Re: [PATCH] Combine location with block using block_locations

[Richard Guenther <richard dot guenther at gmail dot com>]

On Wed, Sep 12, 2012 at 7:20 PM, Dehao Chen <dehao@google.com> wrote:
> There is another bug in the patch (not covered by unittests,
> discovered through spec benchmarks).
>
> When we remove unused locals, we do not mark the block as used for
> debug stmt, but gimple-streamer-out will still stream out blocks for
> debug stmt. There can be 2 fixes:

Because doing so would create code generation differences -g vs. -g0.

> 1.
> --- a/gcc/gimple-streamer-out.c
> +++ b/gcc/gimple-streamer-out.c
> @@ -77,7 +77,8 @@ output_gimple_stmt (struct output_block *ob, gimple stmt)
>    lto_output_location (ob, LOCATION_LOCUS (gimple_location (stmt)));
>
>    /* Emit the lexical block holding STMT.  */
> -  stream_write_tree (ob, gimple_block (stmt), true);
> +  if (!is_gimple_debug (stmt))
> +    stream_write_tree (ob, gimple_block (stmt), true);
>
>    /* Emit the operands.  */
>    switch (gimple_code (stmt))
>
> 2.
> --- a/gcc/tree-ssa-live.c
> +++ b/gcc/tree-ssa-live.c
> @@ -726,9 +726,6 @@ remove_unused_locals (void)
>           gimple stmt = gsi_stmt (gsi);
>           tree b = gimple_block (stmt);
>
> -         if (is_gimple_debug (stmt))
> -           continue;
> -
>           if (gimple_clobber_p (stmt))
>             {
>               have_local_clobbers = true;
>
> Either fix could work. Any suggests which one should we go?

The 2nd one will not work and is not acceptable.  The 1st one - well ...
what happens on trunk right now?  The debug stmt points to a
BLOCK that is possibly removed from the BLOCK tree?  In this case
I think the fix is 3. make sure remove_unused_scope_block_p will
clear BLOCKs from all stmts / expressions that have been removed.

Richard.

> Thanks,
> Dehao
>
> 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.
>> 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.