On Sat, Mar 04, 2017 at 11:15:17AM -0800, H. Peter Anvin wrote:
> On 03/04/17 05:05, Russell King - ARM Linux wrote:
> >>
> >> +static int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
> >> +{
> >> + int op = (encoded_op >> 28) & 7;
> >> + int cmp = (encoded_op >> 24) & 15;
> >> + int oparg = (encoded_op << 8) >> 20;
> >> + int cmparg = (encoded_op << 20) >> 20;
> >
> > Hmm. oparg and cmparg look like they're doing these shifts to get sign
> > extension of the 12-bit values by assuming that "int" is 32-bit -
> > probably worth a comment, or for safety, they should be "s32" so it's
> > not dependent on the bit-width of "int".
> >
>
> For readability, perhaps we should make sign- and zero-extension an
> explicit facility?
There is some of this in already here, 32 and 64 bit versions:
include/linux/bitops.h
Do we really need zero extension? It seems the same.
Example implementation from bitops.h
static inline __s32 sign_extend32(__u32 value, int index)
{
__u8 shift = 31 - index;
return (__s32)(value << shift) >> shift;
}
> /*
> * Truncate an integer x to n bits, using sign- or
> * zero-extension, respectively.
> */
> static inline __const_func__ s32 sex32(s32 x, int n)
> {
> return (x << (32-n)) >> (32-n);
> }
>
> static inline __const_func__ s64 sex64(s64 x, int n)
> {
> return (x << (64-n)) >> (64-n);
> }
>
> #define sex(x,y) \
> ((__typeof__(x)) \
> (((__builtin_constant_p(y) && ((y) <= 32)) || \
> (sizeof(x) <= sizeof(s32))) \
> ? sex32((x),(y)) : sex64((x),(y))))
>
> static inline __const_func__ u32 zex32(u32 x, int n)
> {
> return (x << (32-n)) >> (32-n);
> }
>
> static inline __const_func__ u64 zex64(u64 x, int n)
> {
> return (x << (64-n)) >> (64-n);
> }
>
> #define zex(x,y) \
> ((__typeof__(x)) \
> (((__builtin_constant_p(y) && ((y) <= 32)) || \
> (sizeof(x) <= sizeof(u32))) \
> ? zex32((x),(y)) : zex64((x),(y))))
>
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