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A Safer Scheme Interpreter, Part 2

Posted by kernelbob on January 3, 2011

In my last post, I promised to explain how my Scheme interpreter, Schetoo, can automatically verify that its instructions are restartable.  But first, some background.

In the last post, I proposed Rules 1, 2 and 2a.

Rule 1.  Any instruction of the interpreter may be abandoned and restarted.

Rule 2.  Do everything that might fail before doing anything with side effects.

Rule 2a.  “Side effects” only means memory that was already reachable before the current instruction (and I/O).

Schetoo allocates memory sequentially.  The pointer next_alloc points to the beginning of free memory to allocate.  Here’s the allocator, somewhat simplified.

void *next_alloc, *alloc_end;

heap_object_t *mem_alloc_obj(size_t alloc_size)
    if (next_alloc + alloc_size > alloc_end)
        send_heap_full();      /* calls longjmp() */
    heap_object_t *fresh_object = next_alloc;
    next_alloc += alloc_size;
    return fresh_object;

That means that memory allocated by the current instruction is contiguous.  Before each instruction starts, the interpreter calls a macro called COMMIT(), which saves a copy of next_alloc in a variable called committed.  Any object whose address is between committed and next_alloc must have been freshly allocated in the current instruction.  As the name implies, COMMIT() commits the freshly created objects after the instruction successfully completes.

When memory allocation fails, we raise an “exception”.  C doesn’t have real exceptions, but the setjmp/longjmp facility is good enough.

An instruction has two phases: Can Fail and Can’t Fail.  In the former, the instruction can check arguments and allocate memory, and any of those operations can raise an exception that fails the instruction.  As soon as the instruction has a side effect, it moves into Can’t Fail phase.

Schetoo has a global flag, retry_okay, that records which phase the current instruction is in.  (The name is a little misleading.  Think of it as failure_okay, since some failures don’t lead to retry.)

Side effects occur in a small number of places.  Each of those places sets the current phase to Can’t Fail and records the current function and line number.  This is done through a macro, SIDE_EFFECT().

Object mutations use a higher-level macro, MUTATE(obj)MUTATE calls SIDE_EFFECT, but only if the object is committed.

Failures can occur in an even smaller number of places.  One is the allocator.  Another is a macro called CHECK that is used for all argument checking.  Those call a macro called COULD_RETRY()COULD_RETRY verifies that the instruction is in Can Fail phase, and if it isn’t, it raises an assertion error immediately, with the function and line number where the most recent side effect occurred. (Once again, the name isn’t right.  It should be called COULD_FAIL, I think.)

All of these checks happen in a debug build.  In a production build, the four macros, COMMIT, SIDE_EFFECT, MUTATE, and COULD_RETRY, are all redefined to do nothing, and no checking is done.  So there is no overhead at all in a production build.

I’ve mentioned that exceptions use longjmp().  In my next post, I’ll explain how the interpreter’s main loop catches them.

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One Response to “A Safer Scheme Interpreter, Part 2”

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