Hi Mark,
Mark H Weaver <m...@netris.org> skribis:
>> commit d851e32fdc3d14804108f0389faa75a57599ced4
>> Author: Andy Wingo <wi...@pobox.com>
>> Date: Fri Sep 23 18:02:05 2011 +0200
>>
>> prevent propagation for memory-dependent operations like `car'
>>
>> * module/language/tree-il/primitives.scm (*primitive-constructors*):
>> Record car, cdr, vector-ref, and struct-ref as "constructors".
>> Comment to come later.
>
> If car, cdr, vector-ref and struct-ref are to be included in this set of
> operations, it seems to me that the set should be renamed to something
> other than "constructors".
>
> Note that peval should not perform (in advance) any operations that
> access _mutable_ memory, but accessing non-mutable memory should be
> fine.
These operations need special care because they return a mutable object,
which must be protected against copy propagation, as in:
(let ((x (cons (list 1) (list 2))))
(set-car! (car x) 0)
(car x))
We discussed this on IRC and failed to come up with a nice name, which
is why Andy kept this one.
> Instead of moving those operations into the *primitive-constructors*
> set, perhaps we should make a new set of primitives called something
> like *primitive-mutable-accessors* ?
Yes, though it’s not the accessor that’s mutable. :-)
‘mutable-object-returning-primitive’ would be descriptive but is hard to
read...
> If you're going to move car and cdr from one set to the other, shouldn't
> you do the same for caar, cadr, etc?
No because they get reduced to a sequence of car & cdr.
> Also, if I'm correct in guessing the reason for this change (accessing
> mutable memory), shouldn't the bytevector-*-ref operations go as well?
No because they return an immutable object.
Thanks,
Ludo’.
