Re: [PATCH 1/3] futex: remove duplicated code

[Stafford Horne]

On Sat, Mar 04, 2017 at 03:08:50PM -0800, H. Peter Anvin wrote:
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> From: hpa@xxxxxxxxx
> Message-ID: <CF18535E-39E7-44D3-88D0-80B9961E6681@xxxxxxxxx>
> 
> On March 4, 2017 1:38:05 PM PST, Stafford Horne <shorne@xxxxxxxxx> 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.