Auto merge of #70447 - ecstatic-morse:storage-live-always, r=tmandry

Add utility to find locals that don't use `StorageLive` annotations and use it for `MaybeStorageLive`

Addresses #70004 (comment) (cc @RalfJung).

The only dataflow analysis that is incorrect in this case is `MaybeStorageLive`. `transform/generator.rs` implemented custom handling for this class of locals, but other consumers of this analysis (there's one in [clippy](https://github.com/rust-lang/rust-clippy/blob/513b46793e98ce5b412d388a91f6371d6a9b290b/clippy_lints/src/redundant_clone.rs#L402)) would be incorrect.

r? @tmandry

Author: bors

src/librustc_mir/dataflow/impls/storage_liveness.rs

@@ -2,12 +2,21 @@ pub use super::*;
 
 use crate::dataflow::BottomValue;
 use crate::dataflow::{self, GenKill, Results, ResultsRefCursor};
+use crate::util::storage::AlwaysLiveLocals;
 use rustc_middle::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use std::cell::RefCell;
 
-#[derive(Copy, Clone)]
-pub struct MaybeStorageLive;
+#[derive(Clone)]
+pub struct MaybeStorageLive {
+    always_live_locals: AlwaysLiveLocals,
+}
+
+impl MaybeStorageLive {
+    pub fn new(always_live_locals: AlwaysLiveLocals) -> Self {
+        MaybeStorageLive { always_live_locals }
+    }
+}
 
 impl dataflow::AnalysisDomain<'tcx> for MaybeStorageLive {
     type Idx = Local;
@@ -19,9 +28,12 @@ impl dataflow::AnalysisDomain<'tcx> for MaybeStorageLive {
     }
 
     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) {
+        assert_eq!(body.local_decls.len(), self.always_live_locals.domain_size());
+        for local in self.always_live_locals.iter() {
+            on_entry.insert(local);
+        }
+
+        for arg in body.args_iter() {
             on_entry.insert(arg);
         }
     }

src/librustc_mir/interpret/eval_context.rs

@@ -24,6 +24,7 @@ use super::{
     Immediate, MPlaceTy, Machine, MemPlace, MemPlaceMeta, Memory, OpTy, Operand, Place, PlaceTy,
     ScalarMaybeUndef, StackPopJump,
 };
+use crate::util::storage::AlwaysLiveLocals;
 
 pub struct InterpCx<'mir, 'tcx, M: Machine<'mir, 'tcx>> {
     /// Stores the `Machine` instance.
@@ -610,17 +611,17 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             // Now mark those locals as dead that we do not want to initialize
             match self.tcx.def_kind(instance.def_id()) {
                 // statics and constants don't have `Storage*` statements, no need to look for them
+                //
+                // FIXME: The above is likely untrue. See
+                // <https://github.com/rust-lang/rust/pull/70004#issuecomment-602022110>. Is it
+                // okay to ignore `StorageDead`/`StorageLive` annotations during CTFE?
                 Some(DefKind::Static) | Some(DefKind::Const) | Some(DefKind::AssocConst) => {}
                 _ => {
-                    for block in body.basic_blocks() {
-                        for stmt in block.statements.iter() {
-                            use rustc_middle::mir::StatementKind::{StorageDead, StorageLive};
-                            match stmt.kind {
-                                StorageLive(local) | StorageDead(local) => {
-                                    locals[local].value = LocalValue::Dead;
-                                }
-                                _ => {}
-                            }
+                    // Mark locals that use `Storage*` annotations as dead on function entry.
+                    let always_live = AlwaysLiveLocals::new(self.body());
+                    for local in locals.indices() {
+                        if !always_live.contains(local) {
+                            locals[local].value = LocalValue::Dead;
                         }
                     }
                 }

src/librustc_mir/transform/generator.rs

@@ -56,12 +56,13 @@ use crate::transform::simplify;
 use crate::transform::{MirPass, MirSource};
 use crate::util::dump_mir;
 use crate::util::liveness;
+use crate::util::storage;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_index::bit_set::{BitMatrix, BitSet};
 use rustc_index::vec::{Idx, IndexVec};
-use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
+use rustc_middle::mir::visit::{MutVisitor, PlaceContext};
 use rustc_middle::mir::*;
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::GeneratorSubsts;
@@ -222,6 +223,9 @@ struct TransformVisitor<'tcx> {
     // A list of suspension points, generated during the transform
     suspension_points: Vec<SuspensionPoint<'tcx>>,
 
+    // The set of locals that have no `StorageLive`/`StorageDead` annotations.
+    always_live_locals: storage::AlwaysLiveLocals,
+
     // The original RETURN_PLACE local
     new_ret_local: Local,
 }
@@ -416,19 +420,6 @@ fn replace_local<'tcx>(
     new_local
 }
 
-struct StorageIgnored(liveness::LiveVarSet);
-
-impl<'tcx> Visitor<'tcx> for StorageIgnored {
-    fn visit_statement(&mut self, statement: &Statement<'tcx>, _location: Location) {
-        match statement.kind {
-            StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
-                self.0.remove(l);
-            }
-            _ => (),
-        }
-    }
-}
-
 struct LivenessInfo {
     /// Which locals are live across any suspension point.
     ///
@@ -454,23 +445,19 @@ fn locals_live_across_suspend_points(
     tcx: TyCtxt<'tcx>,
     body: ReadOnlyBodyAndCache<'_, 'tcx>,
     source: MirSource<'tcx>,
+    always_live_locals: &storage::AlwaysLiveLocals,
     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
+    let mut storage_live = MaybeStorageLive::new(always_live_locals.clone())
         .into_engine(tcx, body_ref, def_id)
         .iterate_to_fixpoint()
         .into_results_cursor(body_ref);
 
-    // Find the MIR locals which do not use StorageLive/StorageDead statements.
-    // The storage of these locals are always live.
-    let mut ignored = StorageIgnored(BitSet::new_filled(body.local_decls.len()));
-    ignored.visit_body(&body);
-
     // Calculate the MIR locals which have been previously
     // borrowed (even if they are still active).
     let borrowed_locals_results =
@@ -515,11 +502,14 @@ fn locals_live_across_suspend_points(
             }
 
             storage_live.seek_before(loc);
-            let storage_liveness = storage_live.get();
+            let mut storage_liveness = storage_live.get().clone();
+
+            // Later passes handle the generator's `self` argument separately.
+            storage_liveness.remove(SELF_ARG);
 
             // Store the storage liveness for later use so we can restore the state
             // after a suspension point
-            storage_liveness_map.insert(block, storage_liveness.clone());
+            storage_liveness_map.insert(block, storage_liveness);
 
             requires_storage_cursor.seek_before(loc);
             let storage_required = requires_storage_cursor.get().clone();
@@ -551,8 +541,12 @@ fn locals_live_across_suspend_points(
         .map(|live_here| renumber_bitset(&live_here, &live_locals))
         .collect();
 
-    let storage_conflicts =
-        compute_storage_conflicts(body_ref, &live_locals, &ignored, requires_storage_results);
+    let storage_conflicts = compute_storage_conflicts(
+        body_ref,
+        &live_locals,
+        always_live_locals.clone(),
+        requires_storage_results,
+    );
 
     LivenessInfo {
         live_locals,
@@ -590,18 +584,18 @@ fn renumber_bitset(
 fn compute_storage_conflicts(
     body: &'mir Body<'tcx>,
     stored_locals: &liveness::LiveVarSet,
-    ignored: &StorageIgnored,
+    always_live_locals: storage::AlwaysLiveLocals,
     requires_storage: dataflow::Results<'tcx, MaybeRequiresStorage<'mir, 'tcx>>,
 ) -> BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal> {
-    assert_eq!(body.local_decls.len(), ignored.0.domain_size());
     assert_eq!(body.local_decls.len(), stored_locals.domain_size());
+
     debug!("compute_storage_conflicts({:?})", body.span);
-    debug!("ignored = {:?}", ignored.0);
+    debug!("always_live = {:?}", always_live_locals);
 
-    // Storage ignored locals are not eligible for overlap, since their storage
-    // is always live.
-    let mut ineligible_locals = ignored.0.clone();
-    ineligible_locals.intersect(&stored_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);
 
     // Compute the storage conflicts for all eligible locals.
     let mut visitor = StorageConflictVisitor {
@@ -697,6 +691,7 @@ fn compute_layout<'tcx>(
     source: MirSource<'tcx>,
     upvars: &Vec<Ty<'tcx>>,
     interior: Ty<'tcx>,
+    always_live_locals: &storage::AlwaysLiveLocals,
     movable: bool,
     body: &mut BodyAndCache<'tcx>,
 ) -> (
@@ -710,7 +705,13 @@ fn compute_layout<'tcx>(
         live_locals_at_suspension_points,
         storage_conflicts,
         storage_liveness,
-    } = locals_live_across_suspend_points(tcx, read_only!(body), source, movable);
+    } = locals_live_across_suspend_points(
+        tcx,
+        read_only!(body),
+        source,
+        always_live_locals,
+        movable,
+    );
 
     // Erase regions from the types passed in from typeck so we can compare them with
     // MIR types
@@ -1180,7 +1181,10 @@ fn create_cases<'tcx>(
                     }
 
                     let l = Local::new(i);
-                    if point.storage_liveness.contains(l) && !transform.remap.contains_key(&l) {
+                    let needs_storage_live = point.storage_liveness.contains(l)
+                        && !transform.remap.contains_key(&l)
+                        && !transform.always_live_locals.contains(l);
+                    if needs_storage_live {
                         statements
                             .push(Statement { source_info, kind: StatementKind::StorageLive(l) });
                     }
@@ -1276,11 +1280,13 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
             },
         );
 
+        let always_live_locals = storage::AlwaysLiveLocals::new(&body);
+
         // Extract locals which are live across suspension point into `layout`
         // `remap` gives a mapping from local indices onto generator struct indices
         // `storage_liveness` tells us which locals have live storage at suspension points
         let (remap, layout, storage_liveness) =
-            compute_layout(tcx, source, &upvars, interior, movable, body);
+            compute_layout(tcx, source, &upvars, interior, &always_live_locals, movable, body);
 
         let can_return = can_return(tcx, body);
 
@@ -1294,6 +1300,7 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
             state_substs,
             remap,
             storage_liveness,
+            always_live_locals,
             suspension_points: Vec::new(),
             new_ret_local,
             discr_ty,

src/librustc_mir/util/mod.rs

@@ -3,6 +3,7 @@ pub mod borrowck_errors;
 pub mod def_use;
 pub mod elaborate_drops;
 pub mod patch;
+pub mod storage;
 
 mod alignment;
 pub mod collect_writes;

src/librustc_mir/util/storage.rs

@@ -0,0 +1,47 @@
+use rustc_index::bit_set::BitSet;
+use rustc_middle::mir::visit::Visitor;
+use rustc_middle::mir::{self, Local, Location};
+
+/// The set of locals in a MIR body that do not have `StorageLive`/`StorageDead` annotations.
+///
+/// These locals have fixed storage for the duration of the body.
+//
+// FIXME: Currently, we need to traverse the entire MIR to compute this. We should instead store it
+// as a field in the `LocalDecl` for each `Local`.
+#[derive(Debug, Clone)]
+pub struct AlwaysLiveLocals(BitSet<Local>);
+
+impl AlwaysLiveLocals {
+    pub fn new(body: &mir::Body<'tcx>) -> Self {
+        let mut ret = AlwaysLiveLocals(BitSet::new_filled(body.local_decls.len()));
+
+        let mut vis = StorageAnnotationVisitor(&mut ret);
+        vis.visit_body(body);
+
+        ret
+    }
+
+    pub fn into_inner(self) -> BitSet<Local> {
+        self.0
+    }
+}
+
+impl std::ops::Deref for AlwaysLiveLocals {
+    type Target = BitSet<Local>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+/// Removes locals that have `Storage*` annotations from `AlwaysLiveLocals`.
+struct StorageAnnotationVisitor<'a>(&'a mut AlwaysLiveLocals);
+
+impl Visitor<'tcx> for StorageAnnotationVisitor<'_> {
+    fn visit_statement(&mut self, statement: &mir::Statement<'tcx>, _location: Location) {
+        use mir::StatementKind::{StorageDead, StorageLive};
+        if let StorageLive(l) | StorageDead(l) = statement.kind {
+            (self.0).0.remove(l);
+        }
+    }
+}