precise gc of stack vars

shadow-stack.md

###IDEA keep PMC pointers in explicit shadow-stack, a linked list of frames

###IMPLEMENTATION

struct frame
{
    struct frame *prev;
    size_t size;
    PMC **pmcs;
};

• function prolog (add frame to stack):

    PMC *pmcs[42] = { 0 };
    struct frame frame = { interp->last_frame, sizeof pmcs / sizeof *pmcs, pmcs };
    interp->last_frame = &frame;
  

• function epilogue (remove frame from stack):

    interp->last_frame = frame.prev;
  

###PRO

• can be done in standard C
• resembles current approach, but portable

###CONTRA

• awkward: PMC variables could never be named
• not general enough for all use cases

###ALTERNATIVES

• add ref count to PMCs which tracks external references; such a more general approach would make things like easy C++ integration via smart pointers possible