Re: GCC 3.1.1

[Jack Lloyd <lloyd at acm dot jhu dot edu>]

On Thu, 27 Jun 2002, Toon Moene wrote:

> Mark Mitchell wrote:
>
> > We have now got a pretty long list of regressions that we need to fix
> > before GCC 3.1.1.
>
> This is at least partly because Ordinary Users can set High Priority
> bugs (again) in GNATS.
>
> Consider 7139 for instance.  I'm not a C Guru, but shifting 1 over 64
> bits seems like invoking undefined behaviour to me (i.e., a User Error).

The problem in this code is that it is assuming promotion from 1 (which
IIRC, will be an int) to long long if the shift larger than #bits of int.
It seems so reasonable to expect, too. :( There is no warning because the
shift is a loop variable, 0...63

Basically:

unsigned long long x;
x = 1 << 32;

results in x == 0 rather than 0x100000000 as the code expects.

Simon: Changing "1 << i" to "(unsigned long long)1 << i" seems to fix the
problem you're having. Here is the output of your program when i = 32 with
those casts in:

yuck(32):  y = 1ffffffff, a[0] = ffffffff, a[1] = 1
fooie(32):  y = 100000000, a[0] = 0, a[1] = 1

which looks to be what you were expecting.

-Jack