Auto merge of #108178 - cjgillot:ssa-deref, r=oli-obk

Do not consider `&mut *x` as mutating `x` in `CopyProp`

This PR removes an unfortunate overly cautious case from the current implementation.

Found by #105274 cc `@saethlin`

compiler/rustc_mir_transform/src/ssa.rs

@@ -53,7 +53,7 @@ impl SsaLocals {
         body: &Body<'tcx>,
         borrowed_locals: &BitSet<Local>,
     ) -> SsaLocals {
-        let assignment_order = Vec::new();
+        let assignment_order = Vec::with_capacity(body.local_decls.len());
 
         let assignments = IndexVec::from_elem(Set1::Empty, &body.local_decls);
         let dominators =
@@ -179,37 +179,61 @@ struct SsaVisitor {
     assignment_order: Vec<Local>,
 }
 
+impl SsaVisitor {
+    fn check_assignment_dominates(&mut self, local: Local, loc: Location) {
+        let set = &mut self.assignments[local];
+        let assign_dominates = match *set {
+            Set1::Empty | Set1::Many => false,
+            Set1::One(LocationExtended::Arg) => true,
+            Set1::One(LocationExtended::Plain(assign)) => {
+                assign.successor_within_block().dominates(loc, &self.dominators)
+            }
+        };
+        // We are visiting a use that is not dominated by an assignment.
+        // Either there is a cycle involved, or we are reading for uninitialized local.
+        // Bail out.
+        if !assign_dominates {
+            *set = Set1::Many;
+        }
+    }
+}
+
 impl<'tcx> Visitor<'tcx> for SsaVisitor {
     fn visit_local(&mut self, local: Local, ctxt: PlaceContext, loc: Location) {
         match ctxt {
             PlaceContext::MutatingUse(MutatingUseContext::Store) => {
                 self.assignments[local].insert(LocationExtended::Plain(loc));
-                self.assignment_order.push(local);
+                if let Set1::One(_) = self.assignments[local] {
+                    // Only record if SSA-like, to avoid growing the vector needlessly.
+                    self.assignment_order.push(local);
+                }
             }
             // Anything can happen with raw pointers, so remove them.
             PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
             | PlaceContext::MutatingUse(_) => self.assignments[local] = Set1::Many,
             // Immutable borrows are taken into account in `SsaLocals::new` by
             // removing non-freeze locals.
             PlaceContext::NonMutatingUse(_) => {
-                let set = &mut self.assignments[local];
-                let assign_dominates = match *set {
-                    Set1::Empty | Set1::Many => false,
-                    Set1::One(LocationExtended::Arg) => true,
-                    Set1::One(LocationExtended::Plain(assign)) => {
-                        assign.successor_within_block().dominates(loc, &self.dominators)
-                    }
-                };
-                // We are visiting a use that is not dominated by an assignment.
-                // Either there is a cycle involved, or we are reading for uninitialized local.
-                // Bail out.
-                if !assign_dominates {
-                    *set = Set1::Many;
-                }
+                self.check_assignment_dominates(local, loc);
             }
             PlaceContext::NonUse(_) => {}
         }
     }
+
+    fn visit_place(&mut self, place: &Place<'tcx>, ctxt: PlaceContext, loc: Location) {
+        if place.projection.first() == Some(&PlaceElem::Deref) {
+            // Do not do anything for storage statements and debuginfo.
+            if ctxt.is_use() {
+                // A use through a `deref` only reads from the local, and cannot write to it.
+                let new_ctxt = PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection);
+
+                self.visit_projection(place.as_ref(), new_ctxt, loc);
+                self.check_assignment_dominates(place.local, loc);
+            }
+            return;
+        }
+        self.super_place(place, ctxt, loc);
+    }
 }
 
 #[instrument(level = "trace", skip(ssa, body))]

tests/mir-opt/copy-prop/reborrow.demiraw.CopyProp.diff

@@ -0,0 +1,56 @@
+- // MIR for `demiraw` before CopyProp
++ // MIR for `demiraw` after CopyProp
+
+  fn demiraw(_1: u8) -> () {
+      debug x => _1;                       // in scope 0 at $DIR/reborrow.rs:+0:12: +0:17
+      let mut _0: ();                      // return place in scope 0 at $DIR/reborrow.rs:+0:23: +0:23
+      let _2: *mut u8;                     // in scope 0 at $DIR/reborrow.rs:+1:9: +1:10
+      let mut _4: &mut u8;                 // in scope 0 at $DIR/reborrow.rs:+2:22: +2:29
+      let _6: ();                          // in scope 0 at $DIR/reborrow.rs:+4:5: +4:14
+      let mut _7: *mut u8;                 // in scope 0 at $DIR/reborrow.rs:+4:12: +4:13
+      scope 1 {
+          debug a => _2;                   // in scope 1 at $DIR/reborrow.rs:+1:9: +1:10
+          let _3: &mut u8;                 // in scope 1 at $DIR/reborrow.rs:+2:9: +2:10
+          scope 2 {
+              debug b => _3;               // in scope 2 at $DIR/reborrow.rs:+2:9: +2:10
+              let _5: *mut u8;             // in scope 2 at $DIR/reborrow.rs:+3:9: +3:10
+              scope 4 {
+-                 debug c => _5;           // in scope 4 at $DIR/reborrow.rs:+3:9: +3:10
++                 debug c => _2;           // in scope 4 at $DIR/reborrow.rs:+3:9: +3:10
+              }
+          }
+          scope 3 {
+          }
+      }
+
+      bb0: {
+-         StorageLive(_2);                 // scope 0 at $DIR/reborrow.rs:+1:9: +1:10
+          _2 = &raw mut _1;                // scope 0 at $DIR/reborrow.rs:+1:13: +1:23
+          StorageLive(_3);                 // scope 1 at $DIR/reborrow.rs:+2:9: +2:10
+          StorageLive(_4);                 // scope 1 at $DIR/reborrow.rs:+2:22: +2:29
+          _4 = &mut (*_2);                 // scope 3 at $DIR/reborrow.rs:+2:22: +2:29
+          _3 = &mut (*_4);                 // scope 1 at $DIR/reborrow.rs:+2:22: +2:29
+          StorageDead(_4);                 // scope 1 at $DIR/reborrow.rs:+2:31: +2:32
+-         StorageLive(_5);                 // scope 2 at $DIR/reborrow.rs:+3:9: +3:10
+-         _5 = _2;                         // scope 2 at $DIR/reborrow.rs:+3:13: +3:14
+          StorageLive(_6);                 // scope 4 at $DIR/reborrow.rs:+4:5: +4:14
+-         StorageLive(_7);                 // scope 4 at $DIR/reborrow.rs:+4:12: +4:13
+-         _7 = _5;                         // scope 4 at $DIR/reborrow.rs:+4:12: +4:13
+-         _6 = opaque::<*mut u8>(move _7) -> bb1; // scope 4 at $DIR/reborrow.rs:+4:5: +4:14
++         _6 = opaque::<*mut u8>(_2) -> bb1; // scope 4 at $DIR/reborrow.rs:+4:5: +4:14
+                                           // mir::Constant
+                                           // + span: $DIR/reborrow.rs:38:5: 38:11
+                                           // + literal: Const { ty: fn(*mut u8) {opaque::<*mut u8>}, val: Value(<ZST>) }
+      }
+
+      bb1: {
+-         StorageDead(_7);                 // scope 4 at $DIR/reborrow.rs:+4:13: +4:14
+          StorageDead(_6);                 // scope 4 at $DIR/reborrow.rs:+4:14: +4:15
+          _0 = const ();                   // scope 0 at $DIR/reborrow.rs:+0:23: +5:2
+-         StorageDead(_5);                 // scope 2 at $DIR/reborrow.rs:+5:1: +5:2
+          StorageDead(_3);                 // scope 1 at $DIR/reborrow.rs:+5:1: +5:2
+-         StorageDead(_2);                 // scope 0 at $DIR/reborrow.rs:+5:1: +5:2
+          return;                          // scope 0 at $DIR/reborrow.rs:+5:2: +5:2
+      }
+  }
+

tests/mir-opt/copy-prop/reborrow.miraw.CopyProp.diff

@@ -0,0 +1,52 @@
+- // MIR for `miraw` before CopyProp
++ // MIR for `miraw` after CopyProp
+
+  fn miraw(_1: u8) -> () {
+      debug x => _1;                       // in scope 0 at $DIR/reborrow.rs:+0:10: +0:15
+      let mut _0: ();                      // return place in scope 0 at $DIR/reborrow.rs:+0:21: +0:21
+      let _2: *mut u8;                     // in scope 0 at $DIR/reborrow.rs:+1:9: +1:10
+      let _5: ();                          // in scope 0 at $DIR/reborrow.rs:+4:5: +4:14
+      let mut _6: *mut u8;                 // in scope 0 at $DIR/reborrow.rs:+4:12: +4:13
+      scope 1 {
+          debug a => _2;                   // in scope 1 at $DIR/reborrow.rs:+1:9: +1:10
+          let _3: *mut u8;                 // in scope 1 at $DIR/reborrow.rs:+2:9: +2:10
+          scope 2 {
+              debug b => _3;               // in scope 2 at $DIR/reborrow.rs:+2:9: +2:10
+              let _4: *mut u8;             // in scope 2 at $DIR/reborrow.rs:+3:9: +3:10
+              scope 4 {
+-                 debug c => _4;           // in scope 4 at $DIR/reborrow.rs:+3:9: +3:10
++                 debug c => _2;           // in scope 4 at $DIR/reborrow.rs:+3:9: +3:10
+              }
+          }
+          scope 3 {
+          }
+      }
+
+      bb0: {
+-         StorageLive(_2);                 // scope 0 at $DIR/reborrow.rs:+1:9: +1:10
+          _2 = &raw mut _1;                // scope 0 at $DIR/reborrow.rs:+1:13: +1:23
+          StorageLive(_3);                 // scope 1 at $DIR/reborrow.rs:+2:9: +2:10
+          _3 = &raw mut (*_2);             // scope 3 at $DIR/reborrow.rs:+2:22: +2:33
+-         StorageLive(_4);                 // scope 2 at $DIR/reborrow.rs:+3:9: +3:10
+-         _4 = _2;                         // scope 2 at $DIR/reborrow.rs:+3:13: +3:14
+          StorageLive(_5);                 // scope 4 at $DIR/reborrow.rs:+4:5: +4:14
+-         StorageLive(_6);                 // scope 4 at $DIR/reborrow.rs:+4:12: +4:13
+-         _6 = _4;                         // scope 4 at $DIR/reborrow.rs:+4:12: +4:13
+-         _5 = opaque::<*mut u8>(move _6) -> bb1; // scope 4 at $DIR/reborrow.rs:+4:5: +4:14
++         _5 = opaque::<*mut u8>(_2) -> bb1; // scope 4 at $DIR/reborrow.rs:+4:5: +4:14
+                                           // mir::Constant
+                                           // + span: $DIR/reborrow.rs:30:5: 30:11
+                                           // + literal: Const { ty: fn(*mut u8) {opaque::<*mut u8>}, val: Value(<ZST>) }
+      }
+
+      bb1: {
+-         StorageDead(_6);                 // scope 4 at $DIR/reborrow.rs:+4:13: +4:14
+          StorageDead(_5);                 // scope 4 at $DIR/reborrow.rs:+4:14: +4:15
+          _0 = const ();                   // scope 0 at $DIR/reborrow.rs:+0:21: +5:2
+-         StorageDead(_4);                 // scope 2 at $DIR/reborrow.rs:+5:1: +5:2
+          StorageDead(_3);                 // scope 1 at $DIR/reborrow.rs:+5:1: +5:2
+-         StorageDead(_2);                 // scope 0 at $DIR/reborrow.rs:+5:1: +5:2
+          return;                          // scope 0 at $DIR/reborrow.rs:+5:2: +5:2
+      }
+  }
+

tests/mir-opt/copy-prop/reborrow.remut.CopyProp.diff

@@ -0,0 +1,50 @@
+- // MIR for `remut` before CopyProp
++ // MIR for `remut` after CopyProp
+
+  fn remut(_1: u8) -> () {
+      debug x => _1;                       // in scope 0 at $DIR/reborrow.rs:+0:10: +0:15
+      let mut _0: ();                      // return place in scope 0 at $DIR/reborrow.rs:+0:21: +0:21
+      let _2: &mut u8;                     // in scope 0 at $DIR/reborrow.rs:+1:9: +1:10
+      let _5: ();                          // in scope 0 at $DIR/reborrow.rs:+4:5: +4:14
+      let mut _6: &mut u8;                 // in scope 0 at $DIR/reborrow.rs:+4:12: +4:13
+      scope 1 {
+          debug a => _2;                   // in scope 1 at $DIR/reborrow.rs:+1:9: +1:10
+          let _3: &mut u8;                 // in scope 1 at $DIR/reborrow.rs:+2:9: +2:10
+          scope 2 {
+              debug b => _3;               // in scope 2 at $DIR/reborrow.rs:+2:9: +2:10
+              let _4: &mut u8;             // in scope 2 at $DIR/reborrow.rs:+3:9: +3:10
+              scope 3 {
+-                 debug c => _4;           // in scope 3 at $DIR/reborrow.rs:+3:9: +3:10
++                 debug c => _2;           // in scope 3 at $DIR/reborrow.rs:+3:9: +3:10
+              }
+          }
+      }
+
+      bb0: {
+-         StorageLive(_2);                 // scope 0 at $DIR/reborrow.rs:+1:9: +1:10
+          _2 = &mut _1;                    // scope 0 at $DIR/reborrow.rs:+1:13: +1:19
+          StorageLive(_3);                 // scope 1 at $DIR/reborrow.rs:+2:9: +2:10
+          _3 = &mut (*_2);                 // scope 1 at $DIR/reborrow.rs:+2:13: +2:20
+-         StorageLive(_4);                 // scope 2 at $DIR/reborrow.rs:+3:9: +3:10
+-         _4 = move _2;                    // scope 2 at $DIR/reborrow.rs:+3:13: +3:14
+          StorageLive(_5);                 // scope 3 at $DIR/reborrow.rs:+4:5: +4:14
+-         StorageLive(_6);                 // scope 3 at $DIR/reborrow.rs:+4:12: +4:13
+-         _6 = move _4;                    // scope 3 at $DIR/reborrow.rs:+4:12: +4:13
+-         _5 = opaque::<&mut u8>(move _6) -> bb1; // scope 3 at $DIR/reborrow.rs:+4:5: +4:14
++         _5 = opaque::<&mut u8>(move _2) -> bb1; // scope 3 at $DIR/reborrow.rs:+4:5: +4:14
+                                           // mir::Constant
+                                           // + span: $DIR/reborrow.rs:14:5: 14:11
+                                           // + literal: Const { ty: fn(&mut u8) {opaque::<&mut u8>}, val: Value(<ZST>) }
+      }
+
+      bb1: {
+-         StorageDead(_6);                 // scope 3 at $DIR/reborrow.rs:+4:13: +4:14
+          StorageDead(_5);                 // scope 3 at $DIR/reborrow.rs:+4:14: +4:15
+          _0 = const ();                   // scope 0 at $DIR/reborrow.rs:+0:21: +5:2
+-         StorageDead(_4);                 // scope 2 at $DIR/reborrow.rs:+5:1: +5:2
+          StorageDead(_3);                 // scope 1 at $DIR/reborrow.rs:+5:1: +5:2
+-         StorageDead(_2);                 // scope 0 at $DIR/reborrow.rs:+5:1: +5:2
+          return;                          // scope 0 at $DIR/reborrow.rs:+5:2: +5:2
+      }
+  }
+

tests/mir-opt/copy-prop/reborrow.reraw.CopyProp.diff

@@ -0,0 +1,50 @@
+- // MIR for `reraw` before CopyProp
++ // MIR for `reraw` after CopyProp
+
+  fn reraw(_1: u8) -> () {
+      debug x => _1;                       // in scope 0 at $DIR/reborrow.rs:+0:10: +0:15
+      let mut _0: ();                      // return place in scope 0 at $DIR/reborrow.rs:+0:21: +0:21
+      let _2: &mut u8;                     // in scope 0 at $DIR/reborrow.rs:+1:9: +1:10
+      let _5: ();                          // in scope 0 at $DIR/reborrow.rs:+4:5: +4:14
+      let mut _6: &mut u8;                 // in scope 0 at $DIR/reborrow.rs:+4:12: +4:13
+      scope 1 {
+          debug a => _2;                   // in scope 1 at $DIR/reborrow.rs:+1:9: +1:10
+          let _3: *mut u8;                 // in scope 1 at $DIR/reborrow.rs:+2:9: +2:10
+          scope 2 {
+              debug b => _3;               // in scope 2 at $DIR/reborrow.rs:+2:9: +2:10
+              let _4: &mut u8;             // in scope 2 at $DIR/reborrow.rs:+3:9: +3:10
+              scope 3 {
+-                 debug c => _4;           // in scope 3 at $DIR/reborrow.rs:+3:9: +3:10
++                 debug c => _2;           // in scope 3 at $DIR/reborrow.rs:+3:9: +3:10
+              }
+          }
+      }
+
+      bb0: {
+-         StorageLive(_2);                 // scope 0 at $DIR/reborrow.rs:+1:9: +1:10
+          _2 = &mut _1;                    // scope 0 at $DIR/reborrow.rs:+1:13: +1:19
+          StorageLive(_3);                 // scope 1 at $DIR/reborrow.rs:+2:9: +2:10
+          _3 = &raw mut (*_2);             // scope 1 at $DIR/reborrow.rs:+2:13: +2:24
+-         StorageLive(_4);                 // scope 2 at $DIR/reborrow.rs:+3:9: +3:10
+-         _4 = move _2;                    // scope 2 at $DIR/reborrow.rs:+3:13: +3:14
+          StorageLive(_5);                 // scope 3 at $DIR/reborrow.rs:+4:5: +4:14
+-         StorageLive(_6);                 // scope 3 at $DIR/reborrow.rs:+4:12: +4:13
+-         _6 = move _4;                    // scope 3 at $DIR/reborrow.rs:+4:12: +4:13
+-         _5 = opaque::<&mut u8>(move _6) -> bb1; // scope 3 at $DIR/reborrow.rs:+4:5: +4:14
++         _5 = opaque::<&mut u8>(move _2) -> bb1; // scope 3 at $DIR/reborrow.rs:+4:5: +4:14
+                                           // mir::Constant
+                                           // + span: $DIR/reborrow.rs:22:5: 22:11
+                                           // + literal: Const { ty: fn(&mut u8) {opaque::<&mut u8>}, val: Value(<ZST>) }
+      }
+
+      bb1: {
+-         StorageDead(_6);                 // scope 3 at $DIR/reborrow.rs:+4:13: +4:14
+          StorageDead(_5);                 // scope 3 at $DIR/reborrow.rs:+4:14: +4:15
+          _0 = const ();                   // scope 0 at $DIR/reborrow.rs:+0:21: +5:2
+-         StorageDead(_4);                 // scope 2 at $DIR/reborrow.rs:+5:1: +5:2
+          StorageDead(_3);                 // scope 1 at $DIR/reborrow.rs:+5:1: +5:2
+-         StorageDead(_2);                 // scope 0 at $DIR/reborrow.rs:+5:1: +5:2
+          return;                          // scope 0 at $DIR/reborrow.rs:+5:2: +5:2
+      }
+  }
+

tests/mir-opt/copy-prop/reborrow.rs

@@ -0,0 +1,46 @@
+// Check that CopyProp considers reborrows as not mutating the pointer.
+// unit-test: CopyProp
+
+#![feature(raw_ref_op)]
+
+#[inline(never)]
+fn opaque(_: impl Sized) {}
+
+// EMIT_MIR reborrow.remut.CopyProp.diff
+fn remut(mut x: u8) {
+    let a = &mut x;
+    let b = &mut *a; //< this cannot mutate a.
+    let c = a; //< so `c` and `a` can be merged.
+    opaque(c);
+}
+
+// EMIT_MIR reborrow.reraw.CopyProp.diff
+fn reraw(mut x: u8) {
+    let a = &mut x;
+    let b = &raw mut *a; //< this cannot mutate a.
+    let c = a; //< so `c` and `a` can be merged.
+    opaque(c);
+}
+
+// EMIT_MIR reborrow.miraw.CopyProp.diff
+fn miraw(mut x: u8) {
+    let a = &raw mut x;
+    let b = unsafe { &raw mut *a }; //< this cannot mutate a.
+    let c = a; //< so `c` and `a` can be merged.
+    opaque(c);
+}
+
+// EMIT_MIR reborrow.demiraw.CopyProp.diff
+fn demiraw(mut x: u8) {
+    let a = &raw mut x;
+    let b = unsafe { &mut *a }; //< this cannot mutate a.
+    let c = a; //< so `c` and `a` can be merged.
+    opaque(c);
+}
+
+fn main() {
+    remut(0);
+    reraw(0);
+    miraw(0);
+    demiraw(0);
+}