Re: Bug in bitfield handling?

[Andrew Pinski <pinskia at gmail dot com>]

On Fri, Sep 7, 2012 at 10:57 AM,  <Paul_Koning@dell.com> wrote:
> This seems to be a bug:
>
> struct bug
> {
>     int f1:1;
>     unsigned long long f2:31;
> };
>
> struct bug test = { 1, 0x8000ULL };
>
> int main (int c, char **v)
> {
>     unsigned long long tf2;
>
>     tf2 = test.f2 << 16;
>     if (tf2 == 0x80000000ULL)
>         return 0;
>     return 1;
> }
>
> Since the underlying type of field f2 is unsigned long long, I would not expect the expression "test.f2 << 16" to do sign extending of a 32 bit intermediate result.  But that is in fact what GCC produces, for x86_64-linux (gcc 4.7.0).
>
> If I explicitly cast test.f2 to unsigned long long before the shift, the expected result appears.  But I shouldn't have to do that cast, given that the type of f2 is already unsigned long long, correct?

The type of  "test.f2" is a 31 bit unsigned integer (the declared type
is used for alignment) and that fits in a 32bit signed integer so when
doing test.f2 << 16, it is promoted to only an 32bit signed integer.

Thanks,
Andrew

>
>         paul
>