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Commit 4efcac3a authored by David S. Miller's avatar David S. Miller
Browse files

sparc: Optimize strncpy_from_user() zero byte search. 



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: default avatarDavid S. Miller <davem@davemloft.net>
parent ff06dffb

arch/sparc/lib/usercopy.c

+21 −29
Original line number Diff line number Diff line
@@ -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);

	}