qimalloc.c

/*
 * QIMalloc - Quick, incremental malloc.
 *
 * This is a dummy malloc implementation, without the option to free memory space.
 */
 
static int __malloc_ptr = 0;
static int __malloc_end = 0;

void *malloc(size_t size) {
  if (__malloc_end == 0) {
    __malloc_end = memsize();
  }

  // FIXME: skip the stack space

  // Increase allocated memory space
  if ((__malloc_ptr + size) > __malloc_end) {
    __malloc_end = sbrk(size);

    if (__malloc_end == -1) {
      __builtin_unreachable();
    }

    __malloc_end += size;
  }
  
  const int ret = __malloc_ptr;

  __malloc_ptr += size;

  return ret;
}

// NOTE: no need to zero out space as WASM memory is pre-set to 0 and we don't reuse blocks
void *calloc(size_t count, size_t size) {
  return malloc(count * size);
}

void free(void *ptr) {
  __builtin_unreachable();
}

void *realloc(void *ptr, size_t size) {
  __builtin_unreachable();
}

sbrk.wast

;;
;; Very basic sbrk implementation in wasm. Written in 2016.
;;

(module
  (memory 1)
  (export "sbrk" $__sbrk)
  (export "memsize" $__memsize)

  (func $__sbrk
    (param $inc i32)
    (result i32)

    (local $end i32)  

    (set_local $end (call $__memsize))
    (if (i32.eq (grow_memory (i32.div_u (get_local $inc) (i32.const 65536))) (i32.const -1))
      (then (i32.const -1))
      (else (get_local $end))
    )
  )

  (func $__memsize
    (result i32)
    (i32.mul (current_memory) (i32.const 65536))
  )
)