how to stop bit-field sign extension

how to stop bit-field sign extension

[Kevin Nomura]

What does it mean to have an unsigned bit-field if sign extension
can still occur in a case such as this:


[siml4]$ cat a.c
struct foo {
        unsigned int a:16;
} x;

main()
{
        unsigned long long ll;
        x.a = 0x8000;
        ll = x.a << 16;   /* (unsigned int) x.a does not help */
        printf ("%llx\n", ll);
}


[siml4]$ gcc a.c
[siml4]$ a.out
ffffffff80000000

The target of the assignment is unsigned, so the result of the shift 
of an unsigned bit field is evidently treated as signed.  

Are unsigned semantics of bit fields supported in ANSI C?
The standard mentions that bit fields can have unsigned type,
but are unsigned bit fields then allowed 'implementation
defined' behavior (meaning the unsigned-ness can be ignored)?

Replacing the 16-bit unsigned field with unsigned short avoids
the sign extension when widening to 64 bits.  So this doesn't
seem like a fundamental effect of integral promotion -- not a
consistent one, at least.

Re: how to stop bit-field sign extension

[Falk Hueffner]

Kevin Nomura <nomura@netapp.com> writes:

> What does it mean to have an unsigned bit-field if sign extension
> can still occur in a case such as this:
> 
> [siml4]$ cat a.c
> struct foo {
>         unsigned int a:16;
> } x;
> 
> main()
> {
>         unsigned long long ll;
>         x.a = 0x8000;
>         ll = x.a << 16;   /* (unsigned int) x.a does not help */
>         printf ("%llx\n", ll);
> }
> 
> [siml4]$ gcc a.c
> [siml4]$ a.out
> ffffffff80000000

The rule is very simple -- all values of unsigned int a:16 fit into an
int, so it is promoted to int, just like short or unsigned short are.
Therefore, you need the cast to unsigned. Unfortunately, it does not
help with gcc since due to of the most long-standing bugs in gcc
(http://gcc.gnu.org/PR3325), this cast gets ignored.  As a workaround,
try assigning the bitfield value to an unsigned variable first and
shift it then.

-- 
	Falk

Re: how to stop bit-field sign extension

[Eljay Love-Jensen]

Hi Kevin,

ll = x.a;
ll <<= 16;

HTH,
--Eljay

Re: how to stop bit-field sign extension

[Kevin Nomura]

I added a check to c-typeck.c to find where we are assigning 
unsigned long long from signed int, and this potential problem 
is widespread in our code base, albeit not usually from bit fields.  
Thank you for the response!


On Tue, Sep 30, 2003 at 06:32:09AM -0500, Eljay Love-Jensen wrote:
> Hi Kevin,
> 
> ll = x.a;
> ll <<= 16;
> 
> HTH,
> --Eljay
>