C++ fatal error C1001: An internal error has occurred in the compiler with openMP

Question

I have a program that solves sudoku puzzles and I've gotten it to work sequentially, but now I'm trying to parallelize it using openMP. The function solvePuzzle() includes the algorithm and I want to parallelize the for loop within it however when I add #pragma omp parallel for statement before my for loop i get this error: fatal error C1001: An internal error has occurred in the compiler.

The code for the function is solvePuzzle():

    bool sudoku::solvePuzzle(int grid[CELL][CELL]) {
        int row, col;

        if (!findEmptyCell(grid, row, col))
            return true; 
    #pragma omp parallel for
        for (int num = 1; num <= 9; num++) {
            if (checkAccuracy(grid, row, col, num)) {
                grid[row][col] = num;

                if (solvePuzzle(grid))
                    return true;

                grid[row][col] = EMPTY_CELL;
            }
        }
        return false;
    }

Here is the driver with main if it helps:

#include "SudokuGrid.h"

using namespace std;

int main() {

    sudoku sudoku;
    clock_t t1, t2;
    t1 = clock();

    if (sudoku.readPuzzle("1.txt")) {
        sudoku.printGrid(sudoku.grid);
    }
    else {
        cout << "Incorrect file" << endl;
    }
    cout << endl;
    if (sudoku.solvePuzzle(sudoku.grid) == true)
        sudoku.printGrid(sudoku.grid);
    else
        printf("No solution exists");

    t2 = clock();
    printf("Time to execute = %1d ms\n", (t2 - t1));

    return 0;
}

Full error when built:

1>------ Build started: Project: Sudoku, Configuration: Debug Win32 ------
1>SudokuGrid.cpp
1>c:\users\john\source\repos\sudoku\sudoku\sudokugrid.cpp(8): fatal error 
C1001: An internal error has occurred in the compiler.
1>(compiler file 'f:\dd\vctools\compiler\utc\src\p2\main.c', line 258)
1> To work around this problem, try simplifying or changing the program near 
the locations listed above.
1>Please choose the Technical Support command on the Visual C++
1> Help menu, or open the Technical Support help file for more information
1>
1>Done building project "Sudoku.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

To summarize, the problem is returning from inside the parallelized for loop.

#pragma omp parallel for
    for (int num = 1; num <= 9; num++) {
            ...
            if (solvePuzzle(grid))
                return true;           // BAD

            ...
        }
    }

"OpenMP requires that a loop construct processes each iteration. Breaking out of the loop (using return, goto, break, throw or other means) is not allowed."

-- https://bisqwit.iki.fi/story/howto/openmp/

So the solution is to let the loop loop through all iterations (See also Breaking Out of Loops).

bool solvePuzzle(int grid[CELL][CELL]) {
    bool solved = false;
    int row, col;

    if (!findEmptyCell(grid, row, col))
        return true;

    #pragma omp parallel for
    for (int num = 1; num <= 9; num++) {
        #pragma omp flush (solved)
        if (!solved && checkAccuracy(grid, row, col, num)) {
            grid[row][col] = num;

            if (solvePuzzle(grid)) {
                solved = true;
                #pragma omp flush (solved)
            } else {
                grid[row][col] = EMPTY_CELL;
            }
        }
    }
    return solved;
}

Repro

Demo