Add return_call example for WASM (#2693)

* Add return_call example for WASM

* Apply suggestions from code review

Co-authored-by: Yury Delendik <ydelendik@mozilla.com>

* Update return_call.js

* Add comments

* Add the demo to the meta file

---------

Co-authored-by: Yury Delendik <ydelendik@mozilla.com>
Co-authored-by: Brian Thomas Smith <brian@smith.berlin>

live-examples/wat-examples/statements/call.wat

@@ -1,10 +1,12 @@
 (module
+  ;; Import the `greet` function from the environment
   (import "env" "greet" (func $greet))
 
   (func
-    ;; call the greet function
+    ;; Call the imported `greet` function
     call $greet
   )
 
-  (start 1) ;; run the first function automatically
+  ;; Automatically run the first function when the module starts
+  (start 1)
 )

live-examples/wat-examples/statements/meta.json

@@ -63,6 +63,13 @@
       "title": "Wat Demo: return",
       "type": "wat"
     },
+    "return_call": {
+      "watExampleCode": "./live-examples/wat-examples/statements/return_call.wat",
+      "jsExampleCode": "./live-examples/wat-examples/statements/return_call.js",
+      "fileName": "return_call.html",
+      "title": "Wat Demo: return_call",
+      "type": "wat"
+    },
     "select": {
       "watExampleCode": "./live-examples/wat-examples/statements/select.wat",
       "jsExampleCode": "./live-examples/wat-examples/statements/select.js",

live-examples/wat-examples/statements/return_call.js

@@ -0,0 +1,6 @@
+const url = '{%wasm-url%}';
+const { instance } = await WebAssembly.instantiateStreaming(fetch(url));
+const result = instance.exports.fac(5n);
+
+console.log(result);
+// Expected output: 120n

live-examples/wat-examples/statements/return_call.wat

@@ -0,0 +1,22 @@
+(module
+  ;; Calculate the factorial of a number
+  (func $fac (export "fac") (param $x i64) (result i64)
+    ;; Call the `fac-aux` function with $x and 1 parameters
+    (return_call $fac-aux (local.get $x) (i64.const 1))
+  )
+
+  ;; Perform the factorial calculation
+  (func $fac-aux (param $x i64) (param $r i64) (result i64)
+    ;; If $x is zero, return the accumulated result $r
+    (if (result i64) (i64.eqz (local.get $x))
+      (then (return (local.get $r)))
+      (else
+        ;; Otherwise, recursively call `fac-aux` with $x-1 and $x*$r
+        (return_call $fac-aux
+          (i64.sub (local.get $x) (i64.const 1))
+          (i64.mul (local.get $x) (local.get $r))
+        )
+      )
+    )
+  )
+)