GCC Bugzilla Bug 34454

Testcase :

#include <sys/types.h>
#include <stdio.h>

void foo(ssize_t x)
{
 if (x >= 0) {
   if (x+x < 0) printf("Overflow\n");
}
}

main()
{
  volatile ssize_t x =2147483647;
  foo(x);
}

When compiled with -O2 it doesn't print Overflow, if you omit -O2 it prints
overflow.
gcc 3.4.6 works fine so this is a regression.

This is not a bug, signed overflow is undefined so optimizing away this case is
correct as a positive + positive is always positive.

Use -fwarpv or what ever the strict overflow option is.

Or use unsigned types to do the addition.

FWIW this triggers a crash in Python 2.5, see http://bugs.python.org/issue1608

Note that this gives a warning with -Wstrict-overflow.

And note that you get the desired behaviour if you compile with
-fno-strict-overflow.

As Pinski says, the code is simply incorrect according to standard C, but you
can use the -Wstrict-overflow and -fno-strict-overflow options to help fix
existing code bases.

Guido agreed on using -fwrapv hence a patch submitted, thanks for the
diagnosis.