On Sat, Mar 04, 2017 at 03:08:50PM -0800, H. Peter Anvin wrote:
> <da...@davemloft.net>,Chris Metcalf <cmetc...@mellanox.com>,Thomas Gleixner
> <t...@linutronix.de>,Ingo Molnar <mi...@redhat.com>,Chris Zankel
> <ch...@zankel.net>,Max Filippov <jcmvb...@gmail.com>,Arnd Bergmann
> <a...@arndb.de>,x...@kernel.org,linux-al...@vger.kernel.org,linux-snps-...@lists.infradead.org,linux-arm-ker...@lists.infradead.org,linux-hexa...@vger.kernel.org,linux-i...@vger.kernel.org,linux-m...@linux-mips.org,openr...@lists.librecores.org,linux-par...@vger.kernel.org,linuxppc-...@lists.ozlabs.org,linux-s...@vger.kernel.org,linux...@vger.kernel.org,sparcli...@vger.kernel.org,linux-xte...@linux-xtensa.org,linux-a...@vger.kernel.org
> From: h...@zytor.com
> Message-ID: <cf18535e-39e7-44d3-88d0-80b9961e6...@zytor.com>
>
> On March 4, 2017 1:38:05 PM PST, Stafford Horne <sho...@gmail.com> wrote:
> >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))))
> >>
>
> Also, i strongly believe that making it syntactically cumbersome encodes
> people to open-code it which is bad...
Right, I missed the signed vs unsigned bit.
And it is cumbersome, this would be better
> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.
