[Bug libstdc++/23956] New: Class __mt_alloc overexerts __policy_type::_S_get_pool

[Bug libstdc++/23956] New: Class __mt_alloc overexerts __policy_type::_S_get_pool

[guillaume dot melquiond at ens-lyon dot fr]

When executing the following snippet:

  #include <list>
  int main() { std::list<int> l; }

the function __common_pool_policy<__pool,true>::_S_get_pool() is called three
times! Yet the result of the call is discarded each time. As a matter of fact,
no memory has even been allocated by the program, so the pool is useless. These
three calls are due to the __mt_alloc class. Excerpt from ext/mt_alloc.h:655:

      __mt_alloc() throw()
      { __policy_type::_S_get_pool(); }
      __mt_alloc(const __mt_alloc&) throw()
      { __policy_type::_S_get_pool(); }

I didn't see anything in the online documentation ext/mt_allocator.html that
states that _S_get_pool must be called three times before _S_initialize_once can
even be called (this function is called by allocate). In fact, calling
_S_get_pool before the pool is initialized could be seen as a bug somehow.

Removing these useless __mt_alloc constructors would speed up both compile time
and execution time of programs using STL containers. The pool would then be
created by _S_initialize_once the first time allocate is called.

-- 
           Summary: Class __mt_alloc overexerts __policy_type::_S_get_pool
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: guillaume dot melquiond at ens-lyon dot fr
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23956

[Bug libstdc++/23956] Class __mt_alloc overexerts __policy_type::_S_get_pool

[guillaume dot melquiond at ens-lyon dot fr]

------- Additional Comments From guillaume dot melquiond at ens-lyon dot fr  2005-09-18 21:54 -------
Some additional information. Looking through the CVS log, it appears the
__mt_alloc constructors were empty in GCC 3.4, and they were filled when fixing
PR 17780. The patch was huge and the log is a bit sparse as to why this part was
needed: "(__mt_alloc::__mt_alloc): Call __policy_type::_S_get_pool." The related
thread on the libstdc++ mailing-list does not help either.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23956

[Bug libstdc++/23956] Class __mt_alloc overexerts __policy_type::_S_get_pool

[guillaume dot melquiond at ens-lyon dot fr]

------- Additional Comments From guillaume dot melquiond at ens-lyon dot fr  2005-09-20 05:07 -------
Created an attachment (id=9779)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=9779&action=view)
Remove gratuitous calls to _S_get_pool in __mt_alloc

Since I didn't find any reason for these calls, I removed them (see attached
patch) and ran a few tests. The following testcase has been compiled at -O3
-march=pentium3.

#include <list>

int main() {
  for(int j = 0; j < 10000000; ++j) {
    std::list<int> a, b, c;
    std::list<double> d, e, f;
    std::list<char> g, h, i;
  }
}

Without the calls, the list creation is 70% faster: 2.2s instead of 3.7s on my
computer. When adding an element into each list at each iteration, there still
is a 15% speedup: 15.4s instead of 17.8s. So the cost of these calls to
_S_get_pool is not negligible. And the other STL containers are affected as
well, not just std::list.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23956

[Bug libstdc++/23956] Class __mt_alloc overexerts __policy_type::_S_get_pool

[bkoz at gcc dot gnu dot org]

------- Additional Comments From bkoz at gcc dot gnu dot org  2005-09-21 16:09 -------

I suspect this is an oversight from early patches of PR 17780. I'll look at this.

-benjamin

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23956

[Bug libstdc++/23956] Class __mt_alloc overexerts __policy_type::_S_get_pool

[bkoz at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |bkoz at gcc dot gnu dot org
                   |dot org                     |
             Status|UNCONFIRMED                 |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23956

[Bug libstdc++/23956] Class __mt_alloc overexerts __policy_type::_S_get_pool

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-09-22 08:49 -------
Subject: Bug 23956

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	bkoz@gcc.gnu.org	2005-09-22 08:48:33

Modified files:
	libstdc++-v3   : ChangeLog 
	libstdc++-v3/include/ext: mt_allocator.h 

Log message:
	2005-09-21  Guillaume Melquiond  <guillaume.melquiond@ens-lyon.fr>
	
	PR libstdc++/23956
	* include/ext/mt_allocator.h: Remove excess
	policy_type::_S_get_pool calls in constructors.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/ChangeLog.diff?cvsroot=gcc&r1=1.3110&r2=1.3111
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/include/ext/mt_allocator.h.diff?cvsroot=gcc&r1=1.48&r2=1.49



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23956

[Bug libstdc++/23956] Class __mt_alloc overexerts __policy_type::_S_get_pool

[bkoz at gcc dot gnu dot org]

------- Additional Comments From bkoz at gcc dot gnu dot org  2005-09-22 08:50 -------

in on mainline, queued for 4.0.3

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23956

[Bug libstdc++/23956] Class __mt_alloc overexerts __policy_type::_S_get_pool

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-09-30 20:41 -------
Subject: Bug 23956

CVSROOT:	/cvs/gcc
Module name:	gcc
Branch: 	gcc-4_0-branch
Changes by:	bkoz@gcc.gnu.org	2005-09-30 20:41:41

Modified files:
	libstdc++-v3   : ChangeLog 
	libstdc++-v3/include/ext: mt_allocator.h 

Log message:
	2005-09-30  Guillaume Melquiond  <guillaume.melquiond@ens-lyon.fr>
	
	PR libstdc++/23956
	* include/ext/mt_allocator.h: Remove excess
	policy_type::_S_get_pool calls in constructors.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-4_0-branch&r1=1.2917.2.89&r2=1.2917.2.90
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/include/ext/mt_allocator.h.diff?cvsroot=gcc&only_with_tag=gcc-4_0-branch&r1=1.45.8.3&r2=1.45.8.4



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23956

[Bug libstdc++/23956] Class __mt_alloc overexerts __policy_type::_S_get_pool

[bkoz at gcc dot gnu dot org]

------- Additional Comments From bkoz at gcc dot gnu dot org  2005-09-30 20:42 -------

Fixed in 4.0.3.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.0.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23956