                    match op {
-                        InlineAsmOperand::Out { place, .. }
-                        | InlineAsmOperand::InOut { out_place: place, .. } => {
-                            if let Some(place) = place {
-                                trans.gen(place.local);
-                            }
-                        }
-                        InlineAsmOperand::In { .. }
-                        | InlineAsmOperand::Const { .. }
-                        | InlineAsmOperand::SymFn { .. }
-                        | InlineAsmOperand::SymStatic { .. } => {}
-                    }
-                }
-            }
-
-            // Nothing to do for these. Match exhaustively so this fails to compile when new
-            // variants are added.
-            TerminatorKind::Call { destination: None, .. }
-            | TerminatorKind::Abort
-            | TerminatorKind::Assert { .. }
-            | TerminatorKind::Drop { .. }
-            | TerminatorKind::DropAndReplace { .. }
-            | TerminatorKind::FalseEdges { .. }
-            | TerminatorKind::FalseUnwind { .. }
-            | TerminatorKind::GeneratorDrop
-            | TerminatorKind::Goto { .. }
-            | TerminatorKind::Resume
-            | TerminatorKind::Return
-            | TerminatorKind::SwitchInt { .. }
-            | TerminatorKind::Unreachable => {}
-        }
-    }
-
-    fn terminator_effect(
-        &self,
-        trans: &mut impl GenKill<Self::Idx>,
-        terminator: &mir::Terminator<'tcx>,
-        loc: Location,
-    ) {
-        match &terminator.kind {
-            // For call terminators the destination requires storage for the call
-            // and after the call returns successfully, but not after a panic.
-            // Since `propagate_call_unwind` doesn't exist, we have to kill the
-            // destination here, and then gen it again in `call_return_effect`.
-            TerminatorKind::Call { destination: Some((place, _)), .. } => {
-                trans.kill(place.local);
-            }
-
-            // Nothing to do for these. Match exhaustively so this fails to compile when new
-            // variants are added.
-            TerminatorKind::Call { destination: None, .. }
-            | TerminatorKind::Yield { .. }
-            | TerminatorKind::Abort
-            | TerminatorKind::Assert { .. }
-            | TerminatorKind::Drop { .. }
-            | TerminatorKind::DropAndReplace { .. }
-            | TerminatorKind::FalseEdges { .. }
-            | TerminatorKind::FalseUnwind { .. }
-            | TerminatorKind::GeneratorDrop
-            | TerminatorKind::Goto { .. }
-            | TerminatorKind::InlineAsm { .. }
-            | TerminatorKind::Resume
-            | TerminatorKind::Return
-            | TerminatorKind::SwitchInt { .. }
-            | TerminatorKind::Unreachable => {}
-        }
-
-        self.check_for_move(trans, loc);
-    }
-
-    fn call_return_effect(
-        &self,
-        trans: &mut impl GenKill<Self::Idx>,
-        _block: BasicBlock,
-        _func: &mir::Operand<'tcx>,
-        _args: &[mir::Operand<'tcx>],
-        return_place: mir::Place<'tcx>,
-    ) {
-        trans.gen(return_place.local);
-    }
-
-    fn yield_resume_effect(
-        &self,
-        trans: &mut impl GenKill<Self::Idx>,
-        _resume_block: BasicBlock,
-        resume_place: mir::Place<'tcx>,
-    ) {
-        trans.gen(resume_place.local);
-    }
-}
-
-impl<'mir, 'tcx> MaybeRequiresStorage<'mir, 'tcx> {
-    /// Kill locals that are fully moved and have not been borrowed.
-    fn check_for_move(&self, trans: &mut impl GenKill<Local>, loc: Location) {
-        let mut visitor = MoveVisitor { trans, borrowed_locals: &self.borrowed_locals };
-        visitor.visit_location(&self.body, loc);
-    }
-}
-
-impl<'mir, 'tcx> BottomValue for MaybeRequiresStorage<'mir, 'tcx> {
-    /// bottom = dead
-    const BOTTOM_VALUE: bool = false;
-}
-
-struct MoveVisitor<'a, 'mir, 'tcx, T> {
-    borrowed_locals: &'a RefCell<BorrowedLocalsResults<'mir, 'tcx>>,
-    trans: &'a mut T,
-}
-
-impl<'a, 'mir, 'tcx, T> Visitor<'tcx> for MoveVisitor<'a, 'mir, 'tcx, T>
-where
-    T: GenKill<Local>,
-{
-    fn visit_local(&mut self, local: &Local, context: PlaceContext, loc: Location) {
-        if PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) == context {
-            let mut borrowed_locals = self.borrowed_locals.borrow_mut();
-            borrowed_locals.seek_before_primary_effect(loc);
-            if !borrowed_locals.contains(*local) {
-                self.trans.kill(*local);
-            }
-        }
-    }
-}
diff --git a/src/librustc_mir/transform/generator.rs b/src/librustc_mir/transform/generator.rs
index 4bf2adcd450..5f8104e7934 100644
--- a/src/librustc_mir/transform/generator.rs
+++ b/src/librustc_mir/transform/generator.rs
@@ -50,7 +50,7 @@
 //! Otherwise it drops all the values in scope at the last suspension point.
 
 use crate::dataflow::impls::{
-    MaybeBorrowedLocals, MaybeLiveLocals, MaybeRequiresStorage, MaybeStorageLive,
+    MaybeBorrowedLocals, MaybeInitializedLocals, MaybeLiveLocals, MaybeStorageLive,
 };
 use crate::dataflow::{self, Analysis};
 use crate::transform::no_landing_pads::no_landing_pads;
@@ -444,86 +444,80 @@ fn locals_live_across_suspend_points(
     movable: bool,
 ) -> LivenessInfo {
     let def_id = source.def_id();
-    let body_ref: &Body<'_> = &body;
 
     // Calculate when MIR locals have live storage. This gives us an upper bound of their
     // lifetimes.
     let mut storage_live = MaybeStorageLive::new(always_live_locals.clone())
-        .into_engine(tcx, body_ref, def_id)
+        .into_engine(tcx, body, def_id)
         .iterate_to_fixpoint()
-        .into_results_cursor(body_ref);
-
-    // Calculate the MIR locals which have been previously
-    // borrowed (even if they are still active).
-    let borrowed_locals_results =
-        MaybeBorrowedLocals::all_borrows().into_engine(tcx, body_ref, def_id).iterate_to_fixpoint();
-
-    let mut borrowed_locals_cursor =
-        dataflow::ResultsCursor::new(body_ref, &borrowed_locals_results);
-
-    // Calculate the MIR locals that we actually need to keep storage around
-    // for.
-    let requires_storage_results = MaybeRequiresStorage::new(body, &borrowed_locals_results)
-        .into_engine(tcx, body_ref, def_id)
-        .iterate_to_fixpoint();
-    let mut requires_storage_cursor =
-        dataflow::ResultsCursor::new(body_ref, &requires_storage_results);
-
-    // Calculate the liveness of MIR locals ignoring borrows.
-    let mut liveness = MaybeLiveLocals
-        .into_engine(tcx, body_ref, def_id)
+        .into_results_cursor(body);
+
+    let mut init = MaybeInitializedLocals
+        .into_engine(tcx, body, def_id)
+        .iterate_to_fixpoint()
+        .into_results_cursor(body);
+
+    let mut live = MaybeLiveLocals
+        .into_engine(tcx, body, def_id)
+        .iterate_to_fixpoint()
+        .into_results_cursor(body);
+
+    let mut borrowed = MaybeBorrowedLocals::all_borrows()
+        .into_engine(tcx, body, def_id)
         .iterate_to_fixpoint()
-        .into_results_cursor(body_ref);
+        .into_results_cursor(body);
+
+    // Liveness across yield points is determined by the following boolean equation, where `live`,
+    // `init` and `borrowed` come from dataflow and `movable` is a property of the generator.
+    // Movable generators do not allow borrows to live across yield points, so they don't need to
+    // store a local simply because it is borrowed.
+    //
+    //    live_across_yield := (live & init) | (!movable & borrowed)
+    //
+    let mut locals_live_across_yield_point = |block| {
+        live.seek_to_block_end(block);
+        let mut live_locals = live.get().clone();
+
+        init.seek_to_block_end(block);
+        live_locals.intersect(init.get());
+
+        if !movable {
+            borrowed.seek_to_block_end(block);
+            live_locals.union(borrowed.get());
+        }
+
+        live_locals
+    };
 
     let mut storage_liveness_map = IndexVec::from_elem(None, body.basic_blocks());
     let mut live_locals_at_suspension_points = Vec::new();
     let mut live_locals_at_any_suspension_point = BitSet::new_empty(body.local_decls.len());
 
     for (block, data) in body.basic_blocks().iter_enumerated() {
-        if let TerminatorKind::Yield { .. } = data.terminator().kind {
-            let loc = Location { block, statement_index: data.statements.len() };
-
-            liveness.seek_to_block_end(block);
-            let mut live_locals = liveness.get().clone();
-
-            if !movable {
-                // The `liveness` variable contains the liveness of MIR locals ignoring borrows.
-                // This is correct for movable generators since borrows cannot live across
-                // suspension points. However for immovable generators we need to account for
-                // borrows, so we conseratively assume that all borrowed locals are live until
-                // we find a StorageDead statement referencing the locals.
-                // To do this we just union our `liveness` result with `borrowed_locals`, which
-                // contains all the locals which has been borrowed before this suspension point.
-                // If a borrow is converted to a raw reference, we must also assume that it lives
-                // forever. Note that the final liveness is still bounded by the storage liveness
-                // of the local, which happens using the `intersect` operation below.
-                borrowed_locals_cursor.seek_before_primary_effect(loc);
-                live_locals.union(borrowed_locals_cursor.get());
-            }
-
-            // Store the storage liveness for later use so we can restore the state
-            // after a suspension point
-            storage_live.seek_before_primary_effect(loc);
-            storage_liveness_map[block] = Some(storage_live.get().clone());
-
-            // Locals live are live at this point only if they are used across
-            // suspension points (the `liveness` variable)
-            // and their storage is required (the `storage_required` variable)
-            requires_storage_cursor.seek_before_primary_effect(loc);
-            live_locals.intersect(requires_storage_cursor.get());
+        if !matches!(data.terminator().kind, TerminatorKind::Yield { ..  }) {
+            continue;
+        }
 
-            // The generator argument is ignored.
-            live_locals.remove(SELF_ARG);
+        // Store the storage liveness for later use so we can restore the state
+        // after a suspension point
+        storage_live.seek_to_block_end(block);
+        storage_liveness_map[block] = Some(storage_live.get().clone());
 
-            debug!("loc = {:?}, live_locals = {:?}", loc, live_locals);
+        let mut live_locals = locals_live_across_yield_point(block);
 
-            // Add the locals live at this suspension point to the set of locals which live across
-            // any suspension points
-            live_locals_at_any_suspension_point.union(&live_locals);
+        // The combination of `MaybeInitializedLocals` and `MaybeBorrowedLocals` should be strictly
+        // more precise than `MaybeStorageLive` because they handle `StorageDead` themselves. This
+        // assumes that the MIR forbids locals from being initialized/borrowed before reaching
+        // `StorageLive`.
+        debug_assert!(storage_live.get().superset(&live_locals));
 
-            live_locals_at_suspension_points.push(live_locals);
-        }
+        // Ignore the generator's `self` argument since it is handled seperately.
+        live_locals.remove(SELF_ARG);
+        debug!("block = {:?}, live_locals = {:?}", block, live_locals);
+        live_locals_at_any_suspension_point.union(&live_locals);
+        live_locals_at_suspension_points.push(live_locals);
     }
+
     debug!("live_locals_anywhere = {:?}", live_locals_at_any_suspension_point);
 
     // Renumber our liveness_map bitsets to include only the locals we are
@@ -534,10 +528,11 @@ fn locals_live_across_suspend_points(
         .collect();
 
     let storage_conflicts = compute_storage_conflicts(
-        body_ref,
+        body,
         &live_locals_at_any_suspension_point,
         always_live_locals.clone(),
-        requires_storage_results,
+        init,
+        borrowed,
     );
 
     LivenessInfo {
@@ -569,6 +564,37 @@ fn renumber_bitset(
     out
 }
 
+/// Record conflicts between locals at the current dataflow cursor positions.
+///
+/// You need to seek the cursors before calling this function.
+fn record_conflicts_at_curr_loc(
+    local_conflicts: &mut BitMatrix<Local, Local>,
+    init: &dataflow::ResultsCursor<'mir, 'tcx, MaybeInitializedLocals>,
+    borrowed: &dataflow::ResultsCursor<'mir, 'tcx, MaybeBorrowedLocals>,
+) {
+    // A local requires storage if it is initialized or borrowed. For now, a local
+    // becomes uninitialized if it is moved from, but is still considered "borrowed".
+    //
+    //     requires_storage := init | borrowed
+    //
+    // Just like when determining what locals are live at yield points, there is no need
+    // to look at storage liveness here, since `init | borrowed` is strictly more precise.
+    //
+    // FIXME: This function is called in a loop, so it might be better to pass in a temporary
+    // bitset rather than cloning here.
+    let mut requires_storage = init.get().clone();
+    requires_storage.union(borrowed.get());
+
+    for local in requires_storage.iter() {
+        local_conflicts.union_row_with(&requires_storage, local);
+    }
+
+    // `>1` because the `self` argument always requires storage.
+    if requires_storage.count() > 1 {
+        trace!("requires_storage={:?}", requires_storage);
+    }
+}
+
 /// For every saved local, looks for which locals are StorageLive at the same
 /// time. Generates a bitset for every local of all the other locals that may be
 /// StorageLive simultaneously with that local. This is used in the layout
@@ -577,30 +603,45 @@ fn compute_storage_conflicts(
     body: &'mir Body<'tcx>,
     stored_locals: &BitSet<Local>,
     always_live_locals: storage::AlwaysLiveLocals,
-    requires_storage: dataflow::Results<'tcx, MaybeRequiresStorage<'mir, 'tcx>>,
+    mut init: dataflow::ResultsCursor<'mir, 'tcx, MaybeInitializedLocals>,
+    mut borrowed: dataflow::ResultsCursor<'mir, 'tcx, MaybeBorrowedLocals>,
 ) -> BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal> {
-    assert_eq!(body.local_decls.len(), stored_locals.domain_size());
-
     debug!("compute_storage_conflicts({:?})", body.span);
-    debug!("always_live = {:?}", always_live_locals);
-
-    // Locals that are always live or ones that need to be stored across
-    // suspension points are not eligible for overlap.
-    let mut ineligible_locals = always_live_locals.into_inner();
-    ineligible_locals.intersect(stored_locals);
+    assert_eq!(body.local_decls.len(), stored_locals.domain_size());
 
-    // Compute the storage conflicts for all eligible locals.
-    let mut visitor = StorageConflictVisitor {
-        body,
-        stored_locals: &stored_locals,
-        local_conflicts: BitMatrix::from_row_n(&ineligible_locals, body.local_decls.len()),
-    };
+    // Locals that are always live conflict with all other locals.
+    //
+    // FIXME: Why do we need to handle locals without `Storage{Live,Dead}` specially here?
+    // Shouldn't it be enough to know whether they are initialized?
+    let always_live_locals = always_live_locals.into_inner();
+    let mut local_conflicts = BitMatrix::from_row_n(&always_live_locals, body.local_decls.len());
+
+    // Visit every reachable statement and terminator. The exact order does not matter. When two
+    // locals are live at the same point in time, add an entry in the conflict matrix.
+    for (block, data) in traversal::preorder(body) {
+        // Ignore unreachable blocks.
+        if data.terminator().kind == TerminatorKind::Unreachable {
+            continue;
+        }
 
-    // Visit only reachable basic blocks. The exact order is not important.
-    let reachable_blocks = traversal::preorder(body).map(|(bb, _)| bb);
-    requires_storage.visit_with(body, reachable_blocks, &mut visitor);
+        // Observe the dataflow state *before* all possible locations (statement or terminator) in
+        // each basic block...
+        for statement_index in 0..=data.statements.len() {
+            let loc = Location { block, statement_index };
+            trace!("record conflicts at {:?}", loc);
+            init.seek_before_primary_effect(loc);
+            borrowed.seek_before_primary_effect(loc);
+            record_conflicts_at_curr_loc(&mut local_conflicts, &init, &borrowed);
+        }
 
-    let local_conflicts = visitor.local_conflicts;
+        // ...and then observe the state *after* the terminator effect is applied. As long as
+        // neither `init` nor `borrowed` has a "before" effect, we will observe all possible
+        // dataflow states here or in the loop above.
+        trace!("record conflicts at end of {:?}", block);
+        init.seek_to_block_end(block);
+        borrowed.seek_to_block_end(block);
+        record_conflicts_at_curr_loc(&mut local_conflicts, &init, &borrowed);
+    }
 
     // Compress the matrix using only stored locals (Local -> GeneratorSavedLocal).
     //
@@ -612,7 +653,7 @@ fn compute_storage_conflicts(
     let mut storage_conflicts = BitMatrix::new(stored_locals.count(), stored_locals.count());
     for (idx_a, local_a) in stored_locals.iter().enumerate() {
         let saved_local_a = GeneratorSavedLocal::new(idx_a);
-        if ineligible_locals.contains(local_a) {
+        if always_live_locals.contains(local_a) {
             // Conflicts with everything.
             storage_conflicts.insert_all_into_row(saved_local_a);
         } else {
@@ -628,56 +669,6 @@ fn compute_storage_conflicts(
     storage_conflicts
 }
 
-struct StorageConflictVisitor<'mir, 'tcx, 's> {
-    body: &'mir Body<'tcx>,
-    stored_locals: &'s BitSet<Local>,
-    // FIXME(tmandry): Consider using sparse bitsets here once we have good
-    // benchmarks for generators.
-    local_conflicts: BitMatrix<Local, Local>,
-}
-
-impl dataflow::ResultsVisitor<'mir, 'tcx> for StorageConflictVisitor<'mir, 'tcx, '_> {
-    type FlowState = BitSet<Local>;
-
-    fn visit_statement_before_primary_effect(
-        &mut self,
-        state: &Self::FlowState,
-        _statement: &'mir Statement<'tcx>,
-        loc: Location,
-    ) {
-        self.apply_state(state, loc);
-    }
-
-    fn visit_terminator_before_primary_effect(
-        &mut self,
-        state: &Self::FlowState,
-        _terminator: &'mir Terminator<'tcx>,
-        loc: Location,
-    ) {
-        self.apply_state(state, loc);
-    }
-}
-
-impl<'body, 'tcx, 's> StorageConflictVisitor<'body, 'tcx, 's> {
-    fn apply_state(&mut self, flow_state: &BitSet<Local>, loc: Location) {
-        // Ignore unreachable blocks.
-        if self.body.basic_blocks()[loc.block].terminator().kind == TerminatorKind::Unreachable {
-            return;
-        }
-
-        let mut eligible_storage_live = flow_state.clone();
-        eligible_storage_live.intersect(&self.stored_locals);
-
-        for local in eligible_storage_live.iter() {
-            self.local_conflicts.union_row_with(&eligible_storage_live, local);
-        }
-
-        if eligible_storage_live.count() > 1 {
-            trace!("at {:?}, eligible_storage_live={:?}", loc, eligible_storage_live);
-        }
-    }
-}
-
 fn compute_layout<'tcx>(
     tcx: TyCtxt<'tcx>,
     source: MirSource<'tcx>,
diff --git a/src/test/ui/async-await/async-fn-size-moved-locals.rs b/src/test/ui/async-await/async-fn-size-moved-locals.rs
index 636fafc2bc4..000acf14a3f 100644
--- a/src/test/ui/async-await/async-fn-size-moved-locals.rs
+++ b/src/test/ui/async-await/async-fn-size-moved-locals.rs
@@ -114,5 +114,5 @@ fn main() {
     assert_eq!(1026, std::mem::size_of_val(&single_with_noop()));
     assert_eq!(3078, std::mem::size_of_val(&joined()));
     assert_eq!(3079, std::mem::size_of_val(&joined_with_noop()));
-    assert_eq!(7181, std::mem::size_of_val(&mixed_sizes()));
+    assert_eq!(6157, std::mem::size_of_val(&mixed_sizes()));
 }
diff --git a/src/test/ui/generator/size-moved-locals.rs b/src/test/ui/generator/size-moved-locals.rs
index 74c60d98154..a5786c2999e 100644
--- a/src/test/ui/generator/size-moved-locals.rs
+++ b/src/test/ui/generator/size-moved-locals.rs
@@ -72,6 +72,6 @@ fn overlap_x_and_y() -> impl Generator<Yield = (), Return = ()> {
 fn main() {
     assert_eq!(1025, std::mem::size_of_val(&move_before_yield()));
     assert_eq!(1026, std::mem::size_of_val(&move_before_yield_with_noop()));
-    assert_eq!(2051, std::mem::size_of_val(&overlap_move_points()));
+    assert_eq!(1027, std::mem::size_of_val(&overlap_move_points()));
     assert_eq!(1026, std::mem::size_of_val(&overlap_x_and_y()));
 }