On Sun, 7 May 2017, tbsaunde+...@tbsaunde.org wrote:
This is a start at warning about various resource allocation issues that can be
improved. Currently it only warns about functions that call malloc and then
always pass the resulting pointer to free().
It should be pretty simple to extend this to new/delete and new[]/delete[], as
well as checking that in simple cases the pairing is correct. However it
wasn't obvious to me how to tell if a function call is to a allocating operator
new. We probably don't want to warn about placement new since complicated
things can be happening there. There is the DECL_IS_OPERATOR_NEW() but that
doesn't seem to cover class specific operator new overloads, and maybe even
custom ones at global scope?
Other things that may be reasonable to try and include would be open / close
and locks.
It might also be good to warn about functions that take or return unique
ownership of resources, but I'm not quiet sure how to handle functions that
allocate or deallocate shared resources.
Interesting.
1) How do you avoid warning for code that already uses unique_ptr? After
inlining, it will look just like code calling new, and delete on all
paths. Well, currently you only handle malloc/free, but if you provide an
inline implementation of new/delete that forwards to malloc/free, the
issue should already appear. Or maybe the pass is very early? But then the
compiler will likely seldom notice that the argument to free is actually
the return of malloc.
2) In some sense, we would want to warn when some path is missing the call
to free, but it seems very hard to distinguish the cases where it is
accidentally missing from those where it is done on purpose. Maybe detect
cases where the only paths missing free are through exceptions?
3) This seems very similar to the conditions needed to transform a call to
malloc into a stack allocation. I was experimenting with this some years
ago, see for instance
https://gcc.gnu.org/ml/gcc-patches/2013-11/msg01049.html (it is probably
not the most recent message on the subject, since the main code moved to
CCP afterwards
http://geometrica.saclay.inria.fr/team/Marc.Glisse/tmp/heapstack, but I
can't locate more recent messages at the moment, and it was more a
prototype than a ready patch). Do you think the infrastructure could be
shared between your warning and such an optimization? Or are the goals
likely to diverge too soon for sharing to be practical?
--
Marc Glisse
