On Tue, Apr 29, 2014 at 3:21 PM, Trevor Saunders <tsaund...@mozilla.com> wrote:
> On Tue, Apr 29, 2014 at 02:58:11PM +0200, Richard Biener wrote:
>> On Tue, Apr 29, 2014 at 1:08 PM, <tsaund...@mozilla.com> wrote:
>> > From: Trevor Saunders <tsaund...@mozilla.com>
>> >
>> > Hi,
>> >
>> > This implements finalizers by keeping a list of registered finalizers
>> > and after every mark but before sweeping check to see if any of them are
>> > for unmarked blocks.
>> >
>> > bootstrapped + regtested on x86_64-unknown-linux-gnu, ok?
>> >
>> > Trev
>> >
>> > gcc/ChangeLog:
>> >
>> > * ggc-common.c (ggc_internal_cleared_alloc): Adjust.
>> > * ggc-none.c (ggc_internal_alloc): Assert if a finalizer is passed.
>> > (ggc_internal_cleared_alloc): Likewise.
>> > * ggc-page.c (finalizer): New class.
>> > (globals::finalizer_lists): New member.
>> > (ggc_internal_alloc): Record the finalizer if any for the block
>> > being
>> > allocated.
>> > (ggc_handle_finalizers): New function.
>> > (ggc_collect): Call ggc_handle_finalizers.
>> > * ggc.h (ggc_internal_alloc):Add arguments to allow installing a
>> > finalizer.
>> > (ggc_internal_cleared_alloc): Likewise.
>> > (finalize): New function.
>> > (need_finalization_p): Likewise.
>> > (ggc_alloc): Install the type's destructor as the finalizer if it
>> > might do something.
>> > (ggc_cleared_alloc): Likewise.
>> > (ggc_vec_alloc): Likewise.
>> > (ggc_cleared_vec_alloc): Likewise.
>> > ---
>> > gcc/ggc-common.c | 5 +++--
>> > gcc/ggc-none.c | 8 ++++++--
>> > gcc/ggc-page.c | 50 +++++++++++++++++++++++++++++++++++++++++++++++++-
>> > gcc/ggc.h | 52 ++++++++++++++++++++++++++++++++++++++++++++++------
>> > 4 files changed, 104 insertions(+), 11 deletions(-)
>> >
>> > diff --git a/gcc/ggc-common.c b/gcc/ggc-common.c
>> > index e89cc64..b11a10c 100644
>> > --- a/gcc/ggc-common.c
>> > +++ b/gcc/ggc-common.c
>> > @@ -174,9 +174,10 @@ ggc_mark_roots (void)
>> >
>> > /* Allocate a block of memory, then clear it. */
>> > void *
>> > -ggc_internal_cleared_alloc (size_t size MEM_STAT_DECL)
>> > +ggc_internal_cleared_alloc (size_t size, void (*f)(void *), size_t s,
>> > size_t n
>> > + MEM_STAT_DECL)
>> > {
>> > - void *buf = ggc_internal_alloc (size PASS_MEM_STAT);
>> > + void *buf = ggc_internal_alloc (size, f, s, n PASS_MEM_STAT);
>> > memset (buf, 0, size);
>> > return buf;
>> > }
>> > diff --git a/gcc/ggc-none.c b/gcc/ggc-none.c
>> > index aad89bf..97d3566 100644
>> > --- a/gcc/ggc-none.c
>> > +++ b/gcc/ggc-none.c
>> > @@ -41,14 +41,18 @@ ggc_round_alloc_size (size_t requested_size)
>> > }
>> >
>> > void *
>> > -ggc_internal_alloc (size_t size MEM_STAT_DECL)
>> > +ggc_internal_alloc (size_t size, void (*f)(void *), size_t, size_t
>> > + MEM_STAT_DECL)
>> > {
>> > + gcc_assert (!f); // ggc-none doesn't support finalizers
>> > return xmalloc (size);
>> > }
>> >
>> > void *
>> > -ggc_internal_cleared_alloc (size_t size MEM_STAT_DECL)
>> > +ggc_internal_cleared_alloc (size_t size, void (*f)(void *), size_t, size_t
>> > + MEM_STAT_DECL)
>> > {
>> > + gcc_assert (!f); // ggc-none doesn't support finalizers
>> > return xcalloc (size, 1);
>> > }
>> >
>> > diff --git a/gcc/ggc-page.c b/gcc/ggc-page.c
>> > index ae5e88a..208b526 100644
>> > --- a/gcc/ggc-page.c
>> > +++ b/gcc/ggc-page.c
>> > @@ -332,6 +332,27 @@ typedef struct page_table_chain
>> >
>> > #endif
>> >
>> > +class finalizer
>> > +{
>> > +public:
>> > + finalizer (uintptr_t addr, void (*f)(void *), size_t s, size_t n) :
>> > + m_addr (addr), m_function (f), m_object_size (s), m_n_objects (n) {}
>> > +
>> > + void call () const
>> > + {
>> > + for (size_t i = 0; i < m_n_objects; i++)
>> > + m_function (reinterpret_cast<void *> (m_addr + (i *
>> > m_object_size)));
>> > + }
>> > +
>> > + uintptr_t addr () const { return m_addr; }
>> > +
>> > +private:
>> > + uintptr_t m_addr;
>> > + void (*m_function)(void *);
>> > + size_t m_object_size;
>> > + size_t m_n_objects;
>> > +};
>> > +
>> > #ifdef ENABLE_GC_ALWAYS_COLLECT
>> > /* List of free objects to be verified as actually free on the
>> > next collection. */
>> > @@ -425,6 +446,9 @@ static struct globals
>> > better runtime data access pattern. */
>> > unsigned long **save_in_use;
>> >
>> > + /* finalizer_lists[i] is the list of finalizers for blocks of order i.
>> > */
>> > + vec<finalizer> finalizer_lists[NUM_ORDERS];
>> > +
>> > #ifdef ENABLE_GC_ALWAYS_COLLECT
>> > /* List of free objects to be verified as actually free on the
>> > next collection. */
>> > @@ -1202,7 +1226,8 @@ ggc_round_alloc_size (size_t requested_size)
>> > /* Allocate a chunk of memory of SIZE bytes. Its contents are undefined.
>> > */
>> >
>> > void *
>> > -ggc_internal_alloc (size_t size MEM_STAT_DECL)
>> > +ggc_internal_alloc (size_t size, void (*f)(void *), size_t s, size_t n
>> > + MEM_STAT_DECL)
>> > {
>> > size_t order, word, bit, object_offset, object_size;
>> > struct page_entry *entry;
>> > @@ -1345,6 +1370,10 @@ ggc_internal_alloc (size_t size MEM_STAT_DECL)
>> > /* For timevar statistics. */
>> > timevar_ggc_mem_total += object_size;
>> >
>> > + if (f)
>> > + G.finalizer_lists[order]
>> > + .safe_push (finalizer (reinterpret_cast<uintptr_t> (result), f, s,
>> > n));
>> > +
>> > if (GATHER_STATISTICS)
>> > {
>> > size_t overhead = object_size - size;
>> > @@ -1811,6 +1840,24 @@ clear_marks (void)
>> > }
>> > }
>> >
>> > +static void
>> > +ggc_handle_finalizers ()
>> > +{
>> > + for (unsigned int order = 2; order < NUM_ORDERS; order++)
>> > + {
>> > + unsigned int length = G.finalizer_lists[order].length ();
>> > + for (unsigned int i = length - 1; i < length; i--)
>>
>> I suppose you are walking backwards here to handle dependences
>> properly.
>
> I only did it because it seemed epsilon easier than iterating forward
> and dealing with removals from the list.
Heh, but iterating forward is as easy as
unsigned keep = 0, pick = 0;
for (; pick < length; ++pick)
{
if (!ggc_marked (...))
{
...
}
else if (pick != keep)
G.finalizer_list[order][keep++] = G.finalizer_list[order][pick];
}
G.finalizer_list[oder].truncate (keep);
either as 2nd loop doing the removal (with empty ggc_marked () case)
or by doing the walk forward in the first place.
With a single vector we can then impose a "reasonable"
order of destruction.
Or, as you say, we can also use unordered remove which should
be even more efficient as it avoids all copying.
> I suppose dependancies between objects could be a reason to do it
> backwards, but I tend to think getting dependancies of that sort right
> is impossible and you shouldn't rely on any given ordering. So just
> write itempotent dtors and all's good ;)
>
>> But it doesn't address dependencies across different allocation
>> orders (what's the reason to even have one vector per order?).
>
> I think I did it for locality, but otherwise I don't think there's a
> great reason other than copying what other stuff does with orders.
Hmm, I'd reduce it to a single vector (otherwise you'd eventually
even want a vector per GC page).
>> > + {
>> > + finalizer &f = G.finalizer_lists[order][i];
>> > + if (!ggc_marked_p (reinterpret_cast<void *> (f.addr ())))
>> > + {
>> > + f.call ();
>> > + G.finalizer_lists[order].ordered_remove (i);
>>
>> But this clearly will be an efficiency issue. Can you delay
>> removing the element from the list and instead do it in
>> a second forward run over all finalizers of that order (if there
>> were any finalizers run in the first)?
>
> Or if we agree to not care about order we can just use the non ordered
> remove.
Right. With a single vector and backward walk I can't think of
a case that we could break though? Of course better make
sure any dependence on order will break immediately
(and thus finalize in the "wrong" forward order).
>> I suppose that the overall complexity of walking the finalizers isn't
>> that bad as we walked all reachable ggc memory anyway.
>
> yeah, its bad but not as bad as gc itself is my thought.
>
>> So the only issue would be memory inefficiencies for a lot
>> of small individual objects that need finalization. Thus,
>>
>> > +private:
>> > + uintptr_t m_addr;
>> > + void (*m_function)(void *);
>> > + size_t m_object_size;
>> > + size_t m_n_objects;
>>
>> could be improved by noting that m_function and m_object_size
>> are somewhat redundant (and m_object_size and m_n_objects
>> are unused for non-vectors). So, instead of m_function and
>> m_object_size you could store an index into a global vector
>> of m_function/m_object_size pairs (eventually even statically
>> compute it by gengtype.c to avoid some kind of lookup
>> at allocation time).
>
> deciding if a type needs finalization would mean teaching gengtype about
> a lot of C++ :/ Well actually maybe not, if we don't maybe we could
> just have gengtype emit a blob of C++ that does it if we're really
> clever, but that would take some thought.
Yeah, I think we can improve on this with a followup if it turns out
necessary.
>> As finalization order is currently broken (see above) you could
>> also have two vectors of finalizers, one for the vector case
>> and one for the non-vector case.
>
> that certainly seems doable.
Or go with this in the mean time. Actually that would be my
preference - iterate forward (and document dependences are
not handled) using unordered removes and have overall
two vectors.
>> An optimization would also be to look at the last added
>> finalizer and see if the current object is adjacent to it
>> and only adjust m_n_objects. (not sure if that would trigger
>> often)
>
> What happens if they aren't freed at the same time though?
Good question. Ok, so lets not do this.
>> I suppose given we don't have many uses of finalizers (yet)
>> this all isn't much of an issue. Still please eventually
>> reduce to a single finalizer vector and avoid the ordered remove.
>
> both of those certainly seem doable.
Thanks,
Richard.
> thanks!
>
> Trev
>
>>
>> Thanks,
>> Richard.
>>
>> > + }
>> > + }
>> > + }
>> > +}
>> > +
>> > /* Free all empty pages. Partially empty pages need no attention
>> > because the `mark' bit doubles as an `unused' bit. */
>> >
>> > @@ -2075,6 +2122,7 @@ ggc_collect (void)
>> >
>> > clear_marks ();
>> > ggc_mark_roots ();
>> > + ggc_handle_finalizers ();
>> >
>> > if (GATHER_STATISTICS)
>> > ggc_prune_overhead_list ();
>> > diff --git a/gcc/ggc.h b/gcc/ggc.h
>> > index 50fb199..4c59597 100644
>> > --- a/gcc/ggc.h
>> > +++ b/gcc/ggc.h
>> > @@ -136,13 +136,16 @@ extern void gt_pch_save (FILE *f);
>> > /* Allocation. */
>> >
>> > /* The internal primitive. */
>> > -extern void *ggc_internal_alloc (size_t CXX_MEM_STAT_INFO)
>> > ATTRIBUTE_MALLOC;
>> > +extern void *ggc_internal_alloc (size_t, void (*)(void *) = NULL, size_t
>> > = 0,
>> > + size_t = 1 CXX_MEM_STAT_INFO)
>> > + ATTRIBUTE_MALLOC;
>> >
>> > extern size_t ggc_round_alloc_size (size_t requested_size);
>> >
>> > /* Allocates cleared memory. */
>> > -extern void *ggc_internal_cleared_alloc (size_t CXX_MEM_STAT_INFO)
>> > - ATTRIBUTE_MALLOC;
>> > +extern void *ggc_internal_cleared_alloc (size_t, void (*)(void *) = NULL,
>> > + size_t = 0, size_t = 1
>> > + CXX_MEM_STAT_INFO)
>> > ATTRIBUTE_MALLOC;
>> >
>> > /* Resize a block. */
>> > extern void *ggc_realloc (void *, size_t CXX_MEM_STAT_INFO);
>> > @@ -157,24 +160,55 @@ extern void dump_ggc_loc_statistics (bool);
>> > ((T *) ggc_realloc ((P), (N) * sizeof (T) MEM_STAT_INFO))
>> >
>> > template<typename T>
>> > +void
>> > +finalize (void *p)
>> > +{
>> > + static_cast<T *> (p)->~T ();
>> > +}
>> > +
>> > +template<typename T>
>> > +static inline bool
>> > +need_finalization_p ()
>> > +{
>> > +#if GCC_VERSION >= 4003
>> > + return !__has_trivial_destructor (T);
>> > +#else
>> > + return true;
>> > +#endif
>> > +}
>> > +
>> > +template<typename T>
>> > static inline T *
>> > ggc_alloc (ALONE_CXX_MEM_STAT_INFO)
>> > {
>> > - return static_cast<T *> (ggc_internal_alloc (sizeof (T) PASS_MEM_STAT));
>> > + if (need_finalization_p<T> ())
>> > + return static_cast<T *> (ggc_internal_alloc (sizeof (T), finalize<T>
>> > + PASS_MEM_STAT));
>> > + else
>> > + return static_cast<T *> (ggc_internal_alloc (sizeof (T)
>> > PASS_MEM_STAT));
>> > }
>> >
>> > template<typename T>
>> > static inline T *
>> > ggc_cleared_alloc (ALONE_CXX_MEM_STAT_INFO)
>> > {
>> > - return static_cast<T *> (ggc_internal_cleared_alloc (sizeof (T)
>> > - PASS_MEM_STAT));
>> > + if (need_finalization_p<T> ())
>> > + return static_cast<T *> (ggc_internal_cleared_alloc (sizeof (T),
>> > + finalize<T>
>> > + PASS_MEM_STAT));
>> > + else
>> > + return static_cast<T *> (ggc_internal_cleared_alloc (sizeof (T)
>> > + PASS_MEM_STAT));
>> > }
>> >
>> > template<typename T>
>> > static inline T *
>> > ggc_vec_alloc (size_t c CXX_MEM_STAT_INFO)
>> > {
>> > + if (need_finalization_p<T> ())
>> > + return static_cast<T *> (ggc_internal_alloc (c * sizeof (T),
>> > finalize<T>,
>> > + sizeof (T), c
>> > PASS_MEM_STAT));
>> > + else
>> > return static_cast<T *> (ggc_internal_alloc (c * sizeof (T)
>> > PASS_MEM_STAT));
>> > }
>> > @@ -183,6 +217,12 @@ template<typename T>
>> > static inline T *
>> > ggc_cleared_vec_alloc (size_t c CXX_MEM_STAT_INFO)
>> > {
>> > + if (need_finalization_p<T> ())
>> > + return static_cast<T *> (ggc_internal_cleared_alloc (c * sizeof (T),
>> > + finalize<T>,
>> > + sizeof (T), c
>> > + PASS_MEM_STAT));
>> > + else
>> > return static_cast<T *> (ggc_internal_cleared_alloc (c * sizeof (T)
>> > PASS_MEM_STAT));
>> > }
>> > --
>> > 2.0.0.rc0
>> >
