ELF and garbage collection

[Robin Templeton]

The following program works as expected in Guile 2.0:

(do ((i 0 (1+ i)))
    ((= i 100000))
  (compile ''foo #:from 'scheme #:to 'value))

...but when it's evaluated in Guile from the master branch, Guile
crashes with a "too many root sets" error. This is because
`GC_add_roots' is used in loader.c to instruct the garbage collector to
scan the program constants section of an ELF object, and there's a fixed
limit in libgc on the total number of roots.

AFAICT, `GC_add_roots' used because loaded programs aren't
garbage-collected. This makes normal compilation inconvenient when it's
necessary to compile and execute lots of independent single-expression
programs, like with the Elisp and Common Lisp `load' functions.

The easiest solution is to change nothing and require the use of
interpreters for some applications. Guile-Elisp currently relies on a
Tree-IL interpreter to load files; whole-file compilation is not
possible for some programs.

Maybe it's possible to modify the internal representation of ELF
programs and/or extend libgc so that the garbage collector can manage
them. It would be tricky to do this without scanning arbitrary program
text. Another option could be to define a new program representation for
"small" programs that's amenable to garbage collection, at the cost of
complicating the VM somewhat.

What do you think?

Thanks,
Robin
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