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sparc: Optimize strncpy_from_user() zero byte search.

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Compute a mask that will only have 0x80 in the bytes which
had a zero in them.  The formula is:

	~(((x & 0x7f7f7f7f) + 0x7f7f7f7f) | x | 0x7f7f7f7f)

In the inner word iteration, we have to compute the "x | 0x7f7f7f7f"
part, so we can reuse that in the above calculation.

Once we have this mask, we perform divide and conquer to find the
highest 0x80 location.

Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David S. Miller 2012-05-23 19:20:20 -07:00
parent ff06dffbc8
commit 4efcac3a24
1 changed files with 21 additions and 29 deletions
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50
arch/sparc/lib/usercopy.c
View file

@ -11,35 +11,20 @@ EXPORT_SYMBOL(copy_from_user_overflow);
#define REPEAT_BYTE(x) ((~0ul / 0xff) * (x))
/* Return the high bit set in the first byte that is a zero */
static inline unsigned long has_zero(unsigned long a)
{
return ((a - REPEAT_BYTE(0x01)) & ~a) & REPEAT_BYTE(0x80);
}
static inline long find_zero(unsigned long c)
static inline long find_zero(unsigned long mask)
{
long byte = 0;
#ifdef CONFIG_64BIT
if (!(c & 0xff00000000000000UL))
return 0;
if (!(c & 0x00ff000000000000UL))
return 1;
if (!(c & 0x0000ff0000000000UL))
return 2;
if (!(c & 0x000000ff00000000UL))
return 3;
#define __OFF 4
#else
#define __OFF 0
if (mask >> 32)
mask >>= 32;
else
byte = 4;
#endif
if (!(c & 0xff000000))
return __OFF + 0;
if (!(c & 0x00ff0000))
return __OFF + 1;
if (!(c & 0x0000ff00))
return __OFF + 2;
return __OFF + 3;
#undef __OFF
if (mask >> 16)
mask >>= 16;
else
byte += 2;
return (mask >> 8) ? byte : byte + 1;
}
/*
@ -50,6 +35,8 @@ static inline long find_zero(unsigned long c)
*/
static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
{
const unsigned long high_bits = REPEAT_BYTE(0xfe) + 1;
const unsigned long low_bits = REPEAT_BYTE(0x7f);
long res = 0;
/*
@ -63,14 +50,19 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src, long
goto byte_at_a_time;
while (max >= sizeof(unsigned long)) {
unsigned long c;
unsigned long c, v, rhs;
/* Fall back to byte-at-a-time if we get a page fault */
if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
break;
rhs = c | low_bits;
v = (c + high_bits) & ~rhs;
*(unsigned long *)(dst+res) = c;
if (has_zero(c))
return res + find_zero(c);
if (v) {
v = (c & low_bits) + low_bits;;
v = ~(v | rhs);
return res + find_zero(v);
}
res += sizeof(unsigned long);
max -= sizeof(unsigned long);
}