New partly working cell model

compiler/src/regify/mod.rs

@@ -118,10 +118,11 @@ impl RegFn {
             }
         }
 
-        // A define_field into an object that no creating op of this fn gave
-        // (a class field on `this`, a static field on the class) is a write
-        // like any other: a put_field. So a define_field is always a write
-        // that builds an object before it goes anywhere (see infer)
+        // A define_field into an object that no op of this fn made (a class
+        // field on `this`, a static field on the class) is a write like any
+        // other: a put_field. So a define_field is always a write into an
+        // object before it goes anywhere, and its field is never absent (see
+        // `Cell::write()` in infer)
         let mut created = vec![false; next_val];
 
         for instr in &out {
@@ -265,17 +266,10 @@ fn walk_block(
             continue;
         }
 
-        // A field or element write lands back in the object's own value: the
-        // object with the field is the same object. define_field also leaves
-        // it on the stack, put_field and put_array_el do not
-        if matches!(instr.op, OpCode::PutField | OpCode::DefineField | OpCode::PutArrayEl) {
-            let obj = *out_instr.args.last().unwrap();
-
-            if n_push == 1 {
-                stack.push(obj);
-            }
-
-            out_instr.dsts.push(obj);
+        // define_field leaves the object it wrote on the stack: the same value,
+        // not a new one
+        if instr.op == OpCode::DefineField {
+            stack.push(out_instr.args[1]);
             out[instr_idx] = out_instr;
             continue;
         }