Slow compile - find_symtree_for_symbol()

Sun Apr 16 19:18:00 GMT 2017

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