gcc gives inccorect value for simple expression

[chen]

The following code, when compiled with gcc, it outputs "7".

I have tried it with gcc-4.8.3 and gcc-6.0.1-20160415, both on x86_64 
linux, with options
-O0, -g, -O2. All of them gave the same answer.

When compiled with clang, it gives 6.

I know that it is an ill coded program, but I still can't understand why 
it outputs 7 when compiled by gcc.

/******************************************/
#include <stdio.h>

int main()
{
  int i;

  i = 0;

  printf("%d\n", (++i) + (++i) + (++i));

  return 0;
}