Rollup merge of #95649 - ouz-a:mir-opt, r=oli-obk

New mir-opt deref_separator

This adds a new mir-opt that split certain derefs into this form:
`let x = (*a.b).c;` to => `tmp = a.b; let x = (*tmp).c;`

Huge thanks to ``@oli-obk`` for his patient mentoring.

compiler/rustc_mir_transform/src/deref_separator.rs

@@ -0,0 +1,72 @@
+use crate::MirPass;
+use rustc_middle::mir::patch::MirPatch;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+pub struct Derefer;
+
+pub fn deref_finder<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+    let mut patch = MirPatch::new(body);
+    let (basic_blocks, local_decl) = body.basic_blocks_and_local_decls_mut();
+    for (block, data) in basic_blocks.iter_enumerated_mut() {
+        for (i, stmt) in data.statements.iter_mut().enumerate() {
+            match stmt.kind {
+                StatementKind::Assign(box (og_place, Rvalue::Ref(region, borrow_knd, place))) => {
+                    for (idx, (p_ref, p_elem)) in place.iter_projections().enumerate() {
+                        if p_elem == ProjectionElem::Deref && !p_ref.projection.is_empty() {
+                            // The type that we are derefing.
+                            let ty = p_ref.ty(local_decl, tcx).ty;
+                            let temp = patch.new_temp(ty, stmt.source_info.span);
+
+                            // Because we are assigning this right before original statement
+                            // we are using index i of statement.
+                            let loc = Location { block: block, statement_index: i };
+                            patch.add_statement(loc, StatementKind::StorageLive(temp));
+
+                            // We are adding current p_ref's projections to our
+                            // temp value.
+                            let deref_place =
+                                Place::from(p_ref.local).project_deeper(p_ref.projection, tcx);
+                            patch.add_assign(
+                                loc,
+                                Place::from(temp),
+                                Rvalue::Use(Operand::Move(deref_place)),
+                            );
+
+                            // We are creating a place by using our temp value's location
+                            // and copying derefed values which we need to create new statement.
+                            let temp_place =
+                                Place::from(temp).project_deeper(&place.projection[idx..], tcx);
+                            let new_stmt = Statement {
+                                source_info: stmt.source_info,
+                                kind: StatementKind::Assign(Box::new((
+                                    og_place,
+                                    Rvalue::Ref(region, borrow_knd, temp_place),
+                                ))),
+                            };
+
+                            // Replace current statement with newly created one.
+                            *stmt = new_stmt;
+
+                            // Since our job with the temp is done it should be gone
+                            let loc = Location { block: block, statement_index: i + 1 };
+                            patch.add_statement(loc, StatementKind::StorageDead(temp));
+
+                            // As all projections are off the base projection, if there are
+                            // multiple derefs in the middle of projection, it might cause
+                            // unsoundness, to not let that happen we break the loop.
+                            break;
+                        }
+                    }
+                }
+                _ => (),
+            }
+        }
+    }
+    patch.apply(body);
+}
+
+impl<'tcx> MirPass<'tcx> for Derefer {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        deref_finder(tcx, body);
+    }
+}

compiler/rustc_mir_transform/src/lib.rs

@@ -53,6 +53,7 @@ mod const_prop_lint;
 mod coverage;
 mod deaggregator;
 mod deduplicate_blocks;
+mod deref_separator;
 mod dest_prop;
 pub mod dump_mir;
 mod early_otherwise_branch;
@@ -431,6 +432,7 @@ fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tc
         // `Deaggregator` is conceptually part of MIR building, some backends rely on it happening
         // and it can help optimizations.
         &deaggregator::Deaggregator,
+        &deref_separator::Derefer,
         &Lint(const_prop_lint::ConstProp),
     ];
 

src/test/mir-opt/derefer_test.main.Derefer.diff

@@ -0,0 +1,60 @@
+- // MIR for `main` before Derefer
++ // MIR for `main` after Derefer
+  
+  fn main() -> () {
+      let mut _0: ();                      // return place in scope 0 at $DIR/derefer_test.rs:2:11: 2:11
+      let mut _1: (i32, i32);              // in scope 0 at $DIR/derefer_test.rs:3:9: 3:14
+      let mut _3: &mut (i32, i32);         // in scope 0 at $DIR/derefer_test.rs:4:22: 4:28
++     let mut _6: &mut (i32, i32);         // in scope 0 at $DIR/derefer_test.rs:5:13: 5:26
++     let mut _7: &mut (i32, i32);         // in scope 0 at $DIR/derefer_test.rs:6:13: 6:26
+      scope 1 {
+          debug a => _1;                   // in scope 1 at $DIR/derefer_test.rs:3:9: 3:14
+          let mut _2: (i32, &mut (i32, i32)); // in scope 1 at $DIR/derefer_test.rs:4:9: 4:14
+          scope 2 {
+              debug b => _2;               // in scope 2 at $DIR/derefer_test.rs:4:9: 4:14
+              let _4: &mut i32;            // in scope 2 at $DIR/derefer_test.rs:5:9: 5:10
+              scope 3 {
+                  debug x => _4;           // in scope 3 at $DIR/derefer_test.rs:5:9: 5:10
+                  let _5: &mut i32;        // in scope 3 at $DIR/derefer_test.rs:6:9: 6:10
+                  scope 4 {
+                      debug y => _5;       // in scope 4 at $DIR/derefer_test.rs:6:9: 6:10
+                  }
+              }
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);                 // scope 0 at $DIR/derefer_test.rs:3:9: 3:14
+          (_1.0: i32) = const 42_i32;      // scope 0 at $DIR/derefer_test.rs:3:17: 3:24
+          (_1.1: i32) = const 43_i32;      // scope 0 at $DIR/derefer_test.rs:3:17: 3:24
+          StorageLive(_2);                 // scope 1 at $DIR/derefer_test.rs:4:9: 4:14
+          StorageLive(_3);                 // scope 1 at $DIR/derefer_test.rs:4:22: 4:28
+          _3 = &mut _1;                    // scope 1 at $DIR/derefer_test.rs:4:22: 4:28
+          (_2.0: i32) = const 99_i32;      // scope 1 at $DIR/derefer_test.rs:4:17: 4:29
+          (_2.1: &mut (i32, i32)) = move _3; // scope 1 at $DIR/derefer_test.rs:4:17: 4:29
+          StorageDead(_3);                 // scope 1 at $DIR/derefer_test.rs:4:28: 4:29
+          StorageLive(_4);                 // scope 2 at $DIR/derefer_test.rs:5:9: 5:10
+-         _4 = &mut ((*(_2.1: &mut (i32, i32))).0: i32); // scope 2 at $DIR/derefer_test.rs:5:13: 5:26
++         StorageLive(_6);                 // scope 2 at $DIR/derefer_test.rs:5:13: 5:26
++         _6 = move (_2.1: &mut (i32, i32)); // scope 2 at $DIR/derefer_test.rs:5:13: 5:26
++         _4 = &mut ((*_6).0: i32);        // scope 2 at $DIR/derefer_test.rs:5:13: 5:26
++         StorageDead(_6);                 // scope 3 at $DIR/derefer_test.rs:6:9: 6:10
+          StorageLive(_5);                 // scope 3 at $DIR/derefer_test.rs:6:9: 6:10
+-         _5 = &mut ((*(_2.1: &mut (i32, i32))).1: i32); // scope 3 at $DIR/derefer_test.rs:6:13: 6:26
++         StorageLive(_7);                 // scope 3 at $DIR/derefer_test.rs:6:13: 6:26
++         _7 = move (_2.1: &mut (i32, i32)); // scope 3 at $DIR/derefer_test.rs:6:13: 6:26
++         _5 = &mut ((*_7).1: i32);        // scope 3 at $DIR/derefer_test.rs:6:13: 6:26
++         StorageDead(_7);                 // scope 0 at $DIR/derefer_test.rs:2:11: 7:2
+          _0 = const ();                   // scope 0 at $DIR/derefer_test.rs:2:11: 7:2
+          StorageDead(_5);                 // scope 3 at $DIR/derefer_test.rs:7:1: 7:2
+          StorageDead(_4);                 // scope 2 at $DIR/derefer_test.rs:7:1: 7:2
+          StorageDead(_2);                 // scope 1 at $DIR/derefer_test.rs:7:1: 7:2
+          StorageDead(_1);                 // scope 0 at $DIR/derefer_test.rs:7:1: 7:2
+          return;                          // scope 0 at $DIR/derefer_test.rs:7:2: 7:2
++     }
++ 
++     bb1 (cleanup): {
++         resume;                          // scope 0 at $DIR/derefer_test.rs:2:1: 7:2
+      }
+  }
+  

src/test/mir-opt/derefer_test.rs

@@ -0,0 +1,7 @@
+// EMIT_MIR derefer_test.main.Derefer.diff
+fn main() {
+    let mut a = (42,43);
+    let mut b = (99, &mut a);
+    let x = &mut (*b.1).0;
+    let y = &mut (*b.1).1;
+}

src/test/mir-opt/inline/issue_58867_inline_as_ref_as_mut.b.Inline.after.mir

@@ -10,6 +10,7 @@ fn b(_1: &mut Box<T>) -> &mut T {
         debug self => _4;                // in scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
         let mut _5: &mut T;              // in scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
         let mut _6: &mut T;              // in scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        let mut _7: std::boxed::Box<T>;  // in scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
     }
 
     bb0: {
@@ -19,7 +20,10 @@ fn b(_1: &mut Box<T>) -> &mut T {
         _4 = &mut (*_1);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:5: 8:15
         StorageLive(_5);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
         StorageLive(_6);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
-        _6 = &mut (*(*_4));              // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        StorageLive(_7);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        _7 = move (*_4);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        _6 = &mut (*_7);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        StorageDead(_7);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
         _5 = &mut (*_6);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
         _3 = &mut (*_5);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
         StorageDead(_6);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL

src/test/mir-opt/inline/issue_58867_inline_as_ref_as_mut.d.Inline.after.mir

@@ -7,13 +7,17 @@ fn d(_1: &Box<T>) -> &T {
     let mut _3: &std::boxed::Box<T>;     // in scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:18:5: 18:15
     scope 1 (inlined <Box<T> as AsRef<T>>::as_ref) { // at $DIR/issue-58867-inline-as-ref-as-mut.rs:18:5: 18:15
         debug self => _3;                // in scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        let mut _4: std::boxed::Box<T>;  // in scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
     }
 
     bb0: {
         StorageLive(_2);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:18:5: 18:15
         StorageLive(_3);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:18:5: 18:15
         _3 = &(*_1);                     // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:18:5: 18:15
-        _2 = &(*(*_3));                  // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        StorageLive(_4);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        _4 = move (*_3);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        _2 = &(*_4);                     // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
+        StorageDead(_4);                 // scope 1 at $SRC_DIR/alloc/src/boxed.rs:LL:COL
         _0 = &(*_2);                     // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:18:5: 18:15
         StorageDead(_3);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:18:14: 18:15
         StorageDead(_2);                 // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:19:1: 19:2

src/test/mir-opt/lower_array_len.array_bound.NormalizeArrayLen.diff

@@ -64,5 +64,9 @@
           StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:11:5: 11:6
           return;                          // scope 0 at $DIR/lower_array_len.rs:12:2: 12:2
       }
+  
+      bb6 (cleanup): {
+          resume;                          // scope 0 at $DIR/lower_array_len.rs:6:1: 12:2
+      }
   }
   

src/test/mir-opt/lower_array_len.array_bound_mut.NormalizeArrayLen.diff

@@ -77,5 +77,9 @@
           StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:24:5: 24:6
           return;                          // scope 0 at $DIR/lower_array_len.rs:25:2: 25:2
       }
+  
+      bb7 (cleanup): {
+          resume;                          // scope 0 at $DIR/lower_array_len.rs:17:1: 25:2
+      }
   }
   

src/test/mir-opt/lower_array_len.array_len.NormalizeArrayLen.diff

@@ -26,5 +26,9 @@
           StorageDead(_2);                 // scope 0 at $DIR/lower_array_len.rs:31:13: 31:14
           return;                          // scope 0 at $DIR/lower_array_len.rs:32:2: 32:2
       }
+  
+      bb2 (cleanup): {
+          resume;                          // scope 0 at $DIR/lower_array_len.rs:30:1: 32:2
+      }
   }
   

src/test/mir-opt/lower_array_len.array_len_by_value.NormalizeArrayLen.diff

@@ -26,5 +26,9 @@
           StorageDead(_2);                 // scope 0 at $DIR/lower_array_len.rs:38:13: 38:14
           return;                          // scope 0 at $DIR/lower_array_len.rs:39:2: 39:2
       }
+  
+      bb2 (cleanup): {
+          resume;                          // scope 0 at $DIR/lower_array_len.rs:37:1: 39:2
+      }
   }
   

src/test/mir-opt/lower_slice_len.bound.LowerSliceLenCalls.diff

@@ -59,5 +59,9 @@
           StorageDead(_3);                 // scope 0 at $DIR/lower_slice_len.rs:9:5: 9:6
           return;                          // scope 0 at $DIR/lower_slice_len.rs:10:2: 10:2
       }
+  
+      bb6 (cleanup): {
+          resume;                          // scope 0 at $DIR/lower_slice_len.rs:4:1: 10:2
+      }
   }
   

src/test/mir-opt/uninhabited_fallthrough_elimination.eliminate_fallthrough.UninhabitedEnumBranching.diff

@@ -9,7 +9,7 @@
       bb0: {
           _2 = discriminant(_1);           // scope 0 at $DIR/uninhabited_fallthrough_elimination.rs:22:11: 22:12
 -         switchInt(move _2) -> [1_isize: bb3, 2_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/uninhabited_fallthrough_elimination.rs:22:5: 22:12
-+         switchInt(move _2) -> [1_isize: bb3, 2_isize: bb2, otherwise: bb5]; // scope 0 at $DIR/uninhabited_fallthrough_elimination.rs:22:5: 22:12
++         switchInt(move _2) -> [1_isize: bb3, 2_isize: bb2, otherwise: bb6]; // scope 0 at $DIR/uninhabited_fallthrough_elimination.rs:22:5: 22:12
       }
   
       bb1: {
@@ -29,9 +29,13 @@
   
       bb4: {
           return;                          // scope 0 at $DIR/uninhabited_fallthrough_elimination.rs:27:2: 27:2
+      }
+  
+      bb5 (cleanup): {
+          resume;                          // scope 0 at $DIR/uninhabited_fallthrough_elimination.rs:21:1: 27:2
 +     }
 + 
-+     bb5: {
++     bb6: {
 +         unreachable;                     // scope 0 at $DIR/uninhabited_fallthrough_elimination.rs:25:14: 25:15
       }
   }

src/test/mir-opt/uninhabited_fallthrough_elimination.keep_fallthrough.UninhabitedEnumBranching.diff

@@ -30,5 +30,9 @@
       bb4: {
           return;                          // scope 0 at $DIR/uninhabited_fallthrough_elimination.rs:18:2: 18:2
       }
+  
+      bb5 (cleanup): {
+          resume;                          // scope 0 at $DIR/uninhabited_fallthrough_elimination.rs:12:1: 18:2
+      }
   }
   

src/test/mir-opt/unreachable.main.UnreachablePropagation.diff

@@ -64,6 +64,10 @@
           _0 = const ();                   // scope 0 at $DIR/unreachable.rs:19:6: 19:6
           StorageDead(_1);                 // scope 0 at $DIR/unreachable.rs:20:1: 20:2
           return;                          // scope 0 at $DIR/unreachable.rs:20:2: 20:2
+-     }
+- 
+-     bb7 (cleanup): {
+-         resume;                          // scope 0 at $DIR/unreachable.rs:8:1: 20:2
       }
   }
   

src/test/mir-opt/unreachable_diverging.main.UnreachablePropagation.diff

@@ -69,6 +69,10 @@
           StorageDead(_1);                 // scope 0 at $DIR/unreachable_diverging.rs:20:1: 20:2
           StorageDead(_2);                 // scope 0 at $DIR/unreachable_diverging.rs:20:1: 20:2
           return;                          // scope 0 at $DIR/unreachable_diverging.rs:20:2: 20:2
+-     }
+- 
+-     bb7 (cleanup): {
+-         resume;                          // scope 0 at $DIR/unreachable_diverging.rs:12:1: 20:2
       }
   }
   

src/test/run-make/const_fn_mir/dump.mir

@@ -23,6 +23,10 @@ fn foo() -> i32 {
         _0 = move (_1.0: i32);           // scope 0 at main.rs:5:5: 5:10
         return;                          // scope 0 at main.rs:6:2: 6:2
     }
+
+    bb2 (cleanup): {
+        resume;                          // scope 0 at main.rs:4:1: 6:2
+    }
 }
 
 fn main() -> () {