[PATCH] Combine location with block using block_locations

[Xinliang David Li]

On Wed, Sep 12, 2012 at 10:05 AM, Dehao Chen <dehao@google.com> wrote:
> There are two parts that needs memory management:
>
> 1. The BLOCK structure. This is managed by GC. I originally thought
> that removing blocks from tree.gsbase would paralyze GC. This turned
> out not to be a concern because DECL_INITIAL will still mark those
> used tree nodes. This patch may decrease the memory consumption by
> removing blocks from tree/gimple. However, as it makes more blocks
> become used, they also increase the memory consumption.

You mean when you also make the location table GC root.

Can you share the mem-stats information for the large program with and
without your patch?

thanks,

David

> 2. The data structure in libcpp that maintains the hashtable for the
> location->block mapping. This is relatively minor because for the
> largest source I've seen, it only maintains less than 100K entries in
> the array (less than 1M total memory consumption). However, as it is a
> global data structure, it may make LTO unhappy. Honza is helping
> testing the memory consumption on LTO (but we first need to make this
> patch work for LTO). If the LTO result turns out ok, we probably don't
> want to put these under GC because: 1. it'll make things much more
> complicated. 2. using self managed memory is more efficient (as this
> is frequently used in many passes). 3. not using GC actually saves
> memory because even though the block is in the map, it can still be
> GCed as soon as it's not reachable from DECL_INITIAL.
>
> I've tested this on some very large C++ files (each one takes more
> than 10s to build), the memory consumption does not see noticeable
> increase/decrease.
>
> Thanks,
> Dehao
>
> On Wed, Sep 12, 2012 at 9:39 AM, Xinliang David Li <davidxl@google.com> wrote:
>> On Wed, Sep 12, 2012 at 2:13 AM, Richard Guenther
>> <richard.guenther@gmail.com> wrote:
>>> On Wed, Sep 12, 2012 at 7:06 AM, Dehao Chen <dehao@google.com> wrote:
>>>> Now I think we are facing a more complex problem. The data structure
>>>> we use to store the location_adhoc_data are file-static in linemap.c
>>>> in libcpp. These data structures are not guarded by GTY(()).
>>>> Meanwhile, as we have removed the block data structure from
>>>> gimple.gsbase as well as tree.exp (encoding them into an location_t).
>>>> This could cause block being GCed and the LOCATION_BLOCK becoming
>>>> dangling pointers.
>>>
>>> Uh.  Note that it is quite important that we are able to garbage-collect unused
>>> BLOCKs, this is the whole point of removing unused BLOCK scopes in
>>> remove_unused_locals.  So this indeed becomes much more complicated ...
>>> What would be desired is that the garbage collector can NULL an entry in
>>> the mapping table when it is not referenced in any other way (that other
>>> reference would be the BLOCK tree as stored in a FUNCTION_DECLs DECL_INITIAL).
>>
>> It would be nice to GC those unused BLOCKS. I wonder how many BLOCKS
>> are created for a large C++ program. This patch saves memory by
>> shrinking tree size, is it a net win or loss without GC those BLOCKS?
>>
>> thanks,
>>
>> David
>>
>>
>>>
>>>> I tried to manipulate GTY to make it recognize the LOCATION_BLOCK from
>>>> gimple.gsbase.location. However, neigher nested_ptr nor mark_hook can
>>>> help me.
>>>>
>>>> Another approach would be guard the location_adhoc_data and related
>>>> data structures in GTY(()). However, this is non-trivial because tree
>>>> is not visible in libcpp. At the same time, my implementation heavily
>>>> relies on hashtable to make the code efficient, thus it's quite tricky
>>>> to make "param_is" and "use_params" work.
>>>>
>>>> The final approach, which I'll try tomorrow, would be move all my
>>>> implementation from libcpp to gcc, and guard them with GTY(()). I
>>>> still haven't thought of any potential problem of this approach. Any
>>>> comments?
>>>
>>> I think moving the mapping to GC in a lazy manner as I described above
>>> would be the way to go.  For hashtables GC already supports if_marked,
>>> not sure if similar support is available for arrays/vecs.
>>>
>>> Richard.
>>>
>>>> Thanks,
>>>> Dehao
>>>>
>>>> On Tue, Sep 11, 2012 at 9:00 AM, Dehao Chen <dehao@google.com> wrote:
>>>>> I saw comments in tree-streamer-out.c:
>>>>>
>>>>>   /* Do not stream BLOCK_SOURCE_LOCATION.  We cannot handle debug information
>>>>>      for early inlining so drop it on the floor instead of ICEing in
>>>>>      dwarf2out.c.  */
>>>>>   streamer_write_chain (ob, BLOCK_VARS (expr), ref_p);
>>>>>
>>>>> However, what the code is doing seemed contradictory with the comment.
>>>>> Or am I missing something?
>>>>>
>>>>>
>>>>>
>>>>> On Tue, Sep 11, 2012 at 8:32 AM, Michael Matz <matz@suse.de> wrote:
>>>>>> Hi,
>>>>>>
>>>>>> On Tue, 11 Sep 2012, Dehao Chen wrote:
>>>>>>
>>>>>>> Looks like we have two choices:
>>>>>>>
>>>>>>> 1. Stream out block info, and use LTO_SET_PREVAIL for TREE_CHAIN(t)
>>>>>>
>>>>>> This will actually not work correctly in some cases.  The problem is, if
>>>>>> the prevailing decl is already part of another chain (say in another
>>>>>> block_var list) you would break the current chain.  Hence block vars need
>>>>>> special handling in the lto streamer (another reason why tree_chain is not
>>>>>> the most clever think to use for this chain).  This problem area needs to
>>>>>> be solved somehow if block info is to be preserved correctly.
>>>>>>
>>>>>>> 2. Don't stream out block info for LTO, and still call LTO_NO_PREVAIL
>>>>>>> (TREE_CHAIN (t)).
>>>>>>
>>>>>> That's also a large hammer as it basically will mean no debug info after
>>>>>> LTO :-/ Sigh, at this point I have no good solution that doesn't involve
>>>>>> quite some work, perhaps your hack is good enough for the time being,
>>>>>> though I hate it :)
>>>>>
>>>>> I got it. Then I'll keep the patch as it is (remove the
>>>>> LTO_NO_PREVAIL), and work with Honza to resolve the issue he had, and
>>>>> then we should be good to check in?
>>>>>
>>>>> Thanks,
>>>>> Dehao
>>>>>
>>>>>>
>>>>>>
>>>>>> Ciao,
>>>>>> Michael.