[Bug libstdc++/61667] New: setting max_load_factor of unordered_map cause buckets shrink

[tinrow at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61667

            Bug ID: 61667
           Summary: setting max_load_factor of unordered_map cause buckets
                    shrink
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tinrow at gmail dot com

I want to build an unordered_map with at least 200000 initial buckets and set
max load factor to 0.5. The bucket count shrinks to 2 during call to
max_load_factor.

Reproduce:
$ cat 1.cpp 
#include <unordered_map>
#include <iostream>

using namespace std;

int main () {
  unordered_map<int, int> m(200000);
  cout << m.bucket_count() << endl;
  m.max_load_factor(0.5);
  cout << m.bucket_count() << endl;
  return 0;
}

$ g++ -std=c++11 1.cpp 
$ ./a.out 
202409
2

$ g++ --version
g++ (GCC) 4.8.2

In the ISO/IEC 14882:2011 standard, here is a possible "increase" but no
"shrink".

b.max_load_factor() float
Returns a positive number that the container attempts to keep the load factor
less than or equal to. The container automatically increases the number of
buckets as necessary to keep the load factor below this number.