Problem with std::stable_sort after g++ 9.1 update

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After updating from gcc/g++ 8.1 to 9.1 and recompiling my code, most of its tests failed. Hence, a little digging was made and I have found out that std::stable_sort was the problem.

As it turns out, most calls of my calls to std::stable_sort were not necessary, that is, calling std::sort would be enough. Thus, I have done the substitution where it was possible, and the tests regarding these pieces of code succeed again.

Now, I have only one call to std::stable_sort

void MshReader::determinePhysicalEntitiesRange() {
    // conns is not empty

    std::stable_sort(this->conns.begin(), this->conns.end(), 
        [=](const auto& a, const auto& b){
            return a[this->index] < b[this->index];
        }
    );

    // acess some values of conns
}

Where conns is a std::vector<std::vector<int>> that stores element connectivities. The sorting is done based on column index, its value is assigned in the class header, and all std::vector<int> in conns have that entry.

Another fact worth mentioning is that, on debug builds (compiler flag "-g" is used, "-O3" is NOT) all tests succeed.

Also, on release builds (flag "-O3" is used, "-g" is NOT), by printing the values of conns before and after the call to std::stable_sort, I have discovered that conns is ruined.

Before

row:
  0:    0   2   0   1
  1:    0   2   1   2
  2:    0   1   2   5
  3:    0   1   5   8
  4:    0   3   8   7
  5:    0   3   7   6
  6:    0   0   6   3
  7:    0   0   3   0
  8:    1   4   3   4   9
  9:    1   4   3   9   6
 10:    1   4   4   7   9
 11:    1   4   6   9   7
 12:    1   4   1   2  10
 13:    1   4   1  10   4
 14:    1   4   2   5  10
 15:    1   4   4  10   5
 16:    2   4   4   5   8   7
 17:    2   4   0   1   4   3

After

row:
  0:    0   0   6   3
  1:    0   0   3   0
  2:    0   1   2   5
  3:    0   1   5   8
  4:    0   2   1   2 // there were two rows with column 'index' = 2
  5:    0   3   8   7
  6:    0   3   7   6
  7:    1   4   2   5  10  10  10  10 // this entry was previously on row 14; extra '10's
  8:    1   4   3   4   9
  9:    1   4   3   9   6
 10:    1   4   4   7   9
 11:    1   4   6   9   7
 12:    1   4   1   2  10
 13:    1   4   1  10   4
 14:    1   4   2   5  10
 15:    1   4   4  10   5
 16:    2   4   4   5   8   7
 17:    2   4   0   1   4   3

On debug builds, std::stable_sort outputs the expected result. Besides that, c++17 is being used (compiler flag "-std=c++17").

Therefore,

  • Is something wrong with my call to std::stable_sort?

  • Which changes on g++ led to this behavior?

  • Why this behavior only appears in release builds?

Minimal Example

#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>

template<typename InputIt>
void print2D(InputIt cbegin, InputIt cend, std::string&& message) {
    std::cout << message;
    for (auto i = cbegin; i != cend; ++i) {
        for (auto j = i->cbegin(); j != i->cend(); ++j) {
            std::cout << "\t" << std::setw(3) << std::right << *j;
        }
        std::cout << std::endl;
    }
    std::cout << std::endl;
}

int main() {
    int index = 1;
    std::vector<std::vector<int>> conns{{0,2,0,1},{0,2,1,2},{0,1,2,5},{0,1,5,8},{0,3,8,7},{0,3,7,6},{0,0,6,3},{0,0,3,0},{1,4,3,4,9},{1,4,3,9,6},{1,4,4,7,9},{1,4,6,9,7},{1,4,1,2,10},{1,4,1,10,4},{1,4,2,5,10},{1,4,4,10,5},{2,4,4,5,8,7},{2,4,0,1,4,3}};

    print2D(conns.cbegin(), conns.cend(), "\n\n\tbefore\n");

    std::stable_sort(conns.begin(), conns.end(), 
        [=](const auto& a, const auto& b){
            return a[index] < b[index];
        }
    );

    print2D(conns.cbegin(), conns.cend(), "\n\n\tafter\n");

    return 0;
}

If the above is compiled with

    g++ -o main main.cpp -m64 -std=c++17 -O3

It outputs segmentation fault (core dumped). However, if the flag "-O3" is not employed, the expected results is obtained.

c++17 g++ std c++
1 Answers

The first "b" that, when compiled with g++ 9.1.1, is passed into the lambda has size()==0 which should be impossible. clang++ 8.0.0 runs it without problems using the same compilation flags and does not show anything strange when running valgrind.

I'd say it's a bug in g++. Code with debug output in the lambda function:

#include <algorithm>
#include <iomanip>
#include <iostream>
#include <vector>

template<typename InputIt>
void print2D(InputIt cbegin, InputIt cend, const std::string&& message) {
    std::cout << message;
    for(auto i = cbegin; i != cend; ++i) {
        for(auto j : *i) {
            std::cout << "\t" << std::setw(3) << std::right << j;
        }
        std::cout << "\n";
    }
    std::cout << "\n";
}

int main() {
    size_t index = 1;
    std::vector<std::vector<int>> conns{
        {0, 2, 0, 1},       {0, 2, 1, 2},      {0, 1, 2, 5},     {0, 1, 5, 8},
        {0, 3, 8, 7},       {0, 3, 7, 6},      {0, 0, 6, 3},     {0, 0, 3, 0},
        {1, 4, 3, 4, 9},    {1, 4, 3, 9, 6},   {1, 4, 4, 7, 9},  {1, 4, 6, 9, 7},
        {1, 4, 1, 2, 10},   {1, 4, 1, 10, 4},  {1, 4, 2, 5, 10}, {1, 4, 4, 10, 5},
        {2, 4, 4, 5, 8, 7}, {2, 4, 0, 1, 4, 3}};

    print2D(conns.cbegin(), conns.cend(), "\n\n\tbefore\n");

    std::stable_sort(conns.begin(), conns.end(), [=](const auto& a, const auto& b) {
        std::cout << index << "\ta.size=" << a.size() << " b.size=" << b.size() << "\n";
        return a[index] < b[index];
    });

    print2D(conns.cbegin(), conns.cend(), "\n\n\tafter\n");
}

Output:

   before
     0    2     0   1
     0    2     1   2
     0    1     2   5
     0    1     5   8
     0    3     8   7
     0    3     7   6
     0    0     6   3
     0    0     3   0
     1    4     3   4    9
     1    4     3   9    6
     1    4     4   7    9
     1    4     6   9    7
     1    4     1   2   10
     1    4     1  10    4
     1    4     2   5   10
     1    4     4  10    5
     2    4     4   5    8      7
     2    4     0   1    4      3

1  a.size=4 b.size=0
Segmentation fault (core dumped)
user_75822470
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