Comment by teraflop | Hacker News Reader

Comment by teraflop | Hacker News Reader

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A very simple test case demonstrates that GCC can remove tests in the presence of signed overflow, even in ways that change a program's behavior.

    $ cat undefined.c
    #include <limits.h>
    #include <stdio.h>
    #include <stdlib.h>

    int main() {
        int x = INT_MAX;
        if (x+1 > x) {
            printf("%d > %d\n", x+1, x);
        } else {
            printf("overflow!\n");
        }
    }
    
    $ gcc --version
    gcc (Ubuntu 7.2.0-8ubuntu3.2) 7.2.0
    Copyright (C) 2017 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
    
    $ gcc undefined.c && ./a.out
    overflow!
    
    $ gcc -O3 undefined.c && ./a.out
    -2147483648 > 2147483647

Yes, that example is well-known but different; here, the compiler is assuming that x + 1 will always be greater than x, which is entirely something else than the parent's assertion of assuming that x + small_increment will always be positive.

The difference doesn't matter, you would know better if you weren't clinging so hard to your beliefs. Here's the "difference":

  $ cat undefined.c
  #include <limits.h>
  #include <stdio.h>
  #include <stdlib.h>

  int main() {
      int x = INT_MAX;
      if (x+1 < 0) {
          printf("%d < 0\n", x+1);
      } else {
          printf("overflow!\n");
      }
  }

  $ gcc --version
  gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609
  Copyright (C) 2015 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

  $ gcc undefined.c && ./a.out
  -2147483648 < 0

  $ gcc -O3 undefined.c && ./a.out
  overflow!

The security check is gone all the same.

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