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add example of how stack might be managed

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Rett Berg 2019-11-16 12:38:14 -07:00
parent bdcd2b8146
commit b27d3a088d
1 changed files with 86 additions and 0 deletions
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@ -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))
)
```