debug mode performance patch

[François Dumont <francois dot cppdevs at free dot fr>]

Hi

  This is a patch to improve libstdc++ performance and also limit 
contention when used in debug mode.

  I finally prefered to use a simple approach which is to always 
synchronize access to the lists of safe iterators. I considered that 
trying to make hypothesis on the operation validity in a multithreaded 
environment was error prone because you might do such operations on 
unrelated parts of the sequence which will work in normal mode but not 
with the safe one because of the modifications of safe iterators lists. 
The _M_prior/_M_next safe iterator fields are considered to be part of 
the sequence lists and only safe sequence methods are modifying them 
now. Safe iterator _M_get_mutex is not used anymore but is kept for 
binary compatibility.

  I introduce usage of lambda expressions in forward_list 
implementation rather than introducing new functor like _Equal_to and 
others. I hope that as it is a C++0x container it is fine to use lambdas 
in its implementation.

  To reduce to its minimum the additional contention introduce by the 
debug mode the last modification to achieve would be to add a mutex 
instance to each safe sequence. But as it would break binary 
compatibility I simply changed _M_get_mutex_base function in debug.cc to 
handle a pool of mutexes that are associated to safe sequences depending 
on their memory address. Just tell me if you know a better technique to 
associate a safe sequence instance with a mutex one from the pool.

  Tested on x86_64 linux. Tested also for performance using performance 
testsuite with debug mode activated. After the patch performance are 
equivalent or better, on some tests performance are even much better, 
here is the most impressive enhancement:

Without the patch:
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::new_allocator<int> >
                                                                 669r  
665u    0s        0mem    0pf
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::malloc_allocator<int> >

                                                                 697r  
694u    0s        0mem    0pf
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::__mt_alloc<int, 
__gnu_cxx::__common_pool_policy<__gnu_cxx::__pool, false> > >
                                                                 620r  
618u    0s  6063408mem    0pf
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::bitmap_allocator<int> >

                                                                 578r  
576u    1s 37996912mem    0pf
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::__pool_alloc<int> >
                                                                 548r  
547u    0s  3837568mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::new_allocator<int> >
-thread                                                         8617r 
11847u 5056s     1216mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::malloc_allocator<int> >

-thread                                                         8355r 
11534u 4885s        0mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::__mt_alloc<int, 
__gnu_cxx::__common_pool_policy<__gnu_cxx::__pool, true> > >
-thread                                                         8664r 
11596u 5386s 24891808mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::bitmap_allocator<int> >

-thread                                                         7644r 
10619u 4408s 50607824mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::__pool_alloc<int> >
-thread                                                         8010r 
11055u 4691s 14601856mem    0pf


After the patch:
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::new_allocator<int> >
                                                                 171r  
169u    0s        0mem    0pf
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::malloc_allocator<int> >
                                                                 179r  
180u    0s        0mem    0pf
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::__mt_alloc<int, 
__gnu_cxx::__common_pool_policy<__gnu_cxx::__pool, false> > >
                                                                 125r  
125u    0s  6063408mem    0pf
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::bitmap_allocator<int> >
                                                                  88r   
87u    1s 37996912mem    0pf
sort_search                     type: std::__debug::list<int, 
__gnu_cxx::__pool_alloc<int> >
                                                                  77r   
77u    0s  3837568mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::new_allocator<int> >
-thread                                                          810r 
1554u   10s     1216mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::malloc_allocator<int> >
-thread                                                          820r 
1606u   13s        0mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::__mt_alloc<int, 
__gnu_cxx::__common_pool_policy<__gnu_cxx::__pool, true> > >
-thread                                                          693r 
1357u    7s 24891808mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::bitmap_allocator<int> >
-thread                                                          424r  
820u   11s 50607824mem    0pf
sort_search-thread              type: std::__debug::list<int, 
__gnu_cxx::__pool_alloc<int> >
-thread                                                          568r 
1102u    7s 14601856mem    0pf

François

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Attachment: performance.patch
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