[PING^2] Re: Fix 'hash_table::expand' to destruct stale Value objects
Thomas Schwinge
thomas@codesourcery.com
Fri Sep 17 11:17:45 GMT 2021
Hi!
On 2021-09-10T10:00:25+0200, I wrote:
> On 2021-09-01T19:31:19-0600, Martin Sebor via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>> On 8/30/21 4:46 AM, Thomas Schwinge wrote:
>>> Ping -- we still need to plug the memory leak; see patch attached, and/or
>>> long discussion here:
>>
>> Thanks for answering my questions. I have no concerns with going
>> forward with the patch as is.
>
> Thanks, Martin. Ping for formal approval (and review for using proper
> C++ terminology in the 'gcc/hash-table.h:hash_table::expand' source code
> comment that I'm adding). Patch again attached, for easy reference.
Ping, once again.
Grüße
Thomas
>> Just a suggestion/request: unless
>> this patch fixes all the outstanding problems you know of or suspect
>> in this area (leaks/missing dtor calls) and unless you plan to work
>> on those in the near future, please open a bug for them with a brain
>> dump of what you learned. That should save us time when the day
>> comes to tackle those.
>
> ACK. I'm not aware of any additional known problems. (In our email
> discussion, we did have some "vague ideas" for opportunities of
> clarification/clean-up, but these aren't worth filing PRs for; needs
> someone to gain understanding, taking a look.)
>
>
> Grüße
> Thomas
>
>
>>> On 2021-08-16T14:10:00-0600, Martin Sebor <msebor@gmail.com> wrote:
>>>> On 8/16/21 6:44 AM, Thomas Schwinge wrote:
>>>>> On 2021-08-12T17:15:44-0600, Martin Sebor via Gcc <gcc@gcc.gnu.org> wrote:
>>>>>> On 8/6/21 10:57 AM, Thomas Schwinge wrote:
>>>>>>> So I'm trying to do some C++... ;-)
>>>>>>>
>>>>>>> Given:
>>>>>>>
>>>>>>> /* A map from SSA names or var decls to record fields. */
>>>>>>> typedef hash_map<tree, tree> field_map_t;
>>>>>>>
>>>>>>> /* For each propagation record type, this is a map from SSA names or var decls
>>>>>>> to propagate, to the field in the record type that should be used for
>>>>>>> transmission and reception. */
>>>>>>> typedef hash_map<tree, field_map_t> record_field_map_t;
>>>>>>>
>>>>>>> Thus, that's a 'hash_map<tree, hash_map<tree, tree>>'. (I may do that,
>>>>>>> right?) Looking through GCC implementation files, very most of all uses
>>>>>>> of 'hash_map' boil down to pointer key ('tree', for example) and
>>>>>>> pointer/integer value.
>>>>>>
>>>>>> Right. Because most GCC containers rely exclusively on GCC's own
>>>>>> uses for testing, if your use case is novel in some way, chances
>>>>>> are it might not work as intended in all circumstances.
>>>>>>
>>>>>> I've wrestled with hash_map a number of times. A use case that's
>>>>>> close to yours (i.e., a non-trivial value type) is in cp/parser.c:
>>>>>> see class_to_loc_map_t.
>>>>>
>>>>> Indeed, at the time you sent this email, I already had started looking
>>>>> into that one! (The Fortran test cases that I originally analyzed, which
>>>>> triggered other cases of non-POD/non-trivial destructor, all didn't
>>>>> result in a memory leak, because the non-trivial constructor doesn't
>>>>> actually allocate any resources dynamically -- that's indeed different in
>>>>> this case here.) ..., and indeed:
>>>>>
>>>>>> (I don't remember if I tested it for leaks
>>>>>> though. It's used to implement -Wmismatched-tags so compiling
>>>>>> a few tests under Valgrind should show if it does leak.)
>>>>>
>>>>> ... it does leak memory at present. :-| (See attached commit log for
>>>>> details for one example.)
>>>
>>> (Attached "Fix 'hash_table::expand' to destruct stale Value objects"
>>> again.)
>>>
>>>>> To that effect, to document the current behavior, I propose to
>>>>> "Add more self-tests for 'hash_map' with Value type with non-trivial
>>>>> constructor/destructor"
>>>
>>> (We've done that in commit e4f16e9f357a38ec702fb69a0ffab9d292a6af9b
>>> "Add more self-tests for 'hash_map' with Value type with non-trivial
>>> constructor/destructor", quickly followed by bug fix
>>> commit bb04a03c6f9bacc890118b9e12b657503093c2f8
>>> "Make 'gcc/hash-map-tests.c:test_map_of_type_with_ctor_and_dtor_expand'
>>> work on 32-bit architectures [PR101959]".
>>>
>>>>> (Also cherry-pick into release branches, eventually?)
>>>
>>>>>>> Then:
>>>>>>>
>>>>>>> record_field_map_t field_map ([...]); // see below
>>>>>>> for ([...])
>>>>>>> {
>>>>>>> tree record_type = [...];
>>>>>>> [...]
>>>>>>> bool existed;
>>>>>>> field_map_t &fields
>>>>>>> = field_map.get_or_insert (record_type, &existed);
>>>>>>> gcc_checking_assert (!existed);
>>>>>>> [...]
>>>>>>> for ([...])
>>>>>>> fields.put ([...], [...]);
>>>>>>> [...]
>>>>>>> }
>>>>>>> [stuff that looks up elements from 'field_map']
>>>>>>> field_map.empty ();
>>>>>>>
>>>>>>> This generally works.
>>>>>>>
>>>>>>> If I instantiate 'record_field_map_t field_map (40);', Valgrind is happy.
>>>>>>> If however I instantiate 'record_field_map_t field_map (13);' (where '13'
>>>>>>> would be the default for 'hash_map'), Valgrind complains:
>>>>>>>
>>>>>>> 2,080 bytes in 10 blocks are definitely lost in loss record 828 of 876
>>>>>>> at 0x483DD99: calloc (vg_replace_malloc.c:762)
>>>>>>> by 0x175F010: xcalloc (xmalloc.c:162)
>>>>>>> by 0xAF4A2C: hash_table<hash_map<tree_node*, tree_node*, simple_hashmap_traits<default_hash_traits<tree_node*>, tree_node*> >::hash_entry, false, xcallocator>::hash_table(unsigned long, bool, bool, bool, mem_alloc_origin) (hash-table.h:275)
>>>>>>> by 0x15E0120: hash_map<tree_node*, tree_node*, simple_hashmap_traits<default_hash_traits<tree_node*>, tree_node*> >::hash_map(unsigned long, bool, bool, bool) (hash-map.h:143)
>>>>>>> by 0x15DEE87: hash_map<tree_node*, hash_map<tree_node*, tree_node*, simple_hashmap_traits<default_hash_traits<tree_node*>, tree_node*> >, simple_hashmap_traits<default_hash_traits<tree_node*>, hash_map<tree_node*, tree_node*, simple_hashmap_traits<default_hash_traits<tree_node*>, tree_node*> > > >::get_or_insert(tree_node* const&, bool*) (hash-map.h:205)
>>>>>>> by 0x15DD52C: execute_omp_oacc_neuter_broadcast() (omp-oacc-neuter-broadcast.cc:1371)
>>>>>>> [...]
>>>>>>>
>>>>>>> (That's with '#pragma GCC optimize "O0"' at the top of the 'gcc/*.cc'
>>>>>>> file.)
>>>>>>>
>>>>>>> My suspicion was that it is due to the 'field_map' getting resized as it
>>>>>>> incrementally grows (and '40' being big enough for that to never happen),
>>>>>>> and somehow the non-POD (?) value objects not being properly handled
>>>>>>> during that. Working my way a bit through 'gcc/hash-map.*' and
>>>>>>> 'gcc/hash-table.*' (but not claiming that I understand all that, off
>>>>>>> hand), it seems as if my theory is right: I'm able to plug this memory
>>>>>>> leak as follows:
>>>>>>>
>>>>>>> --- gcc/hash-table.h
>>>>>>> +++ gcc/hash-table.h
>>>>>>> @@ -820,6 +820,8 @@ hash_table<Descriptor, Lazy, Allocator>::expand ()
>>>>>>> {
>>>>>>> value_type *q = find_empty_slot_for_expand (Descriptor::hash (x));
>>>>>>> new ((void*) q) value_type (std::move (x));
>>>>>>> + //BAD Descriptor::remove (x); // (doesn't make sense and) a ton of "Invalid read [...] inside a block of size [...] free'd"
>>>>>>> + x.~value_type (); //GOOD This seems to work! -- but does it make sense?
>>>>>>> }
>>>>>>>
>>>>>>> p++;
>>>>>>>
>>>>>>> However, that doesn't exactly look like a correct fix, does it? I'd
>>>>>>> expect such a manual destructor call in combination with placement new
>>>>>>> (that is being used here, obviously) -- but this is after 'std::move'?
>>>>>>> However, this also survives a smoke-test-like run of parts of the GCC
>>>>>>> testsuite, bootstrap and complete run now ongoing.
>>>>>>
>>>>>> If explicitly calling the dtor on the moved object is the right thing
>>>>>> to do I'd expect to see such invocations elsewhere in hash_table but
>>>>>> I don't. It does seem like removed elements ought to be destroyed,
>>>>>> but it also seems like the destruction should go through some traits
>>>>>> class (e.g., call Descriptor::remove and mark_deleted or do some
>>>>>> similar dance), and be called from other member functions that move
>>>>>> elements.
>>>>>
>>>>> So, we shall assume that any "real C++" use case (that is, C++ class) is
>>>>> using the appropriate C++ method, that is, 'typed_delete_remove', which
>>>>> does 'delete', which does destroy the C++ object, if applicable, else
>>>>> 'typed_noop_remove'.
>>>>>
>>>>> (Shall we look into the few places that use 'typed_free_remove' via
>>>>> 'free_ptr_hash', and potentially replace them with 'typed_delete_remove'?
>>>>> Or is there a reason for the two schemes to co-exist, other than for
>>>>> legacy reasons? Anyway, that's for another day.)
>>>>
>>>> I find all these these traits pretty much impenetrable.
>>>
>>> (Indeed. ... which triggered this reflex with me, to try simplifying
>>> this by getting rid of 'typed_free_remove' etc...)
>>>
>>>> I guess
>>>> intuitively, I'd expect Descriptor::remove() to destroy an element,
>>>> but I have no idea if that would be right given the design.
>>>
>>> So 'typed_delete_remove' does destruct via 'delete'. 'typed_free_remove'
>>> doesn't -- but is only used via 'free_ptr_hash', where this isn't
>>> relevant? 'typed_noop_remove' I haven't considered yet regarding that
>>> issue. Anyway, that's all for another day.
>>>
>>>>> What is different in the 'hash_table::expand' case is that all the Value
>>>>> objects do get 'std::move'd into a new blob of memory via placement new
>>>>> (so a subsequent 'delete' via 'typed_delete_remove' is not appropriate),
>>>>> but then the stale Value objects never get destructed. And indeed an
>>>>> explicit destructor call (which, as I understand is a no-op for primitive
>>>>> types; "pseudo destructor") is the right thing to do; see
>>>>> <https://stackoverflow.com/questions/6730403/how-to-delete-object-constructed-via-placement-new-operator>
>>>>> and others, for example. (Therefore, I don't think this needs to be
>>>>> routed through a "traits" function, but can rather be done in-line here,
>>>>> after each placement new, before deallocation of the original blob of
>>>>> memory. Also, I argue it's the right thing to do also for 'm_ggc',
>>>>> because even if in that case we're not going to leak memory (GC will
>>>>> reclaim), but we still may leak other resources dynamically allocated in
>>>>> a non-trivial constructor.)
>>>>
>>>> Yes, of course, all elements need to be eventually be destroyed.
>>>> My only hesitation was whether it should be done via one of these
>>>> traits classes (like it's done in C++ containers via allocators)
>>>> rather than directly
>>>
>>> Given that there is (apparently) no issue to do a placement new in
>>> 'hash_table::expand', I'd asumme a -- corresponding -- explicit
>>> destructor call would be likewise appropriate? (But I'll of course route
>>> this through a (new) "traits" function if someone explains why this is
>>> the right thing to do.)
>>>
>>>> and whether there might be other places
>>>> where the destruction night need to happen.
>>>
>>> (Possibly, yes, per discussion above -- but that's a separate issue?)
>>>
>>>>> The attached "Fix 'hash_table::expand' to destruct stale Value objects"
>>>>> does prevent my original problem, does address the current 'class2loc'
>>>>> memory leak in 'cp/parser.c' (see commit log for one example), and
>>>>> adjusts the new
>>>>> 'gcc/hash-map-tests.c:test_map_of_type_with_ctor_and_dtor_expand' test
>>>>> case such that number of constructor calls matches the number of
>>>>> destructor calls. After some careful review regarding C++ details
>>>>> (please!), OK to push to master branch? (Also cherry-pick into release
>>>>> branches, eventually?) Is the source code comment that I'm adding
>>>>> sufficiently explanatory and correct in C++ terms?
>>>
>>> Ping.
>>>
>>>> Shouldn't the hash_table dtor (and perhaps also empty()) also
>>>> destroy the elements? (Otherwise, what destroys the elements
>>>> newly constructed here, or anywhere else for that matter, such
>>>> as in the hash_table ctor?)
>>>
>>> Per my understanding, per discussion above, they (eventually) do get
>>> destroyed via 'delete' in 'typed_delete_remove', for example, via
>>> 'Descriptor::remove' calls for all/relevant entries in
>>> 'hash_table::~hash_table', 'hash_table::empty_slow',
>>> 'hash_table::clear_slot', 'hash_table::remove_elt_with_hash'.
>>>
>>> (This means that if there has been an intermediate 'expand', this may
>>> (eventually) destroy objects at a different memory location from where
>>> they originally have been created -- but that's fine.)
>>>
>>> The 'expand' case itself is different: any (live) entries are *moved*
>>> into a new storage memory object via placement new. (And then the
>>> hollow entries in the old storage memory object linger.)
>>>
>>>> Also, shouldn't the destroyed slot be marked either deleted or
>>>> empty?)
>>>
>>> Per my understanding, irrelevant in 'expand': working through 'oentries',
>>> the *move* is only done 'if (!is_empty (x) && !is_deleted (x))' (so
>>> combined with the item above, there is no memory leak for any entries
>>> that have already been 'remove'd -- they have already been destructed),
>>> and the whole (old) storage memory object will be deallocated right after
>>> the 'oentries' loop.
>>>
>>>
>>>> (Sorry, I realize I have more questions than answers.)
>>>
>>> No worries, happens to me most of the times! Thanks for looking into
>>> this.
>>>
>>>
>>> Grüße
>>> Thomas
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