Manually Managed Memory

While core Bril only has simple scalar stack values, the memory extension adds a manually managed heap of array-like allocations. You can create regions, like with malloc in C, and it is the program’s responsibility to delete them, like with free. Programs can manipulate pointers within these regions; a pointer indicates a particular offset within a particular allocated region.

You can read more about the memory extension from its creators, Drew Zagieboylo and Ryan Doenges.

Types

The memory extension adds a parameterized ptr type to Bril:

{"ptr": <Type>}

A pointer value represents a reference to a specific offset within a uniformly-typed region of values.

Operations

These are the operations that manipulate memory allocations:

  • alloc: Create a new memory region. One argument: the number of values to allocate (an integer). The result type is a pointer; the type of the instruction decides the type of the memory region to allocate. For example, this instruction allocates a region of integers:

    {
    "op": "alloc",
    "args": ["size"],
    "dest": "myptr",
    "type": {"ptr": "int"}
}

  • free: Delete an allocation. One argument: a pointer produced by alloc. No return value.

  • store: Write into a memory region. Two arguments: a pointer and a value. The pointer type must agree with the value type (e.g., if the second argument is an int, the first argument must be a ptr<int>). No return value.

  • load: Read from memory. One argument: a pointer. The return type is the pointed-to type for that pointer.

  • ptradd: Adjust the offset for a pointer, producing a new pointer to a different location in the same memory region. Two arguments: a pointer and an offset (an integer, which may be negative). The return type is the same as the original pointer type.

It is an error to access or free a region that has already been freed. It is also an error to access (load or store) a pointer that is out of bounds, i.e., outside the range of valid indices for a given allocation. (Doing a ptradd to produce an out-of-bounds pointer is not an error; subsequently accessing that pointer is.)

Printing

It is not an error to use the core print operation on pointers, but the output is not specified. Implementations can choose to print any representation of the pointer that they deem helpful.