Hi,
Han-Wen Nienhuys <[EMAIL PROTECTED]> writes:
> I don't understand: memoization is only supposed to happen once for
> each piece of code, right? So, the cost of it is not that interesting?
Yes, it's done only once, but if a piece of code hasn't yet been
memoized and is called simultaneously by several threads, then we have a
problem.
> I remember seeing a very scary looking explanation in eval.c about the
> evaluator being unlocked but still thread-safe since the result of memoizing
> was supposed to be confluent (ie. duplicate runs would yield independent
> results.)
>
> /* The Lookup Car Race
> - by Eva Luator
That comment only relates to `scm_lookupcar ()'...
> Is that the only one?
>
> SCM
> scm_m_let (SCM expr, SCM env)
> ...
> /* plain let */
> SCM rvariables;
> SCM inits;
> transform_bindings (bindings, expr, &rvariables, &inits);
>
> {
> const SCM new_body = m_body (SCM_IM_LET, SCM_CDR (cdr_expr));
> const SCM new_tail = scm_cons2 (rvariables, inits, new_body);
> SCM_SETCAR (expr, SCM_IM_LET);
> // ****!!!
> SCM_SETCDR (expr, new_tail);
>
> What happens if another thread tries to evaluate expr at the place marked
> ****!!! ?
You're right, troubles all around!
> At the very least, we should have an atomic SCM_SETCELL() which overwrites
> car and
> cdr atomically.
I'm not sure whether than can be done, since that's two machine words.
At any rate, that wouldn't be sufficient, e.g., to fix `scm_m_letstar ()'.
Thanks,
Ludo'.
