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From: "likan_999.student at sina dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/54075] New: [4.7.1] unordered_map 3x slower than 4.6.2
Date: Mon, 23 Jul 2012 23:07:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54075

             Bug #: 54075
           Summary: [4.7.1] unordered_map 3x slower than 4.6.2
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: likan_999.student@sina.com


The following piece of code shows 3x performance regression from 4.6.2 to
4.7.1.  The reason should come from the change in libstdc++.  The code is
compiled with -O2 -std=c++0x. -O3 doesn't make much difference; with or without
the call to reserve doesn't make much difference.  The attachment contains
profiling using google-perftool.  Looks like the major cost comes from the
rehashing.  Does anyone aware of the issue, or I am the first one to report
this?


#include <unordered_map>

using namespace std;

int main() {
    const int N = 10000000;
    unordered_map<int, int> m;
    m.reserve(2 * N);
    for (int i = 0; i < N; i++) {
        m[i] = i;
    }
}


Timing:
[hidden]$ time ./a-4.6.2.out 

real    0m1.029s
user    0m0.787s
sys    0m0.239s
[hidden]$ time ./a-4.7.1.out 

real    0m3.364s
user    0m2.596s
sys    0m0.761s