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Re: [GSoC] Question about std::map


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 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.

Cheers,
Tobias



Tobias


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