Simplify operands in unions.

Author: cjgillot

compiler/rustc_mir_transform/src/gvn.rs

@@ -865,6 +865,12 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
         }
 
+        let fields: Option<Vec<_>> = fields
+            .iter_mut()
+            .map(|op| self.simplify_operand(op, location).or_else(|| self.new_opaque()))
+            .collect();
+        let fields = fields?;
+
         let (ty, variant_index) = match *kind {
             AggregateKind::Array(..) => {
                 assert!(!fields.is_empty());
@@ -884,12 +890,6 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             AggregateKind::Adt(_, _, _, _, Some(_)) => return None,
         };
 
-        let fields: Option<Vec<_>> = fields
-            .iter_mut()
-            .map(|op| self.simplify_operand(op, location).or_else(|| self.new_opaque()))
-            .collect();
-        let fields = fields?;
-
         if let AggregateTy::Array = ty
             && fields.len() > 4
         {

tests/mir-opt/const_prop/transmute.undef_union_as_integer.GVN.32bit.diff

@@ -12,8 +12,9 @@
           StorageLive(_1);
           StorageLive(_2);
 -         _2 = ();
+-         _1 = Union32 { value: move _2 };
 +         _2 = const ();
-          _1 = Union32 { value: move _2 };
++         _1 = Union32 { value: const () };
           StorageDead(_2);
           _0 = move _1 as u32 (Transmute);
           StorageDead(_1);

tests/mir-opt/const_prop/transmute.undef_union_as_integer.GVN.64bit.diff

@@ -12,8 +12,9 @@
           StorageLive(_1);
           StorageLive(_2);
 -         _2 = ();
+-         _1 = Union32 { value: move _2 };
 +         _2 = const ();
-          _1 = Union32 { value: move _2 };
++         _1 = Union32 { value: const () };
           StorageDead(_2);
           _0 = move _1 as u32 (Transmute);
           StorageDead(_1);

tests/mir-opt/dest-prop/union.main.DestinationPropagation.panic-abort.diff

@@ -5,27 +5,23 @@
       let mut _0: ();
       let _1: main::Un;
       let mut _2: u32;
-      let mut _3: u32;
       scope 1 {
           debug un => _1;
           scope 2 {
           }
           scope 4 (inlined std::mem::drop::<u32>) {
-              debug _x => _3;
+              debug _x => _2;
           }
       }
       scope 3 (inlined val) {
       }
 
       bb0: {
           StorageLive(_1);
-          StorageLive(_2);
-          _2 = const 1_u32;
           _1 = Un { us: const 1_u32 };
+          StorageLive(_2);
+          _2 = (_1.0: u32);
           StorageDead(_2);
-          StorageLive(_3);
-          _3 = (_1.0: u32);
-          StorageDead(_3);
           StorageDead(_1);
           return;
       }

tests/mir-opt/dest-prop/union.main.DestinationPropagation.panic-unwind.diff

@@ -5,27 +5,23 @@
       let mut _0: ();
       let _1: main::Un;
       let mut _2: u32;
-      let mut _3: u32;
       scope 1 {
           debug un => _1;
           scope 2 {
           }
           scope 4 (inlined std::mem::drop::<u32>) {
-              debug _x => _3;
+              debug _x => _2;
           }
       }
       scope 3 (inlined val) {
       }
 
       bb0: {
           StorageLive(_1);
-          StorageLive(_2);
-          _2 = const 1_u32;
           _1 = Un { us: const 1_u32 };
+          StorageLive(_2);
+          _2 = (_1.0: u32);
           StorageDead(_2);
-          StorageLive(_3);
-          _3 = (_1.0: u32);
-          StorageDead(_3);
           StorageDead(_1);
           return;
       }