GVN: Use a VnIndex in Address projection.

compiler/rustc_middle/src/mir/visit.rs

@@ -1415,6 +1415,23 @@ impl PlaceContext {
         )
     }
 
+    /// Returns `true` if this place context may be used to know the address of the given place.
+    pub fn may_observe_address(self) -> bool {
+        matches!(
+            self,
+            PlaceContext::NonMutatingUse(
+                NonMutatingUseContext::SharedBorrow
+                    | NonMutatingUseContext::RawBorrow
+                    | NonMutatingUseContext::FakeBorrow
+            ) | PlaceContext::MutatingUse(
+                MutatingUseContext::Drop
+                    | MutatingUseContext::Borrow
+                    | MutatingUseContext::RawBorrow
+                    | MutatingUseContext::AsmOutput
+            )
+        )
+    }
+
     /// Returns `true` if this place context represents a storage live or storage dead marker.
     #[inline]
     pub fn is_storage_marker(self) -> bool {

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -6,7 +6,7 @@ use rustc_data_structures::fx::{FxHashMap, FxIndexSet, StdEntry};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_index::IndexVec;
 use rustc_index::bit_set::DenseBitSet;
-use rustc_middle::mir::visit::{MutatingUseContext, PlaceContext, Visitor};
+use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use tracing::debug;
@@ -917,12 +917,7 @@ pub fn excluded_locals(body: &Body<'_>) -> DenseBitSet<Local> {
 
     impl<'tcx> Visitor<'tcx> for Collector {
         fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
-            if (context.is_borrow()
-                || context.is_address_of()
-                || context.is_drop()
-                || context == PlaceContext::MutatingUse(MutatingUseContext::AsmOutput))
-                && !place.is_indirect()
-            {
+            if context.may_observe_address() && !place.is_indirect() {
                 // A pointer to a place could be used to access other places with the same local,
                 // hence we have to exclude the local completely.
                 self.result.insert(place.local);

compiler/rustc_mir_transform/src/ssa.rs

@@ -225,6 +225,9 @@ impl SsaVisitor<'_, '_> {
 
 impl<'tcx> Visitor<'tcx> for SsaVisitor<'_, 'tcx> {
     fn visit_local(&mut self, local: Local, ctxt: PlaceContext, loc: Location) {
+        if ctxt.may_observe_address() {
+            self.borrowed_locals.insert(local);
+        }
         match ctxt {
             PlaceContext::MutatingUse(MutatingUseContext::Projection)
             | PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => bug!(),
@@ -237,7 +240,6 @@ impl<'tcx> Visitor<'tcx> for SsaVisitor<'_, 'tcx> {
             PlaceContext::NonMutatingUse(
                 NonMutatingUseContext::SharedBorrow | NonMutatingUseContext::FakeBorrow,
             ) => {
-                self.borrowed_locals.insert(local);
                 self.check_dominates(local, loc);
                 self.direct_uses[local] += 1;
             }

tests/mir-opt/gvn.dereference_indexing.GVN.panic-abort.diff

@@ -43,7 +43,8 @@
 +         nop;
           StorageLive(_8);
           StorageLive(_9);
-          _9 = copy (*_3);
+-         _9 = copy (*_3);
++         _9 = copy _1[_4];
           _8 = opaque::<u8>(move _9) -> [return: bb2, unwind unreachable];
       }
 

tests/mir-opt/gvn.dereference_indexing.GVN.panic-unwind.diff

@@ -43,7 +43,8 @@
 +         nop;
           StorageLive(_8);
           StorageLive(_9);
-          _9 = copy (*_3);
+-         _9 = copy (*_3);
++         _9 = copy _1[_4];
           _8 = opaque::<u8>(move _9) -> [return: bb2, unwind continue];
       }
 

tests/mir-opt/gvn.rs

@@ -1065,8 +1065,8 @@ fn dereference_indexing(array: [u8; 2], index: usize) {
         &array[i]
     };
 
-    // CHECK-NOT: [{{.*}}]
-    // CHECK: [[tmp:_.*]] = copy (*[[a]]);
+    // CHECK-NOT: StorageDead([[i]]);
+    // CHECK: [[tmp:_.*]] = copy _1[[[i]]];
     // CHECK: opaque::<u8>(move [[tmp]])
     opaque(*a);
 }

tests/mir-opt/gvn_repeat.index_place.GVN.diff

@@ -0,0 +1,15 @@
+- // MIR for `index_place` before GVN
++ // MIR for `index_place` after GVN
+
+  fn index_place(_1: usize, _2: usize, _3: [i32; 5]) -> i32 {
+      let mut _0: i32;
+      let mut _4: &i32;
+
+      bb0: {
+          _4 = &_3[_1];
+          _1 = copy _2;
+          _0 = copy (*_4);
+          return;
+      }
+  }
+

tests/mir-opt/gvn_repeat.repeat_local.GVN.diff

@@ -10,8 +10,7 @@
           _4 = [copy _3; 5];
           _5 = &_4[_1];
           _1 = copy _2;
--         _0 = copy (*_5);
-+         _0 = copy _3;
+          _0 = copy (*_5);
           return;
       }
   }

tests/mir-opt/gvn_repeat.rs

@@ -6,6 +6,23 @@
 
 use std::intrinsics::mir::*;
 
+// EMIT_MIR gvn_repeat.index_place.GVN.diff
+#[custom_mir(dialect = "runtime")]
+pub fn index_place(mut idx1: usize, idx2: usize, array: [i32; 5]) -> i32 {
+    // CHECK-LABEL: fn index_place(
+    // CHECK: let mut [[ELEM:.*]]: &i32;
+    // CHECK: _0 = copy (*[[ELEM]])
+    mir! {
+        let elem;
+        {
+            elem = &array[idx1];
+            idx1 = idx2;
+            RET = *elem;
+            Return()
+        }
+    }
+}
+
 // EMIT_MIR gvn_repeat.repeat_place.GVN.diff
 #[custom_mir(dialect = "runtime")]
 pub fn repeat_place(mut idx1: usize, idx2: usize, val: &i32) -> i32 {
@@ -29,7 +46,8 @@ pub fn repeat_place(mut idx1: usize, idx2: usize, val: &i32) -> i32 {
 #[custom_mir(dialect = "runtime")]
 pub fn repeat_local(mut idx1: usize, idx2: usize, val: i32) -> i32 {
     // CHECK-LABEL: fn repeat_local(
-    // CHECK: _0 = copy _3
+    // CHECK: let mut [[ELEM:.*]]: &i32;
+    // CHECK: _0 = copy (*[[ELEM]]);
     mir! {
         let array;
         let elem;
@@ -44,6 +62,7 @@ pub fn repeat_local(mut idx1: usize, idx2: usize, val: i32) -> i32 {
 }
 
 fn main() {
+    assert_eq!(index_place(0, 5, [0; 5]), 0);
     assert_eq!(repeat_place(0, 5, &0), 0);
     assert_eq!(repeat_local(0, 5, 0), 0);
 }