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Re: [GSoC] Question about std::map


On Fri, Jul 04, 2014 at 09:57:11AM +0200, Tobias Grosser wrote:
> On 04/07/2014 04:16, Trevor Saunders wrote:
> >On Thu, Jul 03, 2014 at 07:52:59PM +0200, Tobias Grosser wrote:
> >>On 03/07/2014 19:23, Roman Gareev wrote:
> >>>Dear gcc contributors,
> >>>
> >>>could you please answer a few questions about std::map? Does gcc have
> >>>a policy that forbids using of map in the source code of gcc? Can this
> >>>using create a new installation dependency, which requires libstdc++?
> >>>I would be very grateful for your comments.
> >>
> >>https://gcc.gnu.org/codingconventions.html#Standard_Library
> >>
> >>This suggests that using std::map is allowed. Running "grep 'std::' *"
> >>on gcc, we only find a couple of std::make_pair, std::min and std::max
> >>calls, but I don't see why we should not use std::map. I would say go for it
> >>if there are no vetos. It seems to be the right tool for what you are aiming
> >>for and certainly makes the code a lot more readable.
> >
> >I'm certainly not opposed to using the stl when it makes sense, and
> >reinventing our own stl has a fairly high cost.  However I think the
> >question of if you should use std::map is a complicated one.  First
> >remember std::map is a tree, not a hash table, and consider which
> >performance characteristic you want.  Also remember the stl in some
> >cases trades off possible optimizations to be more generic, if you
> >implement your own version of something you can say ban arrays larger
> >than 2^32 and thereby save a bit of space.  When you use the stl you
> >also need to make sure you don't call things that can throw exceptions
> >since gcc is built with -fno-exceptions and generally isn't exception
> >safe.
> >
> >I think there are some stl things you should basically always prefer
> >e.g. std::swap, there are some things you should think about before
> >using, and some things that are best avoided.  Personally I think you
> >should favor hash tables to std::map most of the time because of there
> >generally better performance.
> >
> >btw if you have specific gripes about the stlish bits gcc already has
> >I'd like to hear them.
> 
> Hi Trev,
> 
> thanks for the feedback. You seem to have a good idea for which use cases a
> std::map is possible. Maybe I can invite you to a patch review currently
> happening on gcc@gcc.gnu.org (Roman, we should do this kind of stuff on
> gcc-patches@ I suppose) under the title "[GSoC] generation of GCC expression
> trees from isl ast expressions". Feel free to ignore most of the code. The
> question I am unsure about is the use of ast_isl_index_hasher() and related
> function, which takes 105 lines of code and alomost the same functionality
> could be implemented by a one-line use of std::map<>. The maps generated are

have you tried the hash_map class I introduced a couple days ago? (maybe
we should teach its generic machinary about strings, but it should
already be better than hash_table for your purpose).  One thing that
confuses me is that you seem to be mapping from ast node to ast node,
but you do the actual mapping with strings, why is that needed?

> commonly small and I doubt this is in the performance critical path. I feel
> very worried about adding so much bloat? What would you suggest.

yeah, that code kind of made my eyes glaze over on the other hand your
talking about going from average constant time to average log time,
which is the sort of thing I'm pretty hesitent to say is a great idea.
Its too bad unordered_map isn't an option.

Trev

> 
> Cheers,
> Tobias
> 
> 
> 
> Tobias
> 


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