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Commit 36126f8f authored by Linus Torvalds's avatar Linus Torvalds
Browse files

word-at-a-time: make the interfaces truly generic 



This changes the interfaces in <asm/word-at-a-time.h> to be a bit more
complicated, but a lot more generic.

In particular, it allows us to really do the operations efficiently on
both little-endian and big-endian machines, pretty much regardless of
machine details.  For example, if you can rely on a fast population
count instruction on your architecture, this will allow you to make your
optimized <asm/word-at-a-time.h> file with that.

NOTE! The "generic" version in include/asm-generic/word-at-a-time.h is
not truly generic, it actually only works on big-endian.  Why? Because
on little-endian the generic algorithms are wasteful, since you can
inevitably do better. The x86 implementation is an example of that.

(The only truly non-generic part of the asm-generic implementation is
the "find_zero()" function, and you could make a little-endian version
of it.  And if the Kbuild infrastructure allowed us to pick a particular
header file, that would be lovely)

The <asm/word-at-a-time.h> functions are as follows:

 - WORD_AT_A_TIME_CONSTANTS: specific constants that the algorithm
   uses.

 - has_zero(): take a word, and determine if it has a zero byte in it.
   It gets the word, the pointer to the constant pool, and a pointer to
   an intermediate "data" field it can set.

   This is the "quick-and-dirty" zero tester: it's what is run inside
   the hot loops.

 - "prep_zero_mask()": take the word, the data that has_zero() produced,
   and the constant pool, and generate an *exact* mask of which byte had
   the first zero.  This is run directly *outside* the loop, and allows
   the "has_zero()" function to answer the "is there a zero byte"
   question without necessarily getting exactly *which* byte is the
   first one to contain a zero.

   If you do multiple byte lookups concurrently (eg "hash_name()", which
   looks for both NUL and '/' bytes), after you've done the prep_zero_mask()
   phase, the result of those can be or'ed together to get the "either
   or" case.

 - The result from "prep_zero_mask()" can then be fed into "find_zero()"
   (to find the byte offset of the first byte that was zero) or into
   "zero_bytemask()" (to find the bytemask of the bytes preceding the
   zero byte).

   The existence of zero_bytemask() is optional, and is not necessary
   for the normal string routines.  But dentry name hashing needs it, so
   if you enable DENTRY_WORD_AT_A_TIME you need to expose it.

This changes the generic strncpy_from_user() function and the dentry
hashing functions to use these modified word-at-a-time interfaces.  This
gets us back to the optimized state of the x86 strncpy that we lost in
the previous commit when moving over to the generic version.

Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 4ae73f2d

arch/openrisc/include/asm/Kbuild

+1 −0
Original line number Diff line number Diff line
@@ -66,3 +66,4 @@ generic-y += topology.h 
generic-y += types.h

generic-y += ucontext.h

generic-y += user.h

generic-y += word-at-a-time.h

arch/sparc/include/asm/Kbuild

+1 −0
Original line number Diff line number Diff line
@@ -21,3 +21,4 @@ generic-y += div64.h 
generic-y += local64.h

generic-y += irq_regs.h

generic-y += local.h

generic-y += word-at-a-time.h

arch/x86/include/asm/word-at-a-time.h

+29 −3
Original line number Diff line number Diff line
@@ -10,6 +10,11 @@ 
 * bit count instruction, that might be better than the multiply

 * and shift, for example.

 */

struct word_at_a_time {

	const unsigned long one_bits, high_bits;

};



#define WORD_AT_A_TIME_CONSTANTS { REPEAT_BYTE(0x01), REPEAT_BYTE(0x80) }



#ifdef CONFIG_64BIT
@@ -37,10 +42,31 @@ static inline long count_masked_bytes(long mask) 


#endif



/* Return the high bit set in the first byte that is a zero */

static inline unsigned long has_zero(unsigned long a)

/* Return nonzero if it has a zero */

static inline unsigned long has_zero(unsigned long a, unsigned long *bits, const struct word_at_a_time *c)

{

	unsigned long mask = ((a - c->one_bits) & ~a) & c->high_bits;

	*bits = mask;

	return mask;

}



static inline unsigned long prep_zero_mask(unsigned long a, unsigned long bits, const struct word_at_a_time *c)

{

	return bits;

}



static inline unsigned long create_zero_mask(unsigned long bits)

{

	bits = (bits - 1) & ~bits;

	return bits >> 7;

}



/* The mask we created is directly usable as a bytemask */

#define zero_bytemask(mask) (mask)



static inline unsigned long find_zero(unsigned long mask)

{

	return ((a - REPEAT_BYTE(0x01)) & ~a) & REPEAT_BYTE(0x80);

	return count_masked_bytes(mask);

}



/*

fs/namei.c

+12 −10
Original line number Diff line number Diff line
@@ -1452,7 +1452,8 @@ EXPORT_SYMBOL(full_name_hash); 
 */

static inline unsigned long hash_name(const char *name, unsigned int *hashp)

{

	unsigned long a, mask, hash, len;

	unsigned long a, b, adata, bdata, mask, hash, len;

	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;



	hash = a = 0;

	len = -sizeof(unsigned long);
@@ -1460,17 +1461,18 @@ static inline unsigned long hash_name(const char *name, unsigned int *hashp) 
		hash = (hash + a) * 9;

		len += sizeof(unsigned long);

		a = load_unaligned_zeropad(name+len);

		/* Do we have any NUL or '/' bytes in this word? */

		mask = has_zero(a) | has_zero(a ^ REPEAT_BYTE('/'));

	} while (!mask);



	/* The mask *below* the first high bit set */

	mask = (mask - 1) & ~mask;

	mask >>= 7;

	hash += a & mask;

		b = a ^ REPEAT_BYTE('/');

	} while (!(has_zero(a, &adata, &constants) | has_zero(b, &bdata, &constants)));



	adata = prep_zero_mask(a, adata, &constants);

	bdata = prep_zero_mask(b, bdata, &constants);



	mask = create_zero_mask(adata | bdata);



	hash += a & zero_bytemask(mask);

	*hashp = fold_hash(hash);



	return len + count_masked_bytes(mask);

	return len + find_zero(mask);

}



#else

include/asm-generic/word-at-a-time.h

0 → 100644
+52 −0
Original line number Diff line number Diff line 
#ifndef _ASM_WORD_AT_A_TIME_H

#define _ASM_WORD_AT_A_TIME_H



/*

 * This says "generic", but it's actually big-endian only.

 * Little-endian can use more efficient versions of these

 * interfaces, see for example

 *	 arch/x86/include/asm/word-at-a-time.h

 * for those.

 */



#include <linux/kernel.h>



struct word_at_a_time {

	const unsigned long high_bits, low_bits;

};



#define WORD_AT_A_TIME_CONSTANTS { REPEAT_BYTE(0xfe) + 1, REPEAT_BYTE(0x7f) }



/* Bit set in the bytes that have a zero */

static inline long prep_zero_mask(unsigned long val, unsigned long rhs, const struct word_at_a_time *c)

{

	unsigned long mask = (val & c->low_bits) + c->low_bits;

	return ~(mask | rhs);

}



#define create_zero_mask(mask) (mask)



static inline long find_zero(unsigned long mask)

{

	long byte = 0;

#ifdef CONFIG_64BIT

	if (mask >> 32)

		mask >>= 32;

	else

		byte = 4;

#endif

	if (mask >> 16)

		mask >>= 16;

	else

		byte += 2;

	return (mask >> 8) ? byte : byte + 1;

}



static inline bool has_zero(unsigned long val, unsigned long *data, const struct word_at_a_time *c)

{

	unsigned long rhs = val | c->low_bits;

	*data = rhs;

	return (val + c->high_bits) & ~rhs;

}



#endif /* _ASM_WORD_AT_A_TIME_H */