[wasm-reduce] Reduce struct.new arguments away when possible (#7118)HEADmain

If all the fields of a struct.new are defaultable, see if replacing it with a
struct.new_default preserves the behavior, and reduce that way if so.

Also add a missing --closed-world to the --remove-unused-types
invocation. Without that, it was erroring and not working, which I
noticed when testing this. The test also checks that.

2 files changed, 44 insertions, 0 deletions

diff --git a/test/reduce/gc.wast b/test/reduce/gc.wast
new file mode 100644
index 000000000..98d1cd075
--- /dev/null
+++ b/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)
+    )
+  )
+)
+
diff --git a/test/reduce/gc.wast.txt b/test/reduce/gc.wast.txt
new file mode 100644
index 000000000..3af5287ce
--- /dev/null
+++ b/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)
+  )
+ )
+)
+