Test failure when ContFrame layout is changed

[shirok]

When the order of fields of ScmContFrame is changed, sometimes test/exception.scm SEGVs on "test unwind-protect (restart)". Specifically, if the size field comes after 3rd word, it happens. It doesn't seem to matter which position the size field is allocated within the 3rd word. It's ok as far as size is somewhere in 1st, 2nd or 3rd word, but on or after 4th word it goes bad.

Because it doesn't matter the position of size within the word (e.g. it can be byte 17-20 or byte 21-24), it doesn't seem to be binary-level mixup. Weird.

[shirok]

Seems that ep->cont contains SCM_NIL when things go wrong. It is supposed to be either NULL or ScmContFrame*.

[shirok]

There's an obsolete cont frame remaining in floating ep pointer. After the stack region of the frame is overwritten, save_cont tries to follow the chain there and fails.

[shirok]

The rabbit hole is deeper. It looks like ep->floating chain isn't always cleared. We always had a possibility that ep points to a bogus continuation frame in the stack, but it doesn't surface unless a special condition is met.

I wonder if it is even possible to guarantee ep->floating relevant for all the times. Its reason is to keep continuation frame pointer updated in case the frames are moved to heap. There may be another way to keep track of cont frames.

[shirok]

Closing. Once we find a lightweight continuation creation, this can be revisited to improve performance.