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

[H. Peter Anvin]

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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...
-- 
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