[off-list ref],Chris Metcalf [off-list ref],Thomas Gleixner [off-list ref],Ingo Molnar [off-list ref],Chris Zankel [off-list ref],Max Filippov [off-list ref],Arnd Bergmann [off-list ref],x86@kernel.org,linux-alpha@vger.kernel.org,linux-snps-arc@lists.infradead.org,linux-arm-kernel@lists.infradead.org,linux-hexagon@vger.kernel.org,linux-ia64@vger.kernel.org,linux-mips@linux-mips.org,openrisc@lists.librecores.org,linux-parisc@vger.kernel.org,linuxppc-dev@lists.ozlabs.org,linux-s390@vger.kernel.org,linux-sh@vger.kernel.org,sparclinux@vger.kernel.org,linux-xtensa@linux-xtensa.org,linux-arch@vger.kernel.org
From: hpa@zytor.com
Message-ID: [off-list ref]

On March 4, 2017 1:38:05 PM PST, Stafford Horne [off-list ref] wrote:

On Sat, Mar 04, 2017 at 11:15:17AM -0800, H. Peter Anvin wrote:

quoted

On 03/04/17 05:05, Russell King - ARM Linux wrote:

quoted

quoted

 
+static int futex_atomic_op_inuser(int encoded_op, u32 __user

*uaddr)

quoted

quoted

quoted

+{
+	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

quoted

quoted

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

quoted

quoted

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;
}

quoted

/*
 * 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...
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.