The body of a while loop that operates on a stack-allocated array is optimized
away. Turning optimizations off gives correct bahviour. I observed this bug with
gcc 3.2.2 on FreeBSD, other people have confirmed this for gcc 3.3.3 20031229 on
Debian.

This program:
                                                                                  
                                                                                  
#include<stdio.h>
                                                                                  
void runBug(int lastElem) {
  int localArray[lastElem];
  int j;
  for(j=0; j<lastElem; j++) localArray[j]=j;
                                                                                  
  int temp=localArray[0];
  size_t current=0;
  while (current<lastElem)
    localArray[current]=localArray[++current];
  localArray[lastElem-1]=temp;
                                                                                  
  for(j=0; j<lastElem; j++) printf("%i, ",localArray[j]);
};
                                                                                  
int main() {
  runBug(4);
  return 0;
};
                                                                                  
compiled with:
                                                                                  
gcc -O0 bug.c
                                                                                  
gives
                                                                                  
./a.out
1, 2, 3, 0,
                                                                                  
(the while loop shuffles the list around, which is what I want). However
with optimizations the loop reduces to incrementing the loop counter and
nothing else:
                                                                                  
gcc -O2 bug.c
                                                                                  
gives:
                                                                                  
./a.out
0, 1, 2, 0,
                                                                                  
Thanks,
Axel.