Re: Slow compile - find_symtree_for_symbol()

[Paul Richard Thomas <paul dot richard dot thomas at gmail dot com>]

Committed as revision 246952.

Thanks for finding this deadwood.

Cheers

Paul

On 16 April 2017 at 20:18, Janus Weil <janus@gcc.gnu.org> wrote:
> Hi Paul,
>
>> Prompted by this thread, I have just tried eliminating the call and
>> the function altogether. No problem, the regtesting went through
>> without a problem. Evidently the need for this function has gone away
>> with some subsequent patch(es).
>>
>> OK for trunk?
>
> yes, ok from my side. Removing unneeded code is certainly a good idea,
> even more so if it is responsible for a performance problem.
>
> I assume this should generate a similar (or even larger?) speedup for
> Andrew's case as his earlier patch. Andrew, can you confirm this?
>
> Cheers,
> Janus
>
>
>
>
>> 2017-04-16  Paul Thomas  <pault@gcc.gnu.org>
>>
>>     PR fortran/80440
>>     * module.c (find_symtree_for_symbol): Delete.
>>     (read_module): Remove the call to the above.
>>
>>
>>
>> On 16 April 2017 at 14:27, Janus Weil <janus@gcc.gnu.org> wrote:
>>> Hi Paul,
>>>
>>>> The reason why the name wass not used is because of the rename
>>>> facility in modules. In this case, the symtree name and the symbol
>>>> names are different.
>>>
>>> thanks for the feedback!
>>>
>>> Do you understand why Andrew's patch (see
>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80440#c0) doesn't show
>>> any testsuite failures then? Not even the test case from the original
>>> r121824 fails (char_array_constructor_2.f90). Don't we have sufficient
>>> coverage of use-renaming?
>>>
>>> Looking at the one occurrence of find_symtree_for_symbol in read_module:
>>>
>>>       /* If possible recycle the symtree that references the symbol.
>>>      If a symtree is not found and the module does not import one,
>>>      a unique-name symtree is found by read_cleanup.  */
>>>       st = find_symtree_for_symbol (gfc_current_ns->sym_root, sym);
>>>       if (st != NULL)
>>>     {
>>>       info->u.rsym.symtree = st;
>>>       info->u.rsym.referenced = 1;
>>>     }
>>>
>>> The comment there sounds a bit like this is just an (optional)
>>> 'optimization'? If it actually slows down things in reality, could one
>>> just get away without this piece of code altogether?
>>>
>>> Cheers,
>>> Janus
>>>
>>>
>>>
>>>> On 15 April 2017 at 20:56, Andrew Benson <abenson@carnegiescience.edu> wrote:
>>>>> Hi Janus,
>>>>>
>>>>> On Saturday, April 15, 2017 1:48:36 PM PDT Janus Weil wrote:
>>>>>> Hi Andrew,
>>>>>>
>>>>>> > Compile times for code that makes extensive USEs of modules seems to be
>>>>>> > very slow in some cases. I've been doing some investigation of the cause
>>>>>> > of this with the hope that I can maybe figure out some way to speed
>>>>>> > things up.
>>>>>> >
>>>>>> > For example, I have a smallish file - 700 lines of code, which takes
>>>>>> > around 3 minutes to compile with a recent build of gfortran.
>>>>>>
>>>>>> whoa, 3 minutes definitely sounds pretty bad for 700 loc :(
>>>>>
>>>>> Yeah, definitely slow! For compiling a large project with many modules it's
>>>>> making development painful.
>>>>>
>>>>>> > Profiling f951 with
>>>>>> > valgrind I find that 63% of that time is spent in
>>>>>> > find_symtree_for_symbol(), which (if I understand correctly) is searching
>>>>>> > for a node in the symtree that already references some symbol being
>>>>>> > imported from a module.
>>>>>> >
>>>>>> > find_symtree_for_symbol() gets called directly 245,658 times in compiling
>>>>>> > this source file (and calls itself recursively almost 19 billion times!).
>>>>>> >
>>>>>> > find_symtree_for_symbol() is just stepping through a binary branching tree
>>>>>> > looking for a reference to a given symbol, but (again, if I understood
>>>>>> > correctly), it can't use the usual bbt search approach because the tree is
>>>>>> > not ordered by the symbol name, so the search is O(n) rather than O(log
>>>>>> > n).
>>>>>> Huh, naively I would say it should be possible to use an ordered tree
>>>>>> here as well, like it is done for the symtree-related functions in
>>>>>> symbol.c (e.g. gfc_find_symtree). There is certainly some reason why
>>>>>> this is not done, but I have too little knowledge of the module.c code
>>>>>> to be much of a help here.
>>>>>
>>>>> This does seem to work. If I ignore my ignorance of why the ordered tree isn't
>>>>> used here and go ahead and search it using the symbol name (ignoring case
>>>>> which seems to differ between the symbol name and the name of the symtree
>>>>> node) then I certainly get a substantial speed-up (the file I mentioned now
>>>>> compiles in 40s), and nothing seems to break. I ran the gfortan test suite
>>>>> which shows two FAILs:
>>>>>
>>>>> gcc/testsuite/gfortran/gfortran.sum:FAIL: gfortran.dg/graphite/pr68279.f90   -
>>>>> O  (internal compiler error)
>>>>> gcc/testsuite/gfortran/gfortran.sum:FAIL: gfortran.dg/graphite/pr68279.f90   -
>>>>> O  (test for excess errors)
>>>>>
>>>>> but those show up when I run the test suite without any change to module.c
>>>>> anyway.
>>>>>
>>>>>> > So, before I dive in and see if I can sufficiently understand how this
>>>>>> > works to figure out if there's an obvious way to make the search more
>>>>>> > efficient, I wanted to ask if anyone else has looked at this, or if
>>>>>> > there's an immediately obvious way to improve the performance of this
>>>>>> > search.
>>>>>>
>>>>>> "svn blame" tells me that find_symtree_for_symbol was introduced by
>>>>>> Paul in this commit in 2007:
>>>>>>
>>>>>> https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=121824
>>>>>>
>>>>>> see also:
>>>>>>
>>>>>> https://gcc.gnu.org/ml/gcc-patches/2007-02/msg00807.html
>>>>>>
>>>>>> So I guess Paul is probably the best person to answer your question ...
>>>>>
>>>>> If Paul can offer any insight into this that would be great.
>>>>>
>>>>> Cheers,
>>>>> Andrew
>>>>>
>>>>> --
>>>>>
>>>>> * Andrew Benson: http://users.obs.carnegiescience.edu/abenson/contact.html
>>>>>
>>>>> * Galacticus: http://sites.google.com/site/galacticusmodel
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> "If you can't explain it simply, you don't understand it well enough"
>>>> - Albert Einstein
>>
>>
>>
>> --
>> "If you can't explain it simply, you don't understand it well enough"
>> - Albert Einstein



-- 
"If you can't explain it simply, you don't understand it well enough"
- Albert Einstein