https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125415
Bug ID: 125415
Summary: [g++][OpenMP] unexpected behavior with iterator loop
update "it = it - 1U" in omp parallel for
Product: gcc
Version: 16.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: itou.tetsuya at fujitsu dot com
Target Milestone: ---
The following C++ program uses a std::vector<int>::iterator in a decreasing
loop.
With OpenMP enabled, the loop using:
it = it - 1U
does not execute as expected, and cnt remains 0.
Equivalent forms work correctly:
- without -fopenmp
- with "it = it - 1"
- with "it -= 1U"
This suggests an OpenMP loop handling/code generation bug for the iterator
update expression "it = it - 1U".
Test case:
#include <iostream>
#include <vector>
#include <omp.h>
int main() {
const int n = 10;
std::vector<int> vec(n);
int cnt = 0;
#pragma omp parallel for schedule(static, 1)
for (std::vector<int>::iterator it = vec.end();
it > vec.begin();
it = it - 1U) {
#pragma omp atomic
cnt++;
}
if (cnt != n)
std::cout << "test2 ng: it=it-1U: cnt=" << cnt << std::endl;
std::cout << "pass" << std::endl;
return 0;
}
Expected result:
pass
Actual result:
test2 ng: it=it-1U: cnt=0
pass
How to reproduce:
g++ -fopenmp test.cc -o test
./test
https://godbolt.org/z/Mv138Pzq9
