Re: [PATCH] gcc: pass-manager: Fix memory leak. [PR jit/63854]

["Marc Nieper-Wißkirchen" <marc@nieper-wisskirchen.de>]

Am Sa., 15. Jan. 2022 um 14:56 Uhr schrieb Marc Nieper-Wißkirchen
<marc@nieper-wisskirchen.de>:
>
> Jeff, David, do you need any more input from my side?
>
> -- Marc
>
> Am Sa., 8. Jan. 2022 um 17:32 Uhr schrieb Jeff Law <jeffreyalaw@gmail.com>:
> >
> >
> >
> > On 1/6/2022 6:53 AM, David Malcolm via Gcc-patches wrote:
> > > On Sun, 2021-12-19 at 22:30 +0100, Marc Nieper-Wißkirchen wrote:
> > >> This patch fixes a memory leak in the pass manager. In the existing
> > >> code,
> > >> the m_name_to_pass_map is allocated in
> > >> pass_manager::register_pass_name, but
> > >> never deallocated.  This is fixed by adding a deletion in
> > >> pass_manager::~pass_manager.  Moreover the string keys in
> > >> m_name_to_pass_map are
> > >> all dynamically allocated.  To free them, this patch adds a new hash
> > >> trait for
> > >> string hashes that are to be freed when the corresponding hash entry
> > >> is removed.
> > >>
> > >> This fix is particularly relevant for using GCC as a library through
> > >> libgccjit.
> > >> The memory leak also occurs when libgccjit is instructed to use an
> > >> external
> > >> driver.
> > >>
> > >> Before the patch, compiling the hello world example of libgccjit with
> > >> the external driver under Valgrind shows a loss of 12,611 (48 direct)
> > >> bytes.  After the patch, no memory leaks are reported anymore.
> > >> (Memory leaks occurring when using the internal driver are mostly in
> > >> the driver code in gcc/gcc.c and have to be fixed separately.)
> > >>
> > >> The patch has been tested by fully bootstrapping the compiler with
> > >> the
> > >> frontends C, C++, Fortran, LTO, ObjC, JIT and running the test suite
> > >> under a x86_64-pc-linux-gnu host.
> > > Thanks for the patch.
> > >
> > > It looks correct to me, given that pass_manager::register_pass_name
> > > does an xstrdup and puts the result in the map.
> > >
> > > That said:
> > > - I'm not officially a reviewer for this part of gcc (though I probably
> > > touched this code last)
> > > - is it cleaner to instead change m_name_to_pass_map's key type from
> > > const char * to char *, to convey that the map "owns" the name?  That
> > > way we probably wouldn't need struct typed_const_free_remove, and (I
> > > hope) works better with the type system.
> > >
> > > Dave
> > >
> > >> gcc/ChangeLog:
> > >>
> > >>          PR jit/63854
> > >>          * hash-traits.h (struct typed_const_free_remove): New.
> > >>          (struct free_string_hash): New.
> > >>          * pass_manager.h: Use free_string_hash.
> > >>          * passes.c (pass_manager::register_pass_name): Use
> > >> free_string_hash.
> > >>          (pass_manager::~pass_manager): Delete allocated
> > >> m_name_to_pass_map.
> > My concern (and what I hadn't had time to dig into) was we initially
> > used nofree_string_hash -- I wanted to make sure there wasn't any path
> > where the name came from the stack (can't be free'd), was saved
> > elsewhere (danging pointer) and the like.  ie, why were we using
> > nofree_string_hash to begin with?  I've never really mucked around with
> > these bits, so the analysis side kept falling off the daily todo list.

The only occurrences of m_name_to_pass_map are in pass-manager.h
(where it is defined as a private field of the class pass_manager) and
in passes.cc. There is just one instance where a name is added to the
map in passes.cc, namely through the put method. There, the name has
been xstrdup'ed.

The name (as a const char *) escapes the pass map in
pass_manager::create_pass_tab through the call to
m_name_pass_map->traverse. This inserts the name into the pass_tab,
which is a static vec of const char *s. The pass_tab does not escape
the translation unit of passes.c. It is used in dump_one_pass where
the name is used as an argument to fprintf. The important point is
that it is not freed and not further copied.

> >
> > If/once you're comfortable with the patch David, then go ahead and apply
> > it on Marc's behalf.
> >
> > jeff
> >