Avoid recording no-op replacements.

Author: cjgillot

compiler/rustc_mir_transform/src/gvn.rs

@@ -637,6 +637,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
         if place.is_indirect()
             && let Some(base) = self.locals[place.local]
             && let Some(new_local) = self.try_as_local(base, location)
+            && place.local != new_local
         {
             place.local = new_local;
             self.reused_locals.insert(new_local);
@@ -646,18 +647,20 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
 
         for i in 0..projection.len() {
             let elem = projection[i];
-            if let ProjectionElem::Index(idx) = elem
-                && let Some(idx) = self.locals[idx]
+            if let ProjectionElem::Index(idx_local) = elem
+                && let Some(idx) = self.locals[idx_local]
             {
                 if let Some(offset) = self.evaluated[idx].as_ref()
                     && let Ok(offset) = self.ecx.read_target_usize(offset)
                     && let Some(min_length) = offset.checked_add(1)
                 {
                     projection.to_mut()[i] =
                         ProjectionElem::ConstantIndex { offset, min_length, from_end: false };
-                } else if let Some(new_idx) = self.try_as_local(idx, location) {
-                    projection.to_mut()[i] = ProjectionElem::Index(new_idx);
-                    self.reused_locals.insert(new_idx);
+                } else if let Some(new_idx_local) = self.try_as_local(idx, location)
+                    && idx_local != new_idx_local
+                {
+                    projection.to_mut()[i] = ProjectionElem::Index(new_idx_local);
+                    self.reused_locals.insert(new_idx_local);
                 }
             }
         }

tests/mir-opt/const_allocation.main.GVN.after.32bit.mir

@@ -7,10 +7,10 @@ fn main() -> () {
 
     bb0: {
         StorageLive(_1);
-        nop;
+        StorageLive(_2);
         _2 = const {ALLOC9: &&[(Option<i32>, &[&str])]};
         _1 = (*_2);
-        nop;
+        StorageDead(_2);
         StorageDead(_1);
         _0 = const ();
         return;

tests/mir-opt/const_allocation.main.GVN.after.64bit.mir

@@ -7,10 +7,10 @@ fn main() -> () {
 
     bb0: {
         StorageLive(_1);
-        nop;
+        StorageLive(_2);
         _2 = const {ALLOC9: &&[(Option<i32>, &[&str])]};
         _1 = (*_2);
-        nop;
+        StorageDead(_2);
         StorageDead(_1);
         _0 = const ();
         return;

tests/mir-opt/const_allocation2.main.GVN.after.32bit.mir

@@ -7,10 +7,10 @@ fn main() -> () {
 
     bb0: {
         StorageLive(_1);
-        nop;
+        StorageLive(_2);
         _2 = const {ALLOC9: &&[(Option<i32>, &[&u8])]};
         _1 = (*_2);
-        nop;
+        StorageDead(_2);
         StorageDead(_1);
         _0 = const ();
         return;

tests/mir-opt/const_allocation2.main.GVN.after.64bit.mir

@@ -7,10 +7,10 @@ fn main() -> () {
 
     bb0: {
         StorageLive(_1);
-        nop;
+        StorageLive(_2);
         _2 = const {ALLOC9: &&[(Option<i32>, &[&u8])]};
         _1 = (*_2);
-        nop;
+        StorageDead(_2);
         StorageDead(_1);
         _0 = const ();
         return;

tests/mir-opt/const_allocation3.main.GVN.after.32bit.mir

@@ -7,10 +7,10 @@ fn main() -> () {
 
     bb0: {
         StorageLive(_1);
-        nop;
+        StorageLive(_2);
         _2 = const {ALLOC4: &&Packed};
         _1 = (*_2);
-        nop;
+        StorageDead(_2);
         StorageDead(_1);
         _0 = const ();
         return;

tests/mir-opt/const_allocation3.main.GVN.after.64bit.mir

@@ -7,10 +7,10 @@ fn main() -> () {
 
     bb0: {
         StorageLive(_1);
-        nop;
+        StorageLive(_2);
         _2 = const {ALLOC2: &&Packed};
         _1 = (*_2);
-        nop;
+        StorageDead(_2);
         StorageDead(_1);
         _0 = const ();
         return;

tests/mir-opt/const_prop/address_of_pair.fn0.GVN.diff

@@ -24,9 +24,8 @@
       bb0: {
           StorageLive(_2);
 -         _2 = (const 1_i32, const false);
--         StorageLive(_3);
 +         _2 = const (1_i32, false);
-+         nop;
+          StorageLive(_3);
           _3 = &raw mut (_2.1: bool);
 -         _2 = (const 1_i32, const false);
 +         _2 = const (1_i32, false);
@@ -42,9 +41,8 @@
           StorageDead(_6);
           _0 = _5;
 -         StorageDead(_5);
--         StorageDead(_3);
-+         nop;
 +         nop;
+          StorageDead(_3);
           StorageDead(_2);
           return;
       }

tests/mir-opt/const_prop/bad_op_unsafe_oob_for_slices.main.GVN.32bit.panic-abort.diff

@@ -22,18 +22,15 @@
       }
 
       bb0: {
--         StorageLive(_1);
-+         nop;
+          StorageLive(_1);
           StorageLive(_2);
--         StorageLive(_3);
-+         nop;
+          StorageLive(_3);
           _9 = const _;
           _3 = &(*_9);
           _2 = &raw const (*_3);
           _1 = move _2 as *const [i32] (PointerCoercion(Unsize));
           StorageDead(_2);
--         StorageDead(_3);
-+         nop;
+          StorageDead(_3);
           StorageLive(_5);
           StorageLive(_6);
           _6 = const 3_usize;
@@ -50,8 +47,7 @@
           StorageDead(_6);
           _0 = const ();
           StorageDead(_5);
--         StorageDead(_1);
-+         nop;
+          StorageDead(_1);
           return;
       }
   }

tests/mir-opt/const_prop/bad_op_unsafe_oob_for_slices.main.GVN.32bit.panic-unwind.diff

@@ -22,18 +22,15 @@
       }
 
       bb0: {
--         StorageLive(_1);
-+         nop;
+          StorageLive(_1);
           StorageLive(_2);
--         StorageLive(_3);
-+         nop;
+          StorageLive(_3);
           _9 = const _;
           _3 = &(*_9);
           _2 = &raw const (*_3);
           _1 = move _2 as *const [i32] (PointerCoercion(Unsize));
           StorageDead(_2);
--         StorageDead(_3);
-+         nop;
+          StorageDead(_3);
           StorageLive(_5);
           StorageLive(_6);
           _6 = const 3_usize;
@@ -50,8 +47,7 @@
           StorageDead(_6);
           _0 = const ();
           StorageDead(_5);
--         StorageDead(_1);
-+         nop;
+          StorageDead(_1);
           return;
       }
   }