Re: GCC optimizes integer overflow: bug or feature?

[Paolo Bonzini]

>       int j;
>       for (j = 1; 0 < j; j *= 2)
>         if (! bigtime_test (j))
>           return 1;
>
> Here it is obvious to a programmer that the comparison is
> intended to do overflow checking, even though the test
> controls the loop.

Well, it's not to me. :-)

> Another question for the GCC experts: would it fix the bug
> if we replaced "j *= 2" with "j <<= 1" in this sample code?

Yes, it will actually compile the code as this:

  int i, j;
  for (i = 0, j = 1; i < 31; i++)
    j <<= 1;

Or you can do, since elsewhere in the code you compute time_t_max:

  for (j = 1; j <= time_t_max / 2 + 1; j *= 2)

Which is IMO more intention revealing.

Paolo