[wasm-reduce] Reduce struct.new arguments away when possible

src/tools/wasm-reduce.cpp

@@ -296,7 +296,7 @@ struct Reducer
       "--simplify-globals",
       "--simplify-locals --vacuum",
       "--strip",
-      "--remove-unused-types",
+      "--remove-unused-types --closed-world",
       "--vacuum"};
     auto oldSize = file_size(working);
     bool more = true;
@@ -609,6 +609,25 @@ struct Reducer
         // children (which would recreate the current state).
         return;
       }
+    } else if (auto* structNew = curr->dynCast<StructNew>()) {
+      // If all the fields are defaultable, try to replace this with a
+      // struct.new_with_default.
+      if (!structNew->isWithDefault() && structNew->type != Type::unreachable) {
+        auto& fields = structNew->type.getHeapType().getStruct().fields;
+        if (std::all_of(fields.begin(), fields.end(), [&](auto& field) {
+              return field.type.isDefaultable();
+            })) {
+          ExpressionList operands(getModule()->allocator);
+          operands.swap(structNew->operands);
+          assert(structNew->isWithDefault());
+          if (tryToReplaceCurrent(structNew)) {
+            return;
+          } else {
+            structNew->operands.swap(operands);
+            assert(!structNew->isWithDefault());
+          }
+        }
+      }
     }
     // Finally, try to replace with a child.
     for (auto* child : ChildIterator(curr)) {

test/reduce/gc.wast

@@ -0,0 +1,28 @@
+(module
+  (rec
+    (type $A (struct (field (mut i32)) (field funcref)))
+    ;; This type can be optimized away.
+    (type $unused (struct))
+  )
+
+  (global $A (ref null $A) (struct.new $A
+    ;; These particular values are not used, and can be removed, leaving the
+    ;; struct.new as struct.new_default.
+    (i32.const 0)
+    (ref.func $use-global)
+  ))
+
+  (func $use-global (export "use-global") (result i32)
+    ;; This function stores 42 in the global struct, then reads and returns
+    ;; that. We don't manage to optimize away anything in this function, which
+    ;; only serves to keep alive the type and the global for the above testing.
+    (struct.set $A 0
+      (global.get $A)
+      (i32.const 42)
+    )
+    (struct.get $A 0
+      (global.get $A)
+    )
+  )
+)
+

test/reduce/gc.wast.txt

@@ -0,0 +1,16 @@
+(module
+ (type $0 (struct (field (mut i32)) (field funcref)))
+ (type $1 (func (result i32)))
+ (global $global$0 (ref null $0) (struct.new_default $0))
+ (export "use-global" (func $0))
+ (func $0 (result i32)
+  (struct.set $0 0
+   (global.get $global$0)
+   (i32.const 42)
+  )
+  (struct.get $0 0
+   (global.get $global$0)
+  )
+ )
+)
+