[Bug c/58019] New: Problem with -O3 optimization

[william.jordan at vbrick dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58019

            Bug ID: 58019
           Summary: Problem with -O3 optimization
           Product: gcc
           Version: 4.7.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: william.jordan at vbrick dot com

I wrote a simple program to solve the 8 queens chess problem (i.e. put 8 queens
on a chessboard so that no queen can take any other queen).  When I compile the
program normally or with -O1 -O2 optimization it finds solutions and prints
them out.  When I compile with -O3 optimization it finds no solutions.

Below is the compiler version and source code.  This was also reproduced using
gcc 4.1.2.

gcc -v output:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.7/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.7.3-1ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.7 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin
--with-system-zlib --enable-objc-gc --with-cloog --enable-cloog-backend=ppl
--disable-cloog-version-check --disable-ppl-version-check --enable-multiarch
--disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.7.3 (Ubuntu/Linaro 4.7.3-1ubuntu1) 


Source code to reproduce problem:
#include <stdio.h>


int TestPiece(int ipos, int jpos, int board[8][8])
{
  int i, j;

  for (i = 0; i < 8; i++) {
    for (j = 0; j < 8; j++) {
      if (board[i][j] && (i != ipos || j!= jpos)) {
    if (i == ipos) {
      return(1);
    } else if (j == jpos) {
      return(2);
    } else if ((ipos - i == jpos - j) || (i - ipos == jpos - j)) {
      return(3);
    }
      }
    }
  }
  return(0);
}


int TestBoard(int board[8][8])
{
  int i, j;

  for (i = 0; i < 8; i++) {
    for (j = 0; j < 8; j++) {
      if (board[i][j]) {
    if (TestPiece(i, j, board)) {
      return(1);
    }
      }
    }
  }
}


int PrintBoard(int board[8][8])
{
  int i, j;

  printf("Solution found\n\n");
  for (i = 0; i < 8; i++) {
    for (j = 0; j < 8; j++) {
      if (board[i][j]) {
    printf("Q");
      } else {
    printf("-");
      }
    }
    printf("\n");
  }
  printf("\n");
}


int main(int argc, char *argv[])
{
  int board[8][8];
  int i, j, k, numchecks = 1;

  for (i = 0; i < 8; i++) {
    numchecks *= 8;
    for (j = 0; j < 8; j++) {
      if (j == 0) {
    board[i][j] = 1;
      } else {
    board[i][j] = 0;
      }
    }
  }


  for (i = 0; i < numchecks; i++) {
    if (!TestBoard(board)) {
      PrintBoard(board);
    }
    for (j = 7; j >= 0; j--) {
      if (board[j][7]) {
    board[j][7] = 0;
    board[j][0] = 1;
      } else {
    for (k = 0; k < 7; k++) {
      if (board[j][k]) {
        board[j][k] = 0;
        board[j][k + 1] = 1;
        break;
      }
    }
    break;
      }
    }
  }
}