`MRB_STACK_EXTEND_DOUBLING`

- If defined doubles the stack size when extending it.

- Otherwise extends stack with `MRB_STACK_GROWTH`.

- Otherwise extends stack with 1.5x growth (minimum `MRB_STACK_GROWTH`).

`MRB_STACK_GROWTH`

- Default value is `128`.

- Used in stack extending.

- Minimum stack growth size when extending.

- Ignored when `MRB_STACK_EXTEND_DOUBLING` is defined.

`MRB_STACK_MAX`

else

size += room;

#else

/* Use linear stack growth.

/* Use 1.5x stack growth.

It is slightly slower than doubling the stack space,

but it saves memory on small devices. */

if (room <= MRB_STACK_GROWTH)

size += MRB_STACK_GROWTH;

else

size += room;

{

size_t newsize = size + (size >> 1); /* 1.5x growth */

if (newsize < size + MRB_STACK_GROWTH)

newsize = size + MRB_STACK_GROWTH;

if (newsize < size + (size_t)room)

newsize = size + room;

size = newsize;

}

#endif

mrb_value *newstack = (mrb_value*)mrb_realloc(mrb, mrb->c->stbase, sizeof(mrb_value) * size);