Simplify operands in unions.

Author: cjgillot

compiler/rustc_mir_transform/src/gvn.rs

@@ -864,6 +864,12 @@ 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());
@@ -883,12 +889,6 @@ 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);