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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>
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1 changed files with 21 additions and 29 deletions
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@ -11,35 +11,20 @@ EXPORT_SYMBOL(copy_from_user_overflow);
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#define REPEAT_BYTE(x) ((~0ul / 0xff) * (x))
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/* Return the high bit set in the first byte that is a zero */
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static inline unsigned long has_zero(unsigned long a)
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{
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return ((a - REPEAT_BYTE(0x01)) & ~a) & REPEAT_BYTE(0x80);
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}
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static inline long find_zero(unsigned long c)
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static inline long find_zero(unsigned long mask)
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{
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long byte = 0;
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#ifdef CONFIG_64BIT
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if (!(c & 0xff00000000000000UL))
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return 0;
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if (!(c & 0x00ff000000000000UL))
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return 1;
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if (!(c & 0x0000ff0000000000UL))
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return 2;
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if (!(c & 0x000000ff00000000UL))
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return 3;
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#define __OFF 4
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#else
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#define __OFF 0
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if (mask >> 32)
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mask >>= 32;
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else
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byte = 4;
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#endif
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if (!(c & 0xff000000))
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return __OFF + 0;
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if (!(c & 0x00ff0000))
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return __OFF + 1;
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if (!(c & 0x0000ff00))
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return __OFF + 2;
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return __OFF + 3;
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#undef __OFF
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if (mask >> 16)
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mask >>= 16;
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else
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byte += 2;
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return (mask >> 8) ? byte : byte + 1;
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}
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/*
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@ -50,6 +35,8 @@ static inline long find_zero(unsigned long c)
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*/
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static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
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{
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const unsigned long high_bits = REPEAT_BYTE(0xfe) + 1;
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const unsigned long low_bits = REPEAT_BYTE(0x7f);
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long res = 0;
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/*
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@ -63,14 +50,19 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src, long
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goto byte_at_a_time;
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while (max >= sizeof(unsigned long)) {
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unsigned long c;
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unsigned long c, v, rhs;
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/* Fall back to byte-at-a-time if we get a page fault */
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if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
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break;
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rhs = c | low_bits;
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v = (c + high_bits) & ~rhs;
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*(unsigned long *)(dst+res) = c;
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if (has_zero(c))
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return res + find_zero(c);
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if (v) {
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v = (c & low_bits) + low_bits;;
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v = ~(v | rhs);
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return res + find_zero(v);
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}
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res += sizeof(unsigned long);
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max -= sizeof(unsigned long);
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}
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