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: David S. Miller <davem@davemloft.net>
diff --git a/arch/sparc/lib/usercopy.c b/arch/sparc/lib/usercopy.c
index 851cb75..87f9645 100644
--- a/arch/sparc/lib/usercopy.c
+++ b/arch/sparc/lib/usercopy.c
@@ -11,35 +11,20 @@
#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)
+static inline long find_zero(unsigned long mask)
{
- return ((a - REPEAT_BYTE(0x01)) & ~a) & REPEAT_BYTE(0x80);
-}
-
-static inline long find_zero(unsigned long c)
-{
+ 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 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 @@
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);
}