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: [email protected]
ReportedBy: [email protected]
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
