Slow compile - find_symtree_for_symbol()

Andrew Benson abenson@carnegiescience.edu
Mon Apr 17 18:32:00 GMT 2017 

No problem - I'm making use of the faster compile times already!

Cheers,
Andrew

On Monday, April 17, 2017 7:23:29 PM PDT Paul Richard Thomas wrote:
> 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


-- 

* Andrew Benson: http://users.obs.carnegiescience.edu/abenson/contact.html

* Galacticus: http://sites.google.com/site/galacticusmodel

More information about the Fortran mailing list