add example of how stack might be managed

1 files changed, 86 insertions, 0 deletions

diff --git a/wasm.html.markdown b/wasm.html.markdown
index 98bfc000..92831e69 100644
--- a/wasm.html.markdown
+++ b/wasm.html.markdown
@@ -222,6 +222,92 @@ contributors:
     )
   )
   (export "apply_cos64" (func $apply_cos64))
+
+  ;; Demonstration of how this C code might be written by hand
+  ;;
+  ;;   typedef struct {
+  ;;       int a;
+  ;;       int b;
+  ;;   } sum_struct_t;
+  ;;
+  ;;   sum_struct_t sum_struct_create(int a, int b) {
+  ;;     return (sum_struct_t){a, b};
+  ;;   }
+  ;;
+  ;;   int sum_local() {
+  ;;     sum_struct_t s = sum_struct_create(40, 2);
+  ;;     return s.a + s.b;
+  ;;   }
+  ;;
+  ;; Wasm is a stack-based language, but for returning values more complicated
+  ;; than an int/float, a memory stack has to be manually managed. One ;;
+  ;; approach is to use a mutable global to store the stack_ptr. We give
+  ;; ourselves 1MiB of mem-stack and grow it downwards.
+  ;;
+  ;; Note: we are differentiating from the memstack (stack stored in memory)
+  ;; and the "stack", which wasm implicitly uses to to pass and return values.
+  (global $memstack_ptr (mut i32) (i32.const 65536))
+
+  ;; structs can only be returned by reference
+  (func $sum_struct_create 
+        (param $sum_struct_ptr i32) 
+        (param $var$a i32) 
+        (param $var$b i32)
+    ;; c// sum_struct_ptr->a = a;
+    (i32.store
+      (get_local $sum_struct_ptr)
+      (get_local $var$a)
+    )
+
+    ;; c// sum_struct_ptr->b = b;
+    (i32.store offset=4
+      (get_local $sum_struct_ptr)
+      (get_local $var$b)
+    )
+  )
+
+  (func $sum_local (result i32)
+    (local $var$sum_struct$a i32)
+    (local $var$sum_struct$b i32)
+    (local $local_memstack_ptr i32)
+
+    ;; reserve stack space
+    (i32.sub
+      (get_global $memstack_ptr)
+      (i32.const 8)
+    )
+    tee_local $local_memstack_ptr ;; tee both stores and returns given value
+    set_global $memstack_ptr
+
+    ;; call the function, storing the result in the stack
+    (call $sum_struct_create
+      ((;$sum_struct_ptr=;) get_local $local_memstack_ptr)
+      ((;$var$a=;) i32.const 40)
+      ((;$var$b=;) i32.const 2)
+    )
+
+    ;; retrieve values from struct
+    (set_local $var$sum_struct$a
+      (i32.load offset=0 (get_local $local_memstack_ptr))
+    )
+    (set_local $var$sum_struct$b
+      (i32.load offset=4 (get_local $local_memstack_ptr))
+    )
+
+    ;; unreserve stack space
+    (set_global $memstack_ptr
+        (i32.add
+          (get_local $local_memstack_ptr)
+          (i32.const 8)
+        )
+    )
+
+    (i32.add
+      (get_local $var$sum_struct$a)
+      (get_local $var$sum_struct$b)
+    )
+  )
+  (export "sum_local" (func $sum_local))
 )
 
 ```