Auto merge of #145541 - cjgillot:dest-prop-live-range, r=Amanieu

Reimplement DestinationPropagation according to live ranges.

This PR reimplements DestinationPropagation as a problem of merging live-ranges of locals. We merge locals that have disjoint live-ranges. This allows merging several locals in the same round by updating live range information.

Live ranges are mainly computed using the `MaybeLiveLocals` analysis. The subtlety is that we split each statement and terminator in 2 positions. The first position is the regular statement. The second position is a shadow, which is always more live. It encodes partial writes and dead writes as a local being live for half a statement. This half statement ensures that writes conflict with another local's writes and regular liveness.

r? `@Amanieu`

3 files changed, 24 insertions, 84 deletions

diff --git a/compiler/rustc_mir_dataflow/src/impls/liveness.rs b/compiler/rustc_mir_dataflow/src/impls/liveness.rs
index 6ec1b03a34e..5eba474a60c 100644
--- a/compiler/rustc_mir_dataflow/src/impls/liveness.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/liveness.rs
@@ -92,7 +92,7 @@ impl<'tcx> Visitor<'tcx> for TransferFunction<'_> {
         }
 
         match DefUse::for_place(*place, context) {
-            Some(DefUse::Def) => {
+            DefUse::Def => {
                 if let PlaceContext::MutatingUse(
                     MutatingUseContext::Call | MutatingUseContext::AsmOutput,
                 ) = context
@@ -105,8 +105,8 @@ impl<'tcx> Visitor<'tcx> for TransferFunction<'_> {
                     self.0.kill(place.local);
                 }
             }
-            Some(DefUse::Use) => self.0.gen_(place.local),
-            None => {}
+            DefUse::Use => self.0.gen_(place.local),
+            DefUse::PartialWrite | DefUse::NonUse => {}
         }
 
         self.visit_projection(place.as_ref(), context, location);
@@ -131,23 +131,29 @@ impl<'tcx> Visitor<'tcx> for YieldResumeEffect<'_> {
 }
 
 #[derive(Eq, PartialEq, Clone)]
-enum DefUse {
+pub enum DefUse {
+    /// Full write to the local.
     Def,
+    /// Read of any part of the local.
     Use,
+    /// Partial write to the local.
+    PartialWrite,
+    /// Non-use, like debuginfo.
+    NonUse,
 }
 
 impl DefUse {
     fn apply(state: &mut DenseBitSet<Local>, place: Place<'_>, context: PlaceContext) {
         match DefUse::for_place(place, context) {
-            Some(DefUse::Def) => state.kill(place.local),
-            Some(DefUse::Use) => state.gen_(place.local),
-            None => {}
+            DefUse::Def => state.kill(place.local),
+            DefUse::Use => state.gen_(place.local),
+            DefUse::PartialWrite | DefUse::NonUse => {}
         }
     }
 
-    fn for_place(place: Place<'_>, context: PlaceContext) -> Option<DefUse> {
+    pub fn for_place(place: Place<'_>, context: PlaceContext) -> DefUse {
         match context {
-            PlaceContext::NonUse(_) => None,
+            PlaceContext::NonUse(_) => DefUse::NonUse,
 
             PlaceContext::MutatingUse(
                 MutatingUseContext::Call
@@ -156,21 +162,20 @@ impl DefUse {
                 | MutatingUseContext::Store
                 | MutatingUseContext::Deinit,
             ) => {
+                // Treat derefs as a use of the base local. `*p = 4` is not a def of `p` but a use.
                 if place.is_indirect() {
-                    // Treat derefs as a use of the base local. `*p = 4` is not a def of `p` but a
-                    // use.
-                    Some(DefUse::Use)
+                    DefUse::Use
                 } else if place.projection.is_empty() {
-                    Some(DefUse::Def)
+                    DefUse::Def
                 } else {
-                    None
+                    DefUse::PartialWrite
                 }
             }
 
             // Setting the discriminant is not a use because it does no reading, but it is also not
             // a def because it does not overwrite the whole place
             PlaceContext::MutatingUse(MutatingUseContext::SetDiscriminant) => {
-                place.is_indirect().then_some(DefUse::Use)
+                if place.is_indirect() { DefUse::Use } else { DefUse::PartialWrite }
             }
 
             // All other contexts are uses...
@@ -188,7 +193,7 @@ impl DefUse {
                 | NonMutatingUseContext::PlaceMention
                 | NonMutatingUseContext::FakeBorrow
                 | NonMutatingUseContext::SharedBorrow,
-            ) => Some(DefUse::Use),
+            ) => DefUse::Use,
 
             PlaceContext::MutatingUse(MutatingUseContext::Projection)
             | PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
diff --git a/compiler/rustc_mir_dataflow/src/impls/mod.rs b/compiler/rustc_mir_dataflow/src/impls/mod.rs
index 3f29b819a6d..6d573e1c00e 100644
--- a/compiler/rustc_mir_dataflow/src/impls/mod.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/mod.rs
@@ -9,7 +9,8 @@ pub use self::initialized::{
     MaybeUninitializedPlaces, MaybeUninitializedPlacesDomain,
 };
 pub use self::liveness::{
-    MaybeLiveLocals, MaybeTransitiveLiveLocals, TransferFunction as LivenessTransferFunction,
+    DefUse, MaybeLiveLocals, MaybeTransitiveLiveLocals,
+    TransferFunction as LivenessTransferFunction,
 };
 pub use self::storage_liveness::{
     MaybeRequiresStorage, MaybeStorageDead, MaybeStorageLive, always_storage_live_locals,
diff --git a/compiler/rustc_mir_dataflow/src/points.rs b/compiler/rustc_mir_dataflow/src/points.rs
index 70d1a34b5fb..e3d1e04a319 100644
--- a/compiler/rustc_mir_dataflow/src/points.rs
+++ b/compiler/rustc_mir_dataflow/src/points.rs
@@ -1,9 +1,5 @@
-use rustc_index::bit_set::DenseBitSet;
-use rustc_index::interval::SparseIntervalMatrix;
 use rustc_index::{Idx, IndexVec};
-use rustc_middle::mir::{self, BasicBlock, Body, Location};
-
-use crate::framework::{Analysis, Results, ResultsVisitor, visit_results};
+use rustc_middle::mir::{BasicBlock, Body, Location};
 
 /// Maps between a `Location` and a `PointIndex` (and vice versa).
 pub struct DenseLocationMap {
@@ -93,65 +89,3 @@ rustc_index::newtype_index! {
     #[debug_format = "PointIndex({})"]
     pub struct PointIndex {}
 }
-
-/// Add points depending on the result of the given dataflow analysis.
-pub fn save_as_intervals<'tcx, N, A>(
-    elements: &DenseLocationMap,
-    body: &mir::Body<'tcx>,
-    mut analysis: A,
-    results: Results<A::Domain>,
-) -> SparseIntervalMatrix<N, PointIndex>
-where
-    N: Idx,
-    A: Analysis<'tcx, Domain = DenseBitSet<N>>,
-{
-    let values = SparseIntervalMatrix::new(elements.num_points());
-    let mut visitor = Visitor { elements, values };
-    visit_results(
-        body,
-        body.basic_blocks.reverse_postorder().iter().copied(),
-        &mut analysis,
-        &results,
-        &mut visitor,
-    );
-    visitor.values
-}
-
-struct Visitor<'a, N: Idx> {
-    elements: &'a DenseLocationMap,
-    values: SparseIntervalMatrix<N, PointIndex>,
-}
-
-impl<'tcx, A, N> ResultsVisitor<'tcx, A> for Visitor<'_, N>
-where
-    A: Analysis<'tcx, Domain = DenseBitSet<N>>,
-    N: Idx,
-{
-    fn visit_after_primary_statement_effect<'mir>(
-        &mut self,
-        _analysis: &mut A,
-        state: &A::Domain,
-        _statement: &'mir mir::Statement<'tcx>,
-        location: Location,
-    ) {
-        let point = self.elements.point_from_location(location);
-        // Use internal iterator manually as it is much more efficient.
-        state.iter().for_each(|node| {
-            self.values.insert(node, point);
-        });
-    }
-
-    fn visit_after_primary_terminator_effect<'mir>(
-        &mut self,
-        _analysis: &mut A,
-        state: &A::Domain,
-        _terminator: &'mir mir::Terminator<'tcx>,
-        location: Location,
-    ) {
-        let point = self.elements.point_from_location(location);
-        // Use internal iterator manually as it is much more efficient.
-        state.iter().for_each(|node| {
-            self.values.insert(node, point);
-        });
-    }
-}